Porphyrins are heterocyclic macrocycles, consisting of four pyrrole subunits (tetrapyrrole) linked by four methine (=CH-) bridges. The extensive conjugated porphyrin macrocycle is chromatic and the name itself, porphyrin, is derived from the Greek word for purple. The aromatic character of porphyrins can be seen by NMR spectroscopy. Porphyrins readily combine with metals by coordinating them in the central cavity. Iron (heme) and magnesium (chlorophyll) are two well known examples although zinc, copper, nickel and cobalt form other known metal-containing phorphyrins. A porphyrin which has no metal in the cavity is called a free base. Some iron-containing porphyrins are called hemes (heme-containing proteins or hemoproteins) and these are found extensively in nature ie. hemoglobin. Hemoglobin is quantitatively the most important hemoprotein. The hemoglobin iron is the transfer site of oxygen and carries it in the blood all round the body for cell respiration. Other examples are cytochromes present in mitochondria and endoplasmic reticulum which takes part in electron transfer events, catalase and peroxidase whic protect the body against the oxidant hydrogen peroxide and tryptophan oxygenase which is present in intermediary metabolism. Hemoproteins are synthesized in all mammalian cells and the major sites are erythropoietic tissue and the liver.
The processes by which heme is synthesized, transported, and metabolized are a critical part of human iron metabolism (Severance and Hamze 2009); here the core processes of heme biosynthesis and catabolism have been annotated.
Erlinger S, Arias IM, Dhumeaux D.; ''Inherited disorders of bilirubin transport and conjugation: new insights into molecular mechanisms and consequences.''; PubMedEurope PMCScholia
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Kamisako T, Kobayashi Y, Takeuchi K, Ishihara T, Higuchi K, Tanaka Y, Gabazza EC, Adachi Y.; ''Recent advances in bilirubin metabolism research: the molecular mechanism of hepatocyte bilirubin transport and its clinical relevance.''; PubMedEurope PMCScholia
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Weisiger RA, Ostrow JD, Koehler RK, Webster CC, Mukerjee P, Pascolo L, Tiribelli C.; ''Affinity of human serum albumin for bilirubin varies with albumin concentration and buffer composition: results of a novel ultrafiltration method.''; PubMedEurope PMCScholia
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Rey MA, Duffy SP, Brown JK, Kennedy JA, Dick JE, Dror Y, Tailor CS.; ''Enhanced alternative splicing of the FLVCR1 gene in Diamond Blackfan anemia disrupts FLVCR1 expression and function that are critical for erythropoiesis.''; PubMedEurope PMCScholia
Troxler RF, Dawber NH, Lester R.; ''Synthesis of urobilinogen by broken cell preparations of intestinal bacteria.''; PubMedEurope PMCScholia
Leung JW, Liu YL, Leung PS, Chan RC, Inciardi JF, Cheng AF.; ''Expression of bacterial beta-glucuronidase in human bile: an in vitro study.''; PubMedEurope PMCScholia
Antonicka H, Mattman A, Carlson CG, Glