Vitamins are a diverse group of organic compounds, required in small amounts in the diet. They have distinct biochemical roles, often as coenzymes, and are either not synthesized or synthesized only in limited amounts by human cells. Vitamins are classified according to their solubility, either fat-soluble or water-soluble. The physiological processes dependent on vitamin-requiring reactions include many aspects of intermediary metabolism, vision, bone formation, and blood coagulation, and vitamin deficiencies are associated with a correspondingly diverse and severe group of diseases.
Water-soluble vitamins include ascorbate (vitamin C) and the members of the B group: thiamin (vitamin B1), riboflavin (B2), niacin (B3), pantothenate (B5), pyridoxine (B6), biotin (B7), folate (B9), and cobalamin (B12). Metabolic processes annotated here include the synthesis of thiamin pyrophosphate (TPP) from thiamin (B1), the synthesis of FMN and FAD from riboflavin (B2), the synthesis of nicotinic acid (niacin - B3) from tryptophan, the synthesis of Coenzyme A from pantothenate (B5), and features of the metabolism of folate (B9).
Board PG, Coggan M, Chelvanayagam G, Easteal S, Jermiin LS, Schulte GK, Danley DE, Hoth LR, Griffor MC, Kamath AV, Rosner MH, Chrunyk BA, Perregaux DE, Gabel CA, Geoghegan KF, Pandit J.; ''Identification, characterization, and crystal structure of the Omega class glutathione transferases.''; PubMedEurope PMCScholia
Quadros EV, Rothenberg SP, Pan YC, Stein S.; ''Purification and molecular characterization of human transcobalamin II.''; PubMedEurope PMCScholia
Goding JW, Grobben B, Slegers H.; ''Physiological and pathophysiological functions of the ecto-nucleotide pyrophosphatase/phosphodiesterase family.''; PubMedEurope PMCScholia
Kim J, Gherasim C, Banerjee R.; ''Decyanation of vitamin B12 by a trafficking chaperone.''; PubMedEurope PMCScholia
Du J, Cullen JJ, Buettner GR.; ''Ascorbic acid: chemistry, biology and the treatment of cancer.''; PubMedEurope PMCScholia
Fyfe JC, Madsen M, Højrup P, Christensen EI, Tanner SM, de la Chapelle A, He Q, Moestrup SK.; ''The functional cobalamin (vitamin B12)-intrinsic factor receptor is a novel complex of cubilin and amnionless.''; PubMedEurope PMCScholia
Maras B, Barra D, Duprè S, Pitari G.; ''Is pantetheinase the actual identity of mouse and human vanin-1 proteins?''; PubMedEurope PMCScholia
Johnson MA, Kuo YM, Westaway SK, Parker SM, Ching KH, Gitschier J, Hayflick SJ.; ''Mitochondrial localization of human PANK2 and hypotheses of secondary iron accumulation in pantothenate kinase-associated neurodegeneration.''; PubMedEurope PMCScholia
Hänzelmann P, Schwarz G, Mendel RR.; ''Functionality of alternative splice forms of the first enzymes involved in human molybdenum cofactor biosynthesis.''; PubMedEurope PMCScholia
Hartmann T, Terao M, Garattini E, Teutloff C, Alfaro JF, Jones JP, Leimkühler S.; ''The impact of single nucleotide polymorphisms on human aldehyde oxidase.''; PubMedEurope PMCScholia
Labay V, Raz T, Baron D, Mandel H, Williams H, Barrett T, Szargel R, McDonald L, Shalata A, Nosaka K, Gregory S, Cohen N.; ''Mutations in SLC19A2 cause thiamine-responsive megaloblastic anaemia associated with diabetes mellitus and deafness.''; PubMedEurope PMCScholia
Wolthers KR, Scrutton NS.; ''Cobalamin uptake and reactivation occurs through specific protein interactions in the methionine synthase-methionine synthase reductase complex.''; PubMedEurope PMCScholia
Hannibal L, Kim J, Brasch NE, Wang S, Rosenblatt DS, Banerjee R, Jacobsen DW.; ''Processing of alkylcobalamins in mammalian cells: A role for the MMACHC (cblC) gene product.''; PubMedEurope PMCScholia
Xiang S, Tong L.; ''Crystal structures of human and Staphylococcus aureus pyruvate carboxylase and molecular insights into the carboxyltransfer reaction.''; PubMedEurope PMCScholia
Rajan DP, Huang W, Dutta B, Devoe LD, Leibach FH, Ganapathy V, Prasad PD.; ''Human placental sodium-dependent vitamin C transporter (SVCT2): molecular cloning and transport function.''; PubMedEurope PMCScholia
Mah W, Deme JC, Watkins D, Fung S, Janer A, Shoubridge EA, Rosenblatt DS, Coulton JW.; ''Subcellular location of MMACHC and MMADHC, two human proteins central to intracellular vitamin B(12) metabolism.''; PubMedEurope PMCScholia
Stallmeyer B, Schwarz G, Schulze J, Nerlich A, Reiss J, Kirsch J, Mendel RR.; ''The neurotransmitter receptor-anchoring protein gephyrin reconstitutes molybdenum cofactor biosynthesis in bacteria, plants, and mammalian cells.''; PubMedEurope PMCScholia
Shah NP, Beech CM, Sturm AC, Tanner SM.; ''Investigation of the ABC transporter MRP1 in selected patients with presumed defects in vitamin B12 absorption.''; PubMedEurope PMCScholia
Fleming JC, Tartaglini E, Steinkamp MP, Schorderet DF, Cohen N, Neufeld EJ.; ''The gene mutated in thiamine-responsive anaemia with diabetes and deafness (TRMA) encodes a functional thiamine transporter.''; PubMedEurope PMCScholia
Wilson A, Leclerc D, Rosenblatt DS, Gravel RA.; ''Molecular basis for methionine synthase reductase deficiency in patients belonging to the cblE complementation group of disorders in folate/cobalamin metabolism.''; PubMedEurope PMCScholia
Subramanian VS, Marchant JS, Said HM.; ''Targeting and trafficking of the human thiamine transporter-2 in epithelial cells.''; PubMedEurope PMCScholia
Hayman AR, Warburton MJ, Pringle JA, Coles B, Chambers TJ.; ''Purification and characterization of a tartrate-resistant acid phosphatase from human osteoclastomas.''; PubMedEurope PMCScholia
Chandler CS, Ballard FJ.; ''Distribution and degradation of biotin-containing carboxylases in human cell lines.''; PubMedEurope PMCScholia
Takahashi-Íñiguez T, García-Arellano H, Trujillo-Roldán MA, Flores ME.; ''Protection and reactivation of human methylmalonyl-CoA mutase by MMAA protein.''; PubMedEurope PMCScholia
Ingaramo M, Beckett D.; ''Selectivity in post-translational biotin addition to five human carboxylases.''; PubMedEurope PMCScholia
Siu VM, Ratko S, Prasad AN, Prasad C, Rupar CA.; ''Amish microcephaly: Long-term survival and biochemical characterization.''; PubMedEurope PMCScholia
Leal NA, Park SD, Kima PE, Bobik TA.; ''Identification of the human and bovine ATP:Cob(I)alamin adenosyltransferase cDNAs based on complementation of a bacterial mutant.''; PubMedEurope PMCScholia
Brizio C, Galluccio M, Wait R, Torchetti EM, Bafunno V, Accardi R, Gianazza E, Indiveri C, Barile M.; ''Over-expression in Escherichia coli and characterization of two recombinant isoforms of human FAD synthetase.''; PubMedEurope PMCScholia
Aminoff M, Carter JE, Chadwick RB, Johnson C, Gräsbeck R, Abdelaal MA, Broch H, Jenner LB, Verroust PJ, Moestrup SK, de la Chapelle A, Krahe R.; ''Mutations in CUBN, encoding the intrinsic factor-vitamin B12 receptor, cubilin, cause hereditary megaloblastic anaemia 1.''; PubMedEurope PMCScholia
Leclerc D, Campeau E, Goyette P, Adjalla CE, Christensen B, Ross M, Eydoux P, Rosenblatt DS, Rozen R, Gravel RA.; ''Human methionine synthase: cDNA cloning and identification of mutations in patients of the cblG complementation group of folate/cobalamin disorders.''; PubMedEurope PMCScholia
Wolf B, Jensen K.; ''Evolutionary conservation of biotinidase: implications for the enzyme's structure and subcellular localization.''; PubMedEurope PMCScholia
Leonardi R, Rock CO, Jackowski S, Zhang YM.; ''Activation of human mitochondrial pantothenate kinase 2 by palmitoylcarnitine.''; PubMedEurope PMCScholia
Batra J, Szabó A, Caulfield TR, Soares AS, Sahin-Tóth M, Radisky ES.; ''Long-range electrostatic complementarity governs substrate recognition by human chymotrypsin C, a key regulator of digestive enzyme activation.''; PubMedEurope PMCScholia
Srikumar K, Premalatha R.; ''Effect of gastrointestinal proteases on purified human intrinsic factor-vitamin B12 (IF-B12) complex.''; PubMedEurope PMCScholia
Prasad PD, Wang H, Huang W, Fei YJ, Leibach FH, Devoe LD, Ganapathy V.; ''Molecular and functional characterization of the intestinal Na+-dependent multivitamin transporter.''; PubMedEurope PMCScholia
Joshi AK, Zhang L, Rangan VS, Smith S.; ''Cloning, expression, and characterization of a human 4'-phosphopantetheinyl transferase with broad substrate specificity.''; PubMedEurope PMCScholia
Wolthers KR, Lou X, Toogood HS, Leys D, Scrutton NS.; ''Mechanism of coenzyme binding to human methionine synthase reductase revealed through the crystal structure of the FNR-like module and isothermal titration calorimetry.''; PubMedEurope PMCScholia
Belli SI, Goding JW.; ''Biochemical characterization of human PC-1, an enzyme possessing alkaline phosphodiesterase I and nucleotide pyrophosphatase activities.''; PubMedEurope PMCScholia
Nabokina SM, Subramanian VS, Said HM.; ''Association of PDZ-containing protein PDZD11 with the human sodium-dependent multivitamin transporter.''; PubMedEurope PMCScholia
Strauss E, Zhai H, Brand LA, McLafferty FW, Begley TP.; ''Mechanistic studies on phosphopantothenoylcysteine decarboxylase: trapping of an enethiolate intermediate with a mechanism-based inactivating agent.''; PubMedEurope PMCScholia
Yonezawa A, Masuda S, Katsura T, Inui K.; ''Identification and functional characterization of a novel human and rat riboflavin transporter, RFT1.''; PubMedEurope PMCScholia
Hänzelmann P, Hernández HL, Menzel C, García-Serres R, Huynh BH, Johnson MK, Mendel RR, Schindelin H.; ''Characterization of MOCS1A, an oxygen-sensitive iron-sulfur protein involved in human molybdenum cofactor biosynthesis.''; PubMedEurope PMCScholia
Kang J, Samuels DC.; ''The evidence that the DNC (SLC25A19) is not the mitochondrial deoxyribonucleotide carrier.''; PubMedEurope PMCScholia
Ashokkumar B, Vaziri ND, Said HM.; ''Thiamin uptake by the human-derived renal epithelial (HEK-293) cells: cellular and molecular mechanisms.''; PubMedEurope PMCScholia
Wolthers KR, Scrutton NS.; ''Protein interactions in the human methionine synthase-methionine synthase reductase complex and implications for the mechanism of enzyme reactivation.''; PubMedEurope PMCScholia
Stucki M, Coelho D, Suormala T, Burda P, Fowler B, Baumgartner MR.; ''Molecular mechanisms leading to three different phenotypes in the cblD defect of intracellular cobalamin metabolism.''; PubMedEurope PMCScholia
Lee HS, Moon BJ, Choi SY, Kwon OS.; ''Human pyridoxal kinase: overexpression and properties of the recombinant enzyme.''; PubMedEurope PMCScholia
Nosaka K, Onozuka M, Kakazu N, Hibi S, Nishimura H, Nishino H, Abe T.; ''Isolation and characterization of a human thiamine pyrophosphokinase cDNA.''; PubMedEurope PMCScholia
Quadros EV.; ''Advances in the understanding of cobalamin assimilation and metabolism.''; PubMedEurope PMCScholia
Quadros EV, Lai SC, Nakayama Y, Sequeira JM, Hannibal L, Wang S, Jacobsen DW, Fedosov S, Wright E, Gallagher RC, Anastasio N, Watkins D, Rosenblatt DS.; ''Positive newborn screen for methylmalonic aciduria identifies the first mutation in TCblR/CD320, the gene for cellular uptake of transcobalamin-bound vitamin B(12).''; PubMedEurope PMCScholia
Wolf B, Grier RE, Allen RJ, Goodman SI, Kien CL.; ''Biotinidase deficiency: the enzymatic defect in late-onset multiple carboxylase deficiency.''; PubMedEurope PMCScholia
Diaz GA, Banikazemi M, Oishi K, Desnick RJ, Gelb BD.; ''Mutations in a new gene encoding a thiamine transporter cause thiamine-responsive megaloblastic anaemia syndrome.''; PubMedEurope PMCScholia
Cole H, Reynolds TR, Lockyer JM, Buck GA, Denson T, Spence JE, Hymes J, Wolf B.; ''Human serum biotinidase. cDNA cloning, sequence, and characterization.''; PubMedEurope PMCScholia
Wuebbens MM, Rajagopalan KV.; ''Mechanistic and mutational studies of Escherichia coli molybdopterin synthase clarify the final step of molybdopterin biosynthesis.''; PubMedEurope PMCScholia
Wilson A, Platt R, Wu Q, Leclerc D, Christensen B, Yang H, Gravel RA, Rozen R.; ''A common variant in methionine synthase reductase combined with low cobalamin (vitamin B12) increases risk for spina bifida.''; PubMedEurope PMCScholia
Hiratsuka M, Sakamoto O, Li X, Suzuki Y, Aoki Y, Narisawa K.; ''Identification of holocarboxylase synthetase (HCS) proteins in human placenta.''; PubMedEurope PMCScholia
Deme JC, Miousse IR, Plesa M, Kim JC, Hancock MA, Mah W, Rosenblatt DS, Coulton JW.; ''Structural features of recombinant MMADHC isoforms and their interactions with MMACHC, proteins of mammalian vitamin B12 metabolism.''; PubMedEurope PMCScholia
Simpson GL, Ortwerth BJ.; ''The non-oxidative degradation of ascorbic acid at physiological conditions.''; PubMedEurope PMCScholia
Shirabe K, Landi MT, Takeshita M, Uziel G, Fedrizzi E, Borgese N.; ''A novel point mutation in a 3' splice site of the NADH-cytochrome b5 reductase gene results in immunologically undetectable enzyme and impaired NADH-dependent ascorbate regeneration in cultured fibroblasts of a patient with type II hereditary methemoglobinemia.''; PubMedEurope PMCScholia
Hymes J, Wolf B.; ''Human biotinidase isn't just for recycling biotin.''; PubMedEurope PMCScholia
Wang H, Dutta B, Huang W, Devoe LD, Leibach FH, Ganapathy V, Prasad PD.; ''Human Na(+)-dependent vitamin C transporter 1 (hSVCT1): primary structure, functional characteristics and evidence for a non-functional splice variant.''; PubMedEurope PMCScholia
Nishino K, Itokawa Y, Nishino N, Piros K, Cooper JR.; ''Enzyme system involved in the synthesis of thiamin triphosphate. I. Purification and characterization of protein-bound thiamin diphosphate: ATP phosphoryltransferase.''; PubMedEurope PMCScholia
Jansen R, Kalousek F, Fenton WA, Rosenberg LE, Ledley FD.; ''Cloning of full-length methylmalonyl-CoA mutase from a cDNA library using the polymerase chain reaction.''; PubMedEurope PMCScholia
Quadros EV, Nakayama Y, Sequeira JM.; ''The binding properties of the human receptor for the cellular uptake of vitamin B12.''; PubMedEurope PMCScholia
Marelja Z, Stöcklein W, Nimtz M, Leimkühler S.; ''A novel role for human Nfs1 in the cytoplasm: Nfs1 acts as a sulfur donor for MOCS3, a protein involved in molybdenum cofactor biosynthesis.''; PubMedEurope PMCScholia
Aghajanian S, Worrall DM.; ''Identification and characterization of the gene encoding the human phosphopantetheine adenylyltransferase and dephospho-CoA kinase bifunctional enzyme (CoA synthase).''; PubMedEurope PMCScholia
Zhyvoloup A, Nemazanyy I, Panasyuk G, Valovka T, Fenton T, Rebholz H, Wang ML, Foxon R, Lyzogubov V, Usenko V, Kyyamova R, Gorbenko O, Matsuka G, Filonenko V, Gout IT.; ''Subcellular localization and regulation of coenzyme A synthase.''; PubMedEurope PMCScholia
Karthikeyan S, Zhou Q, Mseeh F, Grishin NV, Osterman AL, Zhang H.; ''Crystal structure of human riboflavin kinase reveals a beta barrel fold and a novel active site arch.''; PubMedEurope PMCScholia
Suzuki Y, Yang X, Aoki Y, Kure S, Matsubara Y.; ''Mutations in the holocarboxylase synthetase gene HLCS.''; PubMedEurope PMCScholia
Lakaye B, Makarchikov AF, Antunes AF, Zorzi W, Coumans B, De Pauw E, Wins P, Grisar T, Bettendorff L.; ''Molecular characterization of a specific thiamine triphosphatase widely expressed in mammalian tissues.''; PubMedEurope PMCScholia
Said HM, Balamurugan K, Subramanian VS, Marchant JS.; ''Expression and functional contribution of hTHTR-2 in thiamin absorption in human intestine.''; PubMedEurope PMCScholia
Ichida K, Matsumura T, Sakuma R, Hosoya T, Nishino T.; ''Mutation of human molybdenum cofactor sulfurase gene is responsible for classical xanthinuria type II.''; PubMedEurope PMCScholia
Prohl C, Pelzer W, Diekert K, Kmita H, Bedekovics T, Kispal G, Lill R.; ''The yeast mitochondrial carrier Leu5p and its human homologue Graves' disease protein are required for accumulation of coenzyme A in the matrix.''; PubMedEurope PMCScholia
Lerner-Ellis JP, Tirone JC, Pawelek PD, Doré C, Atkinson JL, Watkins D, Morel CF, Fujiwara TM, Moras E, Hosack AR, Dunbar GV, Antonicka H, Forgetta V, Dobson CM, Leclerc D, Gravel RA, Shoubridge EA, Coulton JW, Lepage P, Rommens JM, Morgan K, Rosenblatt DS.; ''Identification of the gene responsible for methylmalonic aciduria and homocystinuria, cblC type.''; PubMedEurope PMCScholia
Quadros EV, Nakayama Y, Sequeira JM.; ''The protein and the gene encoding the receptor for the cellular uptake of transcobalamin-bound cobalamin.''; PubMedEurope PMCScholia
Mathews FS, Gordon MM, Chen Z, Rajashankar KR, Ealick SE, Alpers DH, Sukumar N.; ''Crystal structure of human intrinsic factor: cobalamin complex at 2.6-A resolution.''; PubMedEurope PMCScholia
Weatherly SC, Volrath SL, Elich TD.; ''Expression and characterization of recombinant fungal acetyl-CoA carboxylase and isolation of a soraphen-binding domain.''; PubMedEurope PMCScholia
Hörtnagel K, Prokisch H, Meitinger T.; ''An isoform of hPANK2, deficient in pantothenate kinase-associated neurodegeneration, localizes to mitochondria.''; PubMedEurope PMCScholia
Fan C, Bobik TA.; ''Functional characterization and mutation analysis of human ATP:Cob(I)alamin adenosyltransferase.''; PubMedEurope PMCScholia
Andersen CB, Madsen M, Storm T, Moestrup SK, Andersen GR.; ''Structural basis for receptor recognition of vitamin-B(12)-intrinsic factor complexes.''; PubMedEurope PMCScholia
Zhang YM, Rock CO, Jackowski S.; ''Biochemical properties of human pantothenate kinase 2 isoforms and mutations linked to pantothenate kinase-associated neurodegeneration.''; PubMedEurope PMCScholia
di Salvo ML, Hunt S, Schirch V.; ''Expression, purification, and kinetic constants for human and Escherichia coli pyridoxal kinases.''; PubMedEurope PMCScholia
Coelho D, Suormala T, Stucki M, Lerner-Ellis JP, Rosenblatt DS, Newbold RF, Baumgartner MR, Fowler B.; ''Gene identification for the cblD defect of vitamin B12 metabolism.''; PubMedEurope PMCScholia
Hymes J, Wolf B.; ''Biotinidase and its roles in biotin metabolism.''; PubMedEurope PMCScholia
Bailey LM, Wallace JC, Polyak SW.; ''Holocarboxylase synthetase: correlation of protein localisation with biological function.''; PubMedEurope PMCScholia
Srinivasan B, Baratashvili M, van der Zwaag M, Kanon B, Colombelli C, Lambrechts RA, Schaap O, Nollen EA, Podgoršek A, Kosec G, Petković H, Hayflick S, Tiranti V, Reijngoud DJ, Grzeschik NA, Sibon OC.; ''Extracellular 4'-phosphopantetheine is a source for intracellular coenzyme A synthesis.''; PubMedEurope PMCScholia
Dobson CM, Wai T, Leclerc D, Wilson A, Wu X, Doré C, Hudson T, Rosenblatt DS, Gravel RA.; ''Identification of the gene responsible for the cblA complementation group of vitamin B12-responsive methylmalonic acidemia based on analysis of prokaryotic gene arrangements.''; PubMedEurope PMCScholia
Kang JH, Hong ML, Kim DW, Park J, Kang TC, Won MH, Baek NI, Moon BJ, Choi SY, Kwon OS.; ''Genomic organization, tissue distribution and deletion mutation of human pyridoxine 5'-phosphate oxidase.''; PubMedEurope PMCScholia
Tanner SM, Aminoff M, Wright FA, Liyanarachchi S, Kuronen M, Saarinen A, Massika O, Mandel H, Broch H, de la Chapelle A.; ''Amnionless, essential for mouse gastrulation, is mutated in recessive hereditary megaloblastic anemia.''; PubMedEurope PMCScholia
Rutsch F, Gailus S, Miousse IR, Suormala T, Sagné C, Toliat MR, Nürnberg G, Wittkampf T, Buers I, Sharifi A, Stucki M, Becker C, Baumgartner M, Robenek H, Marquardt T, Höhne W, Gasnier B, Rosenblatt DS, Fowler B, Nürnberg P.; ''Identification of a putative lysosomal cobalamin exporter altered in the cblF defect of vitamin B12 metabolism.''; PubMedEurope PMCScholia
Subramanian VS, Marchant JS, Said HM.; ''Biotin-responsive basal ganglia disease-linked mutations inhibit thiamine transport via hTHTR2: biotin is not a substrate for hTHTR2.''; PubMedEurope PMCScholia
Kristiansen M, Aminoff M, Jacobsen C, de La Chapelle A, Krahe R, Verroust PJ, Moestrup SK.; ''Cubilin P1297L mutation associated with hereditary megaloblastic anemia 1 causes impaired recognition of intrinsic factor-vitamin B(12) by cubilin.''; PubMedEurope PMCScholia
Densupsoontorn N, Sanpakit K, Vijarnsorn C, Pattaragarn A, Kangwanpornsiri C, Jatutipsompol C, Tirapongporn H, Jirapinyo P, Shah NP, Sturm AC, Tanner SM.; ''Imerslund-Gräsbeck syndrome: new mutation in amnionless.''; PubMedEurope PMCScholia
Manoj N, Strauss E, Begley TP, Ealick SE.; ''Structure of human phosphopantothenoylcysteine synthetase at 2.3 A resolution.''; PubMedEurope PMCScholia
Daruwala R, Song J, Koh WS, Rumsey SC, Levine M.; ''Cloning and functional characterization of the human sodium-dependent vitamin C transporters hSVCT1 and hSVCT2.''; PubMedEurope PMCScholia
Borgese N, D'Arrigo A, De Silvestris M, Pietrini G.; ''NADH-cytochrome b5 reductase and cytochrome b5 isoforms as models for the study of post-translational targeting to the endoplasmic reticulum.''; PubMedEurope PMCScholia
Balamurugan K, Ortiz A, Said HM.; ''Biotin uptake by human intestinal and liver epithelial cells: role of the SMVT system.''; PubMedEurope PMCScholia
Liang WJ, Johnson D, Jarvis SM.; ''Vitamin C transport systems of mammalian cells.''; PubMedEurope PMCScholia
Youngdahl-Turner P, Mellman IS, Allen RH, Rosenberg LE.; ''Protein mediated vitamin uptake. Adsorptive endocytosis of the transcobalamin II-cobalamin complex by cultured human fibroblasts.''; PubMedEurope PMCScholia
Leimkuhler S, Freuer A, Araujo JA, Rajagopalan KV, Mendel RR.; ''Mechanistic studies of human molybdopterin synthase reaction and characterization of mutants identified in group B patients of molybdenum cofactor deficiency.''; PubMedEurope PMCScholia
Padovani D, Banerjee R.; ''Assembly and protection of the radical enzyme, methylmalonyl-CoA mutase, by its chaperone.''; PubMedEurope PMCScholia
Abu-Elheiga L, Jayakumar A, Baldini A, Chirala SS, Wakil SJ.; ''Human acetyl-CoA carboxylase: characterization, molecular cloning, and evidence for two isoforms.''; PubMedEurope PMCScholia
Worgan LC, Niles K, Tirone JC, Hofmann A, Verner A, Sammak A, Kucic T, Lepage P, Rosenblatt DS.; ''Spectrum of mutations in mut methylmalonic acidemia and identification of a common Hispanic mutation and haplotype.''; PubMedEurope PMCScholia
Lerner-Ellis JP, Dobson CM, Wai T, Watkins D, Tirone JC, Leclerc D, Doré C, Lepage P, Gravel RA, Rosenblatt DS.; ''Mutations in the MMAA gene in patients with the cblA disorder of vitamin B12 metabolism.''; PubMedEurope PMCScholia
Green P, Wiseman M, Crow YJ, Houlden H, Riphagen S, Lin JP, Raymond FL, Childs AM, Sheridan E, Edwards S, Josifova DJ.; ''Brown-Vialetto-Van Laere syndrome, a ponto-bulbar palsy with deafness, is caused by mutations in c20orf54.''; PubMedEurope PMCScholia
Schmuck EM, Board PG, Whitbread AK, Tetlow N, Cavanaugh JA, Blackburn AC, Masoumi A.; ''Characterization of the monomethylarsonate reductase and dehydroascorbate reductase activities of Omega class glutathione transferase variants: implications for arsenic metabolism and the age-at-onset of Alzheimer's and Parkinson's diseases.''; PubMedEurope PMCScholia
Froese DS, Kochan G, Muniz JR, Wu X, Gileadi C, Ugochukwu E, Krysztofinska E, Gravel RA, Oppermann U, Yue WW.; ''Structures of the human GTPase MMAA and vitamin B12-dependent methylmalonyl-CoA mutase and insight into their complex formation.''; PubMedEurope PMCScholia
Daugherty M, Polanuyer B, Farrell M, Scholle M, Lykidis A, de Crécy-Lagard V, Osterman A.; ''Complete reconstitution of the human coenzyme A biosynthetic pathway via comparative genomics.''; PubMedEurope PMCScholia
Zhou B, Westaway SK, Levinson B, Johnson MA, Gitschier J, Hayflick SJ.; ''A novel pantothenate kinase gene (PANK2) is defective in Hallervorden-Spatz syndrome.''; PubMedEurope PMCScholia
Doolin MT, Barbaux S, McDonnell M, Hoess K, Whitehead AS, Mitchell LE.; ''Maternal genetic effects, exerted by genes involved in homocysteine remethylation, influence the risk of spina bifida.''; PubMedEurope PMCScholia
Yao Y, Yonezawa A, Yoshimatsu H, Masuda S, Katsura T, Inui K.; ''Identification and comparative functional characterization of a new human riboflavin transporter hRFT3 expressed in the brain.''; PubMedEurope PMCScholia
Bosch AM, Abeling NG, Ijlst L, Knoester H, van der Pol WL, Stroomer AE, Wanders RJ, Visser G, Wijburg FA, Duran M, Waterham HR.; ''Brown-Vialetto-Van Laere and Fazio Londe syndrome is associated with a riboflavin transporter defect mimicking mild MADD: a new inborn error of metabolism with potential treatment.''; PubMedEurope PMCScholia
Yoshimatsu H, Yonezawa A, Yao Y, Sugano K, Nakagawa S, Omura T, Matsubara K.; ''Functional involvement of RFVT3/SLC52A3 in intestinal riboflavin absorption.''; PubMedEurope PMCScholia
Plesa M, Kim J, Paquette SG, Gagnon H, Ng-Thow-Hing C, Gibbs BF, Hancock MA, Rosenblatt DS, Coulton JW.; ''Interaction between MMACHC and MMADHC, two human proteins participating in intracellular vitamin B₁₂ metabolism.''; PubMedEurope PMCScholia
Randaccio L, Geremia S, Demitri N, Wuerges J.; ''Vitamin B12: unique metalorganic compounds and the most complex vitamins.''; PubMedEurope PMCScholia
Ramaswamy G, Karim MA, Murti KG, Jackowski S.; ''PPARalpha controls the intracellular coenzyme A concentration via regulation of PANK1alpha gene expression.''; PubMedEurope PMCScholia
Coelho D, Kim JC, Miousse IR, Fung S, du Moulin M, Buers I, Suormala T, Burda P, Frapolli M, Stucki M, Nürnberg P, Thiele H, Robenek H, Höhne W, Longo N, Pasquali M, Mengel E, Watkins D, Shoubridge EA, Majewski J, Rosenblatt DS, Fowler B, Rutsch F, Baumgartner MR.; ''Mutations in ABCD4 cause a new inborn error of vitamin B12 metabolism.''; PubMedEurope PMCScholia
Johnston J, Bollekens J, Allen RH, Berliner N.; ''Structure of the cDNA encoding transcobalamin I, a neutrophil granule protein.''; PubMedEurope PMCScholia
Wuerges J, Garau G, Geremia S, Fedosov SN, Petersen TE, Randaccio L.; ''Structural basis for mammalian vitamin B12 transport by transcobalamin.''; PubMedEurope PMCScholia
Nielsen MJ, Rasmussen MR, Andersen CB, Nexø E, Moestrup SK.; ''Vitamin B12 transport from food to the body's cells--a sophisticated, multistep pathway.''; PubMedEurope PMCScholia
Zhao R, Gao F, Goldman ID.; ''Molecular cloning of human thiamin pyrophosphokinase.''; PubMedEurope PMCScholia
This CandidateSet contains sequences identified by William Pearson's analysis of Reactome catalyst entities. Catalyst entity sequences were used to identify analagous sequences that shared overall homology and active site homology. Sequences in this Candidate set were identified in an April 24, 2012 analysis.
The plasma membrane-associated transport protein SLC5A6 (SMVT) mediates the uptake of one molecule of pantothenate (PanK) and two sodium ions from the extracellular space to the cytosol. Limited Northern blotting studies suggest that SLC5A6 is widely expressed, and abundant in placenta and small intestine. SLC5A6 may thus play a central role in pantothenate uptake. SLC5A6 also mediates the uptake of biotin and lipoate (Prasad et al. 1999; Wang et al. 1999).
The methenyltetrahydrofolate cyclohydrolase activity of the trifunctional MTHFD1 enzyme catalyzes the reversible reaction of 5,10-methenylTHF polyglutamate and H2O to form 10-formylTHF polyglutamate. MTHFD1 is cytosolic and occurs as a dimer. The human enzyme has been identified and partially characterized biochemically (Hum et al. 1988); additional reaction details can be inferred from the properties of the well-studied homologous rabbit enzyme (Villar et al. 1985).
Two transport proteins, SLC19A2 (THTR1) and SLC19A3 (THTR2), associated with the plasma membrane, are each able to mediate the transport of extracellular thiamin into the cytosol. In the body, both transporters are widely distributed, and both are abundant in kidney and intestinal epithelia, consistent with their involvement in thiamin uptake under physiological conditions (Ashokkumar et al. 2006; Said et al. 2004; Subramanian et al. 2006 - J Biol Chem). The observation that mutations in SLC19A2 (THTR1) cause a progressive disorder that can be partially reversed by treatment with high doses of thiamin likewise suggests a role for this protein in thiamin uptake under normal conditions (Diaz et al. 1999; Fleming et al. 1999; Labay et al. 1999).
Two features of this transport process remain incompletely understood, however. First, mutations in SLC19A3 cause a progressive disorder that is responsive to biotin treatment (Zhou et al. 2005), although studies of cultured cells indicate that the protein has no affinity for biotin (Subramanian et al. 2006 - Am J Physiol). Also, studies to date provide little information about the mechanism by which thiamin, once taken up by epithelial cells in the intestine and kidney, is released from these cells into the blood.
Cytosolic folylpolyglutamate synthase catalyzes the reaction of THF-glutamate(n), L-glutamate, and ATP to form THF-glutamate(n+1), ADP, and orthophosphate. (The first glutamate residue is attached to the glutamate moiety of THF itself; for convenience the process is annotated here as if it proceeded in a single concerted step.) The extent of conjugation is variable, but the commonest cytosolic form of THF has five added glutamate residues. Although its properties as a donor of one-carbon units are not affected by glutamate addition, THF that lacks added glutamate residues cannot be retained in the cytosol so this reaction is needed for normal THF function under physiological conditions (Garrow et al. 1992; Chen et al. 1996).
The enzyme, nicotinate nucleotide pyrophosphorylase, is specific for quinolinate. Its activity is strictly dependent on Mg2+ ions being present. A phosphoribosyl group is transferred to quinolinate to form nicotinate D-ribonucleotide. This reaction represents another rate-limiting step of the pathway from tryptophan to NAD+.
The kinase activity of CoA synthase (COASY) catalyzes the phosphorylation of dephospho-CoA to form Coenzyme A (CoA-SH). The enzyme is located in the mitochondrial outer membrane (Daugherty et al. 2002; Zhyvoloup et al. 2003).
Cytosolic omega class glutathione transferases (GSTO1 and GSTO2) catalyze the reaction of dehydroascorbate (DHvitC) and glutathione (GSH) to form ascorbate (VitC) and oxidized glutathione (GSSG). The GSTO enzymes occur as homodimers (Board et al. 2000), and while both have dehydroascorbate reductase activity in vitro, that of GSTO2 is much greater than that of GSTO1 (Schmuck et al. 2005). Polymorphisms affecting the activities of the two enzymes have been described (Whitbread et al. 2005).
Cytosolic folylpolyglutamate synthase catalyzes the reaction of 5-methylTHF-glutamate(n), L-glutamate, and ATP to form 5-methylTHF-glutamate(n+1), ADP, and orthophosphate. (The first glutamate residue is attached to the glutamate moiety of 5-methylTHF itself; for convenience the process is annotated here as if it proceeded in a single concerted step.) The extent of conjugation is variable, but the commonest cytosolic form of 5-methylTHF has five added glutamate residues. Although its properties as a donor of one-carbon units are not affected by glutamate addition, 5-methylTHF that lacks added glutamate residues cannot be retained in the cytosol so this reaction is needed for normal 5-methylTHF function under physiological conditions (Garrow et al. 1992; Chen et al. 1996).
The reduction of cytosolic semidehydroascorbate (SDA) to ascorbate (vitC) is catalyzed by cytochrome B5 (CYB5A) associated with the mitochondrial outer membrane. In the course of the reaction, the heme iron of the cytochrome is oxidized (Linster & Van Schaftingen 2007, Shirabe et al. 1995).
Cytosolic MTHFR dimer catalyzes the reaction of 5,10-methyleneTHF polyglutamate, NADPH, and H+ to form 5-methylTHF polyglutamate and NADP+. The specificity and importance of this reaction in vivo have been established through the study of patients deficient in the enzyme (Goyette et al. 1995).
SLC46A1 protein in the plasma membrane mediates the reversible transport of folate between the extracellular space and the cytosol. Retention of folate within the cell is dependent on polyglutamate addition (Qiu et al. 2006; Chen et al. 1996).
Cytosolic serine hydroxymethyltransferase catalyzes the reversible reaction of 5,10-methyleneTHF polyglutamate and glycine to form tetrahydrofolate polyglutamate (THF polyglutamate) and serine. The active form of the enzyme is a tetramer (Renwick et al. 1998).
The formate-tetrahydrofolate ligase activity of the trifunctional MTHFD1 enzyme catalyzes the reaction of THF polyglutamate, formate, and ATP to form 10-formylTHF polyglutamate, ADP, and orthophosphate. MTHFD1 is cytosolic and occurs as a dimer. The human enzyme has been identified and partially characterized biochemically (Hum et al. 1988); additional reaction details can be inferred from the properties of the well-studied homologous rabbit enzyme (Villar et al. 1985).
The conjugation of cysteine (Cys) and 4'- phosphopantothenate (PPanK) to form 4-phosphopantothenoylcysteine (PPC) , coupled to the conversion of ATP to AMP and pyrophosphate, is catalyzed by cytosolic phosphopantothenate-cysteine ligase (PPCS aka Phosphopantothenoylcysteine synthase or PPC synthase). Mammalian PPCS prefers ATP to CTP, unlike the E. coli ortholog (Daughtery et al. 2002; Manoj et al. 2003).
NMNAT2 catalyzes the reaction of nicotinate D-ribonucleotide and ATP to form deamino-NAD+ (nicotinate adenine dinucleotide) and pyrophosphate (Sorci et al. 2007). The active form of the enzyme is a monomer in vitro (Raffaelli et al. 2002). Although the predicted amino acid sequence of the enzyme lacks an obvious signal sequence or transmembrane domain (Yalowitz et al. 2004), recombinant FLAG-tagged protein expressed in HeLa cells localizes predominantly to the Golgi apparatus (Berger et al. 2005). Its localization within the Golgi apparatus is unknown and the annotation here is based on the plausible but speculative assumption that the enzyme is associated with the Gogi membrane and accessible from the cytosol. Immunostaining studies indicate that the protein is abundant in Islets of Langerhans and in several regions of the brain (Yalowitz et al. 2004).
The plasma membrane-associated transport proteins SVCT1 and SVCT2 are each capable of mediating the uptake of one molecule of ascorbate (VitC) and two sodium ions from the extracellular space to the cytosol (Daruwala et al. 1999; Rajan et al. 1999; Wang et al. 1999). In the body SVCT2 is expressed in most tissues, while SVCT1 is largely confined to epithelial cells (Liang et al. 2001). SVCT2 may mediate fetal uptake of ascorbate from the maternal circulation (Rajan et al. 1999). The transporters responsible for its uptake from the small intestine and for its release from enterocytes into the circulation have not been identified, although both SVCT1 and 2 are expressed in intestinal cells.
FMN can be phosphorylated and adenylated to produce the second cofactor from riboflavin origins, flavin adenine dinucleotide (FAD). The enzyme responsible , FMN adenylyltransferase (FLAD1 aka FAD synthase), is cytosolic and transfers a phosphate and an adenyl group from ATP to form FAD.
Cytosolic pantothenate kinases catalyze the reaction of ATP and pantothenate to form ADP and phosphopantothenate. This enzymatic activity has been demonstrated in crude cell extracts for two isoforms of mouse pantothenate kinase 1 (Rock et al. 2002) and for their human homologues (Ramaswamy and 2004). Two additional human genes, PANK3 and PANK4, encode closely related proteins but pantothenate kinase activity has not been demonstrated experimentally for them (Leonardi et al. 2005; Zhou et al. 2001).
Cytosolic nicotinate phosphoribosyltransferase (NaPRT) catalyzes the Mg++-dependent reaction of nicotinate and phosphoribosyl pyrophosphate to form nicotinate mononucleotide (NaMN, nicotinate D-ribonucleotide) and pyrophosphate. The active form of the enzyme is a homodimer (Preiss and Handler 1958; Niedel and Dietrich 1973; Hara et al. 2007).
Mitochondrial folylpolyglutamate synthase catalyzes the reaction of THF-glutamate(n), L-glutamate, and ATP to form THF-glutamate(n+1), ADP, and orthophosphate. (The first glutamate residue is attached to the glutamate moiety of THF itself; for convenience the process is annotated here as if it proceeded in a single concerted step.) The extent of conjugation is variable, but the commonest mitochondrial form of THF has six added glutamate residues. Although its properties as a donor of one-carbon units are not affected by glutamate addition, THF that lacks added glutamate residues cannot be retained in the mitochondrial matrix so this reaction is needed for normal THF function under physiological conditions. The mitochondrial and cytosolic forms of folylpolyglutamate synthase are encoded by the same gene - alternative splicing generates mRNA with or without an initial exon encoding a mitochondrial targeting sequence (Garrow et al. 1992; Chen et al. 1996).
NAD synthase catalyzes the final step in the biosynthesis of NAD+, both in the de novo synthesis and in the salvage pathways. The enzyme makes use of glutamine as an amide donor in the reaction. NAD synthase exists as a homohexamer in the cytosol. There are two forms of NAD synthase in humans, NADsyn1 and NADsyn2. The major difference between the two forms is that NADsyn1 appears to be glutamine-dependent whereas NADsyn2 is strictly ammonia-dependent.
Nicotinamide deaminase (NAMD) deaminates nicotinamide to nicotinate. There is no literature on the human enzyme but there is evidence showing a marked nicotinamide deaminase activity when red blood cells are infected with Plasmodium falciparum (Zerez C. et al, 1990). What is not clear is whether this activity is stimulated by the parasite or encoded by its genome.
The plasma membrane-associated transport protein SLC5A6 (aka sodium-dependent multivitamin transporter, SMVT) mediates the uptake of one molecule of biotin (Btn) and two sodium ions from the extracellular space to the cytosol. Limited Northern blotting studies suggest that SLC5A6 is widely expressed and abundant in placenta, liver and small intestine. SLC5A6 may thus play a central role in Btn uptake from dietary sources. SLC5A6 also mediates the uptake of pantothenate and lipoate (Prasad et al. 1999; Wang et al. 1999, Balamurugan et al. 2003).
SLC25A32 protein in the inner mitochondrial membrane mediates the reversible transport of tetrahydrofolate between the cytosol and the mitochondrial matrix. Retention of tetrahydrofolate within the mitochondrial matrix is dependent on mitochondrial polyglutamate addition (Titus and Moran 2000; Chen et al. 1996).
Phosphatase action on flavin adenine dinucleotide (FAD) can reform flavin mononucleotide (FMN). The enzyme performing the reaction is nucleotide pyrophosphatase (ENPP1) and it exists as a homodimer on the plasma membrane.
Cytosolic serine hydroxymethyltransferase catalyzes the reversible reaction of tetrahydrofolate polyglutamate (THF polyglutamate) and serine to form 5,10-methyleneTHF polyglutamate and glycine. The active form of the enzyme is a tetramer (Renwick et al. 1998). In the body, this reaction is a major source of 5,10-methyleneTHF, which in turn is a critical precursor in the synthesis of dTMP.
SLC19A1 protein, associated with the plasma membrane, mediates the uptake of extracellular 5-methyltetrahydrofolate and other reduced folates (Williams and Flintoff 1995; Ferguson and Flintoff 1999).
The uptake of extracellular dehydroascorbate into the cytosol is mediated by GLUT1 and GLUT3 associated with the plasma membrane (Rumsey et al. 1997, 2000). This process may play a significant role in ascorbate utilization in the central nervous system (Agus et al. 1997). The process is efficiently competitively inhibited by glucose, leading to the suggestion that inhibited dehydroascorbate uptake may contribute to the pathology of diabetes (Liang et al. 2001; Rumsey et al. 2000).
Cytosolic dihydrofolate reductase catalyzes the reaction of dihydrofolate, NADPH, and H+ to form tetrahydrofolate (THF) and NADP+ (Chen et al. 1984; Davies et al. 1990).
Cytosolic, homodimeric tartrate-resistant acid phosphatase type 5 (TRAP) catalyzes the hydrolysis of flavin mononucleotide (FMN) to yield riboflavin (RIB) and orthophosphate.
Cytosolic AASDHPPT (alpha-aminoadipic semialdehyde dehydrogenase-phosphopantetheinyl transferase) catalyzes the transfer of a phosphopantetheine moiety from coenzyme A to serine 2156 within the ACP domain of FAS (fatty acyl synthase). Only a single human enzyme with phosphopantetheinyl transferase activity has been identified, and its broad substrate specificity suggests that it may be responsible as well for the postranslational modification of enzymes of lysine catabolism (Joshi et al. 2003; Praphanphoj et al. 2001).
Cytochrome b5 reductase (CYB5R3) catalyzes the reduction of cytosolic ferric CYB5A (CYB5A:ferriheme) to ferrous CYPB5A (CYB5A:heme), coupled to the conversion of NADH to NAD+ (Shirabe et al. 1995). CYB5R3 is associated with the outer mitochondrial membrane via a myristoyl group added post-translationally to glycine residue 2 of the protein (Borgese et al. 1993).
The methylenetetrahydrofolate dehydrogenase activity of the trifunctional MTHFD1 enzyme catalyzes the reversible reaction of 5,10-methenylTHF polyglutamate, NADPH, and H+ to form 5,10-methyleneTHF polyglutamate and NADP+. MTHFD1 is cytosolic and occurs as a dimer. The human enzyme has been identified and partially characterized biochemically (Hum et al. 1988); additional reaction details can be inferred from the properties of the well-studied homologous rabbit enzyme (Villar et al. 1985).
SLC46A1 protein in the plasma membrane mediates the reversible transport of folate between the extracellular space and the cytosol. Retention of folate within the cell is dependent on polyglutamate addition. Although the SLC46A1 gene is expressed in several tissues in the body, this transporter appears to be primarily needed for absorption of dietary folate from the intestinal lumen (Qiu et al. 2006; Chen et al. 1996).
The methylenetetrahydrofolate dehydrogenase activity of the trifunctional MTHFD1 enzyme catalyzes the reversible reaction of 5,10-methyleneTHF polyglutamate and NADP+ to form 5,10-methenylTHF polyglutamate, NADPH, and H+. MTHFD1 is cytosolic and occurs as a dimer. The human enzyme has been identified and partially characterized biochemically (Hum et al. 1988); additional reaction details can be inferred from the properties of the well-studied homologous rabbit enzyme (Villar et al. 1985).
NMNAT3 catalyzes the reaction of nicotinate D-ribonucleotide and ATP to form deamino-NAD+ (nicotinate adenine dinucleotide) and pyrophosphate (Sorci et al. 2007). The active form of the enzyme is a tetramer in vitro (Zhang et al. 2003). Recombinant FLAG-tagged protein expressed in HeLa cells localizes both to the cytosol and to mitochondria (Berger et al. 2005). The cytosolic protein is annotated here.
Pantothenate kinase 2 catalyzes the reaction of ATP and pantothenate to form ADP and phosphopantothenate. While pantothenate kinase 2 co-purifies with mitocondria, its precise location within the mitochondrion has not been established (Hortnagel et al. 2003; Johnson et al. 2004). Recent work by Leonardi et al. (2007) supports a model in which the enzyme is located in the intermembrane space, hence freely accessible to small molecules from the cytosol.Pantothenate is phosphorylated by pantothenate kinase (PANK). Deficiencies in PANK2 cause a progressive neurodegenerative disorder associated with iron accumulation in the brain, but the relationship between disease symptoms and pantothenate metabolism remains unclear (Zhou et al. 2001; Zhang et al. 2006).
Cytosolic dihydrofolate reductase catalyzes the reaction of folate, NADPH, and H+ to form dihydrofolate and NADP+ (Chen et al. 1984; Davies et al. 1990).
The methenyltetrahydrofolate cyclohydrolase activity of the trifunctional MTHFD1 enzyme catalyzes the reversible reaction of 10-formylTHF polyglutamate to form 5,10-methenylTHF polyglutamate and H2O. MTHFD1 is cytosolic and occurs as a dimer. The human enzyme has been identified and partially characterized biochemically (Hum et al. 1988); additional reaction details can be inferred from the properties of the well-studied homologous rabbit enzyme (Villar et al. 1985).
Graves disease carrier protein (SLC25A16), associated with the inner mitochondrial membrane, mediates the transport of cytosolic coenzyme A (CoA-SH) into the mitochondrial matrix. Evidence for this event is indirect. The protein has the sequence motifs expected for a transport protein, and yeast cells deficient in its homologue, Leu5p, fail to accumulate mitochondrial CoA-SH and can be rescued by expression of SLC25A16. At the same time, neither the yeast nor the human protein has been shown directly to function as a transporter (Prohl et al. 2001, Leonardi et al. 2007).
SLC25A32 protein in the inner mitochondrial membrane mediates the reversible transport of tetrahydrofolate between the cytosol and the mitochondrial matrix. Retention of tetrahydrofolate within the mitochondrial matrix is dependent on mitochondrial polyglutamate addition (Titus and Moran 2000; Chen et al. 1996).
The adenylyl transferase activity of bifunctional coenzyme A synthase (COASY) catalyzes the transfer of an adenylyl group from ATP to pantetheinephosphate (PPANT) to form dephospho-Coenzyme A (DP-CoA) (Daugherty et al. 2002). The enzyme is associated with the mitochondrial outer membrane (Zhyvoloup et al. 2003).
Nicotinamide phosphoribosyltransferase (NamPRT) catalyzes the condensation of nicotinamide with 5- phosphoribosyl-1-pyrophosphate to yield nicotinamide D-ribonucleotide (NMN), an intermediate in the biosynthesis of NAD. It is the rate limiting component in the mammalian NAD biosynthesis pathway.
The decarboxylation of phosphopantothenoylcysteine (PPC) to 4'-phosphopantetheine (PPANT) is carried out by phosphopantothenoylcysteine decarboxylase (PPCDC). PPCDC is cytosolic and exists as a homotrimer, binding one FMN cofactor per subunit. While a second isoform has been inferred from large-scale sequnceing studies, it lacks the protein's FMN-binding domain and would thus appear to be nonfunctional if it is expressed.
Phosphorylation of riboflavin (RIB) results in the formation of the first cofactor, flavin mononucleotide (FMN). This reaction is catalyzed by riboflavin kinase (RFK), a cytosolic enzyme existing as a monomer. It utilizes either zinc or magnesium ions in the reaction.
Cytosolic thiamin pyrophosphokinase (TPK1) catalyzes the reaction of thiamin (THMN) and ATP to form thiamin diphosphate (ThDP aka thiamin pyrophosphate) and ADP. ThDP is an active cofactor for transketolase, pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, enzymes involved in glycolysis and energy production. The gene encoding the human enzyme has been cloned and its protein product has been shown to have TPK1 activity (Nosaka et al. 2001; Zhao et al. 2001). Its homodomeric structure and association with Mg2+ are inferred from properties of the homologous yeast enzyme (Baker et al. 2001).
NAD+ kinase catalyzes the transfer of a phosphate group from ATP to NAD+, forming NADP+. This is the only way to generate NADP+ in all living organisms. The enzyme requires a divalent metal to be effective. Zn2+ is the best metal for this purpose.
NMNAT1 catalyzes the reaction of nicotinate D-ribonucleotide and ATP to form deamino-NAD+ (nicotinate adenine dinucleotide) and pyrophosphate (Schweiger et al. 2001). The active form of the enzyme in vitro is a hexamer (Zhou et al. 2002), and its activity is substantially greater in the presence of Zn++ than of Mg++ (Sorci et al. 2007). The predicted amino acid sequence of the enzyme contains a nuclear localization domain and the protein is observed to localize to the nucleus (Schweiger et al. 2001; Berger et al. 2005).
Biotin (Btn) acts as a coenzyme to 4 carboxylases which exist in their inactive apo forms. In the cytosol, these apo carboxylases are biotinylated to their active halo forms by the activity of biotin-protein ligase (HCLS) (Ingaramo & Beckett 2012, Hiratsuka et al. 1998, Bailey et al. 2010). Defects in HLCS causes holocarboxylase synthetase deficiency (HLCS deficiency aka biotin-responsive multiple carboxylase deficiency; MIM:253270). HLCS deficiency is an autosomal recessive disorder whereby deficient HLCS activity results in reduced activity of multiple carboxylases. Symptoms include metabolic acidosis, organic aciduria, lethargy, hypotonia, convulsions and dermatitis (Suzuki et al. 2005). Propionyl-CoA carboxylase is most likely functional as a dodecamer, composed of six Btn-containing alpha subunits (PCCA) and six beta subunits (PCCB). The exact order in which this complex is constructed is unknown.
Solute carrier family 52, riboflavin transporter, member 3 (SLC52A3) transports riboflavin (RIB) from the lumen into small intestine epithelial cells (Yao et al. 2010). Activity is inhibited by riboflavin analogues such as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) (Yao et al. 2010). Defects in SLC52A3 cause Brown-Vialetto-Van Laere syndrome type 1 (BVVLS1; MIM:211530). BVVLS1 is a rare autosomal recessive neurologic disorder characterized by sensorineural hearing loss and a variety of cranial nerve palsies (Green et al. 2010). Defects in SLC52A3 also cause Fazio-Londe disease (FALOND; MIM:211500), a rare neurological disease characterized by progressive weakness of the muscles innervated by cranial nerves located at the lower brain stem (Bosch et al. 2011).
Transcobalamin (TCN) is a vitamin B12-binding protein secreted by endothelial cells into plasma that facilitates the transport of cobalamin (Cbl, vitamin B12) into hepatocytes or cells requiring Cbl. Two TCN genes, TCN1 (aka haptocorrin) and TCN2, code for functional proteins that can bind Cbl (Johnston et al. 1989, Quadros et al. 1986, Wuerges et al. 2006). TCN2 can be bound to between 10-30% of the total circulating Cbl, the remaining Cbl bound to TCN1. TCN2 transports Cbl used by tissues. The role of TCN1 carrying between 70-90% of the Cbl serum fraction is unknown.
Methionine synthase (MTR) catalyses the transfer of a methyl group from 5-methyltetrahydrofolate (MTHF) to homocysteine (HCYS) to then form methionine (L-Met). In the first step, MTR mediates the transfer of a methyl group from 5-methyltetrahydrofolate (MTHF) to B12s (bound to the enzyme MTR) to form the cofactor methylcobalamin (MeCbl), the form that activates MTR (Leclerc et al. 1996).
Defects in MTR cause methylcobalamin deficiency type G (cblG; MIM:250940), an autosomal recessive inherited disease that causes mental retardation, macrocytic anemia, and homocystinuria. Mutations causing cblG include P1173L, Ile881, H920D, R585*, E1204* and A1204P (Leclerc et al. 1996, Gulati et al. 1996, Watkins et al. 2002).
Human biotinidase (BTD) (Cole et al. 1994) is a secreted enzyme that catalyzes the hydrolysis of biocytin (BCTN), the product of biotin-dependent carboxylase degradation, to biotin (Btn) and lysine. BTD deficiency, an autosomal recessive disorder, results in a secondary Btn deficiency that leads to late-onset multiple carboxylase deficiency (MIM:253260) (Wolf et al. 1983).
Once in the lysosome, transcobalamin 2:cobalamin (TCN2:Cbl) is degraded to release Cbl (Youngdahl-Turner et al. 1979). Cbl is ready to be exported out of the lysosome to the cytosol by the probable lysosomal cobalamin transporter (LMBRD1) (Rutsch et al. 2009). Once in the cytosol, Cbl can be used in the synthesis of the essential cofactors methylcobalamin (MeCbl) and adenosylcobalamin (AdoCbl) as described in the steps above.
The ubiquitously-expressed CD320 antigen (CD320 aka transcobalamin receptor, TCblR) internalises TCN2:Cbl by endocytosis after binding to it (Quadros et al. 2009, Quadros et al. 2005). Defects in CD320 cause methylmalonic aciduria type TCblR (MMATC aka methylmalonic aciduria; MIM:613646) (Quadros et al. 2010).
Methylmalonic aciduria and homocystinuria type D protein (MMADHC) has sequence homology with a bacterial ATP-binding cassette transporter and contains a putative cobalamin binding motif and a putative mitochondrial targeting sequence which is thought to target cob(II)alamin (B12r) to the mitochondria (Stucki et al. 2012, Coelho et al. 2008). The actual mechanism of transport into the mitochondrion is unknown.
Once biotinylated, three halocarboxylases (hCBXs) are localised to the mitochondrial matrix. The mechanism of transfer is still unclear. Pyruvate carboxylase (PC) is required for gluconeogenesis, lipogenesis, neurotransmitter synthesis and insulin secretion; Methylcrotonyl-CoA carboxylase (MCC) is required for amino acid metabolism; propionyl-CoA carboxylase (PCC) is required for odd-chain fatty acid oxidation (Ingaramo & Beckett 2012, Hiratsuka et al. 1998, Bailey et al. 2010).
Exact details are unclear but it is assumed once the binding proteins (MMADCHC and MMADHC) deliver B12r to its cytosolic destination, they dissociate from B12r (Mah et al. 2013, Plesa et al. 2011, Deme et al. 2012).
Aldehyde oxidase (AOX1) is a complex molybdo-flavoprotein that belongs to the xanthine oxidase family. It is active as a homodimer, with each monomer binding two distinct [2Fe2S] clusters, FAD and the molybdenum cofactor. AOX1 plays an important role in the metabolism of drugs based on its broad substrate specificity oxidising aromatic aza-heterocycles and aldehydes (Hartmann et al. 2012).
Biotin (Btn) acts as a coenzyme to 4 carboxylases which exist in their inactive apo forms. In the cytosol, these apo carboxylases are biotinylated to their active halo forms by the activity of biotin-protein ligase (HCLS) (Ingaramo & Beckett 2012, Hiratsuka et al. 1998, Bailey et al. 2010). Defects in HLCS causes holocarboxylase synthetase deficiency (HLCS deficiency aka biotin-responsive multiple carboxylase deficiency; MIM:253270). HLCS deficiency is an autosomal recessive disorder whereby deficient HLCS activity results in reduced activity of multiple carboxylases. Symptoms include metabolic acidosis, organic aciduria, lethargy, hypotonia, convulsions and dermatitis (Suzuki et al. 2005). Acetyl-CoA carboxylases 1 and 2 (ACACA and ACACB) bind 2 Mn2+ and 1 Btn per subunit (Colbert et al. 2010). They can exist as either monomers, homodimers or homotetramers (latter two not shown in this reaction) (Magnard et al. 2002, Moreau et al. 2006). Unlike the other carboxylases acted upon by HCLS, ACACA and B are located in the cytosol (shown here) as well as the mitochondrial membrane. The exact order in which this complex is constructed is unknown.
In the mucosal cells of the distal ileum, in preparation for internalisation, the gastric intrinsic factor:cobalamin (GIF:Cbl) complex interacts with cubilin (CUBN). CUBN is a cotransporter facilitating uptake of lipoproteins, vitamins and iron (Matthews et al. 2007). CUBN is in complex with protein amnionless (AMN), a necessary component which directs subcellular localization and endocytosis of GIF:Cbl (Fyfe et al. 2004, Anderson et al. 2010). Defects in CUBN and AMN both cause recessive hereditary megaloblastic anemia 1 (RH-MGA1 aka MGA1 Norwegian type or Imerslund-Grasbeck syndrome, I-GS; MIM:261100). The resultant malabsorption of Cbl (vitamin B12) leads to impaired B12-dependent folate metabolism and ultimately impaired thymine synthesis and DNA replication (Aminoff et al. 1999, Kristiansen et al. 2000, Tanner et al. 2003, Densupsoontorn et al. 2012).
Gastric parietal cells secrete gastric intrinsic factor (GIF) which binds tightly to free cobalamin (Cbl) released from transcobalamin (TCN1, haptocorrin) in the proximal intestine (Matthews et al. 2007). Cbl must bind to GIF to be absorbed from the small intestine.
A semi-synthetic form of the vitamin, cyanocobalamin (CNCbl, where a cyanide group is in the upper axial position), is produced from bacterial hydroxocobalamin and used in many pharmaceuticals, supplements and as a food additive. It is presumed to take the same route after ingestion as cobalamin (Cbl) (Randaccio et al. 2010). At this point in the pathway, CNCbl is reductively decyanated by methylmalonic aciduria and homocystinuria type C protein (MMACHC) to produce cob(II)alamin (B12r, vitaman B12r) and hydrogen cyanide (HCN) (Kim et al. 2008). Decyanation of CNCbl is required for it to be made available for conversion to active cofactors.
Transcobalamin 2 (TCN2) degradation is necessary for cobalamin (Cbl) to be released from the complex and made available for binding to Cbl-dependant apoenzymes (Youngdahl-Turner et al. 1979). The TCN2:Cbl complex translocates to lysosomes for degradation.
MMACHC:B12r (cob(II)alamin, vitamin B12r) binding to methylmalonic aciduria and homocystinuria type D protein (MMADHC) represents a branch point in the targeted delivery of B12r to either cytosolic or mitochondrial enzymes requiring this cofactor (Mah et al. 2013, Plesa et al. 2011, Deme et al. 2012). Both MMACHC and MMADHC are implicated in the intracellular transport of cobalamins but exact details of the mechanisms involved remain unclear.
Defects in MMADHC cause methylmalonic aciduria and homocystinuria type cblD (MMAHCD; MIM:277410), a disorder of cobalamin metabolism characterized by decreased levels of the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl) (Coelho et al. 2008).
Biotin (Btn) acts as a coenzyme to 4 carboxylases which exist in their inactive apo forms. In the cytosol, these apo carboxylases are biotinylated to their active halo forms by the activity of biotin-protein ligase (HCLS) (Ingaramo & Beckett 2012, Hiratsuka et al. 1998, Bailey et al. 2010). Defects in HLCS causes holocarboxylase synthetase deficiency (HLCS deficiency aka biotin-responsive multiple carboxylase deficiency; MIM:253270). HLCS deficiency is an autosomal recessive disorder whereby deficient HLCS activity results in reduced activity of multiple carboxylases. Symptoms include metabolic acidosis, organic aciduria, lethargy, hypotonia, convulsions and dermatitis (Suzuki et al. 2005). Methylcrotonoyl-CoA carboxylase is most likely functional as a dodecamer, composed of 6 Btn-containing alpha subunits (MCCC1) and six beta subunits (MCCC2). The exact order in which this complex is constructed is unknown.
Multidrug resistance-associated protein 1 (ABCC1, MRP1) can specifically mediate the ATP-dependant export of free cobalamin (Cbl aka vitamin B12) from small intestine cells to the portal vein (Shah et al. 2011).
The mitochondrial holocarboxylases (hCBXs) propionyl-CoA carboxylase, methylcrotonoyl-CoA carboxylase and pyruvate carboxylase are degraded proteolytically to biocytin (BCTN aka biotinyllysine) or small biotinyl peptides (not shown here) (Chandler & Ballard 1985, Hymes & Wolf 1996, Hymes & Wolf 1999).
Mitochondrial cob(I)yrinic acid a,c-diamide adenosyltransferase (MMAB) is a dual purpose enzyme involved in the reduction and adenosylation of cobalamin. In the first step, MMAB reduces cob(II)alamin (B12r) to cob(I)alamin (B12s) (Fan & Bobik 2008, Leal et al. 2003). Defects in MMAB cause methylmalonic aciduria type cblB (MMAB aka methylmalonic aciduria type B or vitamin B12-responsive methylmalonicaciduria of cblB complementation type; MIM:251110). Affected individuals have methylmalonic aciduria and metabolic ketoacidosis, despite a functional methylmalonyl-CoA mutase. In severe cases, newborns are lethargic, prone to vomiting and fail to thrive (Dobson et al. 2002).
The cubilin:protein amnionless (CUBN:AMN) complex mediates the internalisation and endocytosis of gastric internal factor:cobalamin (GIF:Cbl) into mucosal cells of the distal ileum (Fyfe et al. 2004).
The probable lysosomal cobalamin transporter (LMBRD1) is the most likely candidate to transport cobalamin (Cbl) from inside the lysosome to the cytosol (Rutsch et al. 2009). From here, Cbl can either be used to synthesise the essential cofactors for methionine synthase in the cytosol or methylmalonyl-CoA mutase in the mitochondria or, it can transported out of the cell to tissues that require Cbl. Defects in LMBRD1 cause methylmalonic aciduria and homocystinuria type cblF (MMAHCF; MIM:277380), characterised biochemically by decreased levels of the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MetCbl) (Rutsch et al. 2009, Gailus et al. 2010).
Transcobalamin 1 (TCN1 aka haptocorrin, HC) is a glycoprotein produced by salivary glands in response to food ingestion (Johnston et al. 1989). TCN1 binds strongly to cobalamin (Cbl aka vitamin B12) and its essential function is protection of the acid-sensitive Cbl while it moves through the stomach. Once food is in the stomach, pepsin and the acidic pH degrade food proteins. Cbl is in its +3 oxidation state in dietary sources.
Methionine synthase reductase (MTRR) is involved in reducing B12r (cob(II)alamin) to B12s (cob(I)alamin), the cofactor form used by methionine synthase (MTR) to become functional. MTRR requires 1 FMN and 1 FAD per subunit for activity (Wolthers et al. 2007). MTRR exists a stable complex with MTR, bound through their FMN-binding and C-terminal activation domains respectively (Wolthers & Scrutton 2007, Wolthers & Scrutton 2009).
Once the receptor complex (TCN2:Cbl:CD320) is internalised by endocytosis, the receptor (CD320) dissociates to return to the plasma membrane (Youngdahl-Turner et al. 1979).
Cytosolic acetyl-CoA carboxylases 1 and 2 (Btn-ACACA/B:2Mn2+) are degraded proteolytically to biocytin (BCTN aka biotinyllysine) or small biotinyl peptides (not shown here) by an unknown protease (Chandler & Ballard 1985, Hymes & Wolf 1996, Hymes & Wolf 1999).
In the proximal intestine, pancreatic enzymes degrade transcobalamin 1 (TCN1) to release cobalamin (Cbl). The two major pancreatic proteases are trypsins (PRSSs) and chymotrypsins (CTRBs) (Srikumar & Premalatha 2003, Nielsen et al. 2012).
Mitochondrial cob(I)yrinic acid a,c-diamide adenosyltransferase (MMAB) is a dual purpose enzyme involved in the reduction and adenosylation of cobalamin. In the second step, MMAB transfers an adenosyl group from ATP to cob(I)alamin (B12s) to form adenosylcabalamin (AdoCbl) (Fan & Bobik 2008, Leal et al. 2003). Defects in MMAB cause methylmalonic aciduria type cblB (MMAB aka methylmalonic aciduria type B or vitamin B12-responsive methylmalonicaciduria of cblB complementation type; MIM:251110). Affected individuals have methylmalonic aciduria and metabolic ketoacidosis, despite a functional methylmalonyl-CoA mutase. In severe cases, newborns are lethargic, prone to vomiting and fail to thrive (Dobson et al. 2002).
Transcobalamin 1 (TCN1 aka haptocorrin, HC) is a glycoprotein produced by many cells including gastric cells in response to go food intake (Johnston et al. 1989). It can also bind a large fraction of cobalamin (Cbl) in the general circulation. No functional significance for this general binding is presently known (Quadros 2010, Alpers & Russel-Jones 1999 (in Chemistry and Biochemistry of B12, Banerjee 1999)).
Biotin (Btn) acts as a coenzyme to 4 carboxylases which exist in their inactive apo forms. In the cytosol, these apo carboxylases are biotinylated to their active halo forms by the activity of biotin-protein ligase (HCLS) (Ingaramo & Beckett 2012, Hiratsuka et al. 1998, Bailey et al. 2010). Defects in HLCS causes holocarboxylase synthetase deficiency (HLCS deficiency aka biotin-responsive multiple carboxylase deficiency; MIM:253270). HLCS deficiency is an autosomal recessive disorder whereby deficient HLCS activity results in reduced activity of multiple carboxylases. Symptoms include metabolic acidosis, organic aciduria, lethargy, hypotonia, convulsions and dermatitis (Suzuki et al. 2005). Pyruvate carboxylase (PC) is homotetrameric, binds 1 Mn2+ and 1 Btn per PC subunit (Xiang & Tong 2008). The exact order in which this complex is constructed is unknown.
Methionine synthase reductase (MTRR) is involved in reducing B12r (cob(II)alamin) to B12s (cob(I)alamin), the cofactor form used by methionine synthase (MTR) to become functional. Regeneration of functional MTR requires reductive methylation via a reaction catalyzed by MTRR in which S-adenosylmethionine (SAM) is used as a methyl donor. MTRR requires 1 FMN and 1 FAD per subunit for activity (Wolthers et al. 2007). MTRR exists a stable complex with MTR, bound through their FMN-binding and C-terminal activation domains respectively (Wolthers & Scrutton 2007, Wolthers & Scrutton 2009).
Defects in MTRR cause methylcobalamin deficiency type E (cblE; MIM:236270) (Wilson et al. 1999). Patients with cblE exhibit megaloblastic anemia and hyperhomocysteinemia. SAM is used as a methyl donor in many biological reactions and demethylation of methionine produces homocysteine. Remethylation is carried out by MTR but in cblE patients, MTR cannot be reduced by defective MTRR to form a functional enzyme thus homocysteine accumulates. Mutations in MTRR that cause cblE include Leu576del (Leclerc et al. 1998), S454L (Zavadakova et al. 2005) and the compound heterozygote G487R/M569Ifs*9 (Zavadakova et al. 2002).
Wilson et al. showed that a 66A-G polymorphism, resulting in an Ile22Met (I22M) substitution, is associated with susceptibility to folate-sensitive neural tube defects (FS-NTD; MIM:601634) (Wilson et al. 1999b, Doolin et al. 2002). Serum deficiency of vitamin B12 increased the effect.
Methylmalonyl-CoA mutase (MUT aka MCM) (Jansen et al. 1989) utilises adenosylcobalamin (AdoCbl) as a cofactor and transfers an adenosyl group from AdoCbl to methymalonyl-CoA to form succinyl-CoA, a precursor for the citric acid cycle. MUT has a homodimeric structure and is located in the mitochondrial matrix. Defects in MUT cause methylmalonic aciduria type mut (MMAM; MIM:251000), an often fatal disorder of organic acid metabolism (Worgan et al. 2006).
Methylmalonic aciduria type A protein (MMAA) (Dobson et al. 2002) is thought to act as a chaperone to MUT and is suggested to play a dual role with regards to MUT protection and reactivation.
Defects in MMAA cause methylmalonic aciduria type cblA (MMAA aka methylmalonic aciduria type A or vitamin B12-responsive methylmalonicaciduria of cblA complementation type; MIM:251100). Affected individuals accumulate methylmalonic acid in the blood and urine and are prone to potentially life threatening acidotic crises in infancy or early childhood (Dobson et al. 2002, Lerner-Ellis et al. 2004).
Methylmalonic aciduria and homocystinuria type C protein (MMACHC aka cblC protein) is suggested to be involved in the binding and intracellular transport of cobalamin (Cbl aka vitamin B12). MMACHC can catalyse the removal of the "R" group from Cbl (eg dealkylation of ADOCbl and MeCbl or decyanation of CNCbl). It may also be involved in reducing Cbl (+3 oxidation state) to cob(II)alamin (B12r, vitamin B12r; +2 oxidation state) (Hannibal et al. 2009). B12r is either escorted by MMACHC to its destination enzyme partners in the mitochondria and cytosol or is exported through the basolateral membrane by the ATP-dependent ABCC1.
Defects in MMACHC cause methylmalonic aciduria and homocystinuria type cblC (MMAHCC; MIM:277400). MMAHCC is the most common disorder of cobalamin metabolism and is characterized by decreased levels of the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MetCbl). Affected individuals may have developmental, haematologic, neurologic, metabolic, ophthalmologic, and dermatologic clinical findings (Lerner-Ellis et al. 2006).
Methylmalonyl-CoA mutase (MUT aka MCM) (Jansen et al. 1989) utilises adenosylcobalamin (AdoCbl) as a cofactor and transfers an adenosyl group from AdoCbl to methylmalonyl-CoA to form succinyl-CoA, a precursor for the citric acid cycle. MUT has a homodimeric structure and is located in the mitochondrial matrix. Defects in MUT cause methylmalonic aciduria, mut type (MMAM; MIM:251000), an often fatal disorder of organic acid metabolism (Worgan et al. 2006).
Methylmalonic aciduria type A protein (MMAA) is thought to act as a chaperone to MUT, the enzyme which utilises adenosylcobalamin (AdoCbl) as a cofactor. MMAA is suggested to play a dual role with regards to MUT protection and reactivation. Some AdoCbl-dependent enzymes undergo suicide inactivation after catalysis due to the oxidative inactivation of Cbl. MMAA is thought to play a protective role to prevent MUT being inactivated in this way. After the catalytic cycle when MUT is inactive, MMAA increases the enzymatic activity of MUT through exchange of the damaged cofactor. Whether this happens via GTP-mediated hydrolysis is unknown at present (Takahashi-Iniguez et al. 2011, Froese et al. 2010). Bacterial AdoCbl-containing enzymes possess reactivating factors which release the inactivated cofactor to allow the resulting apoenzyme to reconstitute into an active form. A bacterial orthologue of MMAA, MeaB, forms a stable complex with MUT and plays a role in its protection and reactivation (Padovani & Banerjee 2006).
Defects in MMAA cause methylmalonic aciduria type cblA (cblA aka methylmalonic aciduria type A or vitamin B12-responsive methylmalonicaciduria of cblA complementation type; MIM:251100). Affected individuals accumulate methylmalonic acid in the blood and urine and are prone to potentially life threatening acidotic crises in infancy or early childhood (Dobson et al. 2002, Lerner-Ellis et al. 2004).
In order to get a sulfur atom for subsequent sulfuration reactions, cysteine is first desulfurated by NFS1 which transfers it onto a cysteine of MOCS3, yielding a protein persulfide (Marelja et al, 2008).
Sulfur transfer onto MOCS2A is closely preceded by its adenylylation and deadenylylation. After release of MOCS2A-CO-S(1-), two cysteines on MOCS3 form a disulfide bridge. This means that MOCS3 has to be reduced to be able to participate in the next round. The reducing agent is not known (Marelja et al, 2008).
Gephyrin, which stabilizes receptors on neuronal membranes, also catalyzes the transfer of a molybdenum ion onto the cofactor. The mechanism was elucidated in plants but, as the pathway is highly conserved, human gephyrin can complement missing plant proteins. Doubts remain about the actual molybdenum donor, probably molybdate, and whether a copper ion is possibly bound and removed (Stallmeyer et al, 1999).
GTP cyclizes in a reaction involving radicals of S-adenosylmethionine, catalyzed by the iron-sulfur cluster dimeric MOCS1. The intermediate result is called precursor Z (Hanzelmann et al, 2002; Hanzelmann et al, 2004).
Thiamin triphosphate (ThTP) is believed to be synthesized from thiamin diphosphate (ThDP), catalyzed by ThDP kinase (TDPK), an enzyme that remains poorly characterized (Nishino et al, 1983).
Pyridoxal kinase (PDXK) catalyzes the ATP-dependent phosphorylation of pyridoxine (PDX) to form pyridoxine phosphate (PDXP) (Lee et al. 2000, di Salvo et al. 2004).
While the biosynthesis of the molybdenum cofactor for sulfite oxidase is finished after molybdenum ion insertion, human xanthine oxidase and aldehyde oxidase will only show activity with this cofactor when one of the oxygens bound to molybdenum is replaced with sulfur. The exchange is catalyzed by the MOCOS cysteine desulfurase (Ichida et al, 2001).
Thiamin triphosphate (ThTP) can transfer phosphate to a few proteins. Animal tissues contain a membrane-associated as well as a soluble thiamine triphosphatase that can dephosphorylate ThTP. Only the soluble enzyme was characterized in calf (Lakaye et al, 2002).
Pyridoxal kinase (PDXK) catalyzes the ATP-dependent phosphorylation of pyridoxal (PXL) to form pyridoxal 5'-phosphate (PXLP) (Lee et al. 2000, di Salvo et al. 2004).
Pyridoxal kinase (PDXK) catalyzes the ATP-dependent phosphorylation of pyridoxamine (PXA) to form pyridoxamine phosphate (PXAP) (Lee et al. 2000, di Salvo et al. 2004).
After the MOCS2A dimer is loaded with two sulfur atoms, their sequential deposition on the precursor Z molecule, with ring cleavage, is catalyzed by the MOCS2B half of the MOCS2 tetramer (Leimkuhler et al, 2003; Wuebbens & Rajagopalan, 2002).
A methyl group from methylcobalamin (MeCbl) is transferred to homocysteine (HCYS), forming methionine (L-Met) (Leclerc et al. 1996). MTR is now in an inactive state as its cofactor state, B12r (cob(II)alamin), requires reduction to B12s (cob(I)alamin) to reactivate the enzyme. This is performed by methionine synthase reducatse (MTRR).
Water-soluble vitamins include ascorbate (vitamin C) and the members of the B group: thiamin (vitamin B1), riboflavin (B2), niacin (B3), pantothenate (B5), pyridoxine (B6), biotin (B7), folate (B9), and cobalamin (B12). Metabolic processes annotated here include the synthesis of thiamin pyrophosphate (TPP) from thiamin (B1), the synthesis of FMN and FAD from riboflavin (B2), the synthesis of nicotinic acid (niacin - B3) from tryptophan, the synthesis of Coenzyme A from pantothenate (B5), and features of the metabolism of folate (B9).
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2xMUT
AdoCblAMN GIF
CblMMADHC
B12rCbl
CD320Annotated Interactions
2xMUT
AdoCblTwo features of this transport process remain incompletely understood, however. First, mutations in SLC19A3 cause a progressive disorder that is responsive to biotin treatment (Zhou et al. 2005), although studies of cultured cells indicate that the protein has no affinity for biotin (Subramanian et al. 2006 - Am J Physiol). Also, studies to date provide little information about the mechanism by which thiamin, once taken up by epithelial cells in the intestine and kidney, is released from these cells into the blood.
Defects in MTR cause methylcobalamin deficiency type G (cblG; MIM:250940), an autosomal recessive inherited disease that causes mental retardation, macrocytic anemia, and homocystinuria. Mutations causing cblG include P1173L, Ile881, H920D, R585*, E1204* and A1204P (Leclerc et al. 1996, Gulati et al. 1996, Watkins et al. 2002).
Defects in MMADHC cause methylmalonic aciduria and homocystinuria type cblD (MMAHCD; MIM:277410), a disorder of cobalamin metabolism characterized by decreased levels of the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl) (Coelho et al. 2008).
Defects in MTRR cause methylcobalamin deficiency type E (cblE; MIM:236270) (Wilson et al. 1999). Patients with cblE exhibit megaloblastic anemia and hyperhomocysteinemia. SAM is used as a methyl donor in many biological reactions and demethylation of methionine produces homocysteine. Remethylation is carried out by MTR but in cblE patients, MTR cannot be reduced by defective MTRR to form a functional enzyme thus homocysteine accumulates. Mutations in MTRR that cause cblE include Leu576del (Leclerc et al. 1998), S454L (Zavadakova et al. 2005) and the compound heterozygote G487R/M569Ifs*9 (Zavadakova et al. 2002).
Wilson et al. showed that a 66A-G polymorphism, resulting in an Ile22Met (I22M) substitution, is associated with susceptibility to folate-sensitive neural tube defects (FS-NTD; MIM:601634) (Wilson et al. 1999b, Doolin et al. 2002). Serum deficiency of vitamin B12 increased the effect.
Methylmalonic aciduria type A protein (MMAA) (Dobson et al. 2002) is thought to act as a chaperone to MUT and is suggested to play a dual role with regards to MUT protection and reactivation.
Defects in MMAA cause methylmalonic aciduria type cblA (MMAA aka methylmalonic aciduria type A or vitamin B12-responsive methylmalonicaciduria of cblA complementation type; MIM:251100). Affected individuals accumulate methylmalonic acid in the blood and urine and are prone to potentially life threatening acidotic crises in infancy or early childhood (Dobson et al. 2002, Lerner-Ellis et al. 2004).
Defects in MMACHC cause methylmalonic aciduria and homocystinuria type cblC (MMAHCC; MIM:277400). MMAHCC is the most common disorder of cobalamin metabolism and is characterized by decreased levels of the coenzymes adenosylcobalamin (AdoCbl) and methylcobalamin (MetCbl). Affected individuals may have developmental, haematologic, neurologic, metabolic, ophthalmologic, and dermatologic clinical findings (Lerner-Ellis et al. 2006).
Methylmalonic aciduria type A protein (MMAA) is thought to act as a chaperone to MUT, the enzyme which utilises adenosylcobalamin (AdoCbl) as a cofactor. MMAA is suggested to play a dual role with regards to MUT protection and reactivation. Some AdoCbl-dependent enzymes undergo suicide inactivation after catalysis due to the oxidative inactivation of Cbl. MMAA is thought to play a protective role to prevent MUT being inactivated in this way. After the catalytic cycle when MUT is inactive, MMAA increases the enzymatic activity of MUT through exchange of the damaged cofactor. Whether this happens via GTP-mediated hydrolysis is unknown at present (Takahashi-Iniguez et al. 2011, Froese et al. 2010). Bacterial AdoCbl-containing enzymes possess reactivating factors which release the inactivated cofactor to allow the resulting apoenzyme to reconstitute into an active form. A bacterial orthologue of MMAA, MeaB, forms a stable complex with MUT and plays a role in its protection and reactivation (Padovani & Banerjee 2006).
Defects in MMAA cause methylmalonic aciduria type cblA (cblA aka methylmalonic aciduria type A or vitamin B12-responsive methylmalonicaciduria of cblA complementation type; MIM:251100). Affected individuals accumulate methylmalonic acid in the blood and urine and are prone to potentially life threatening acidotic crises in infancy or early childhood (Dobson et al. 2002, Lerner-Ellis et al. 2004).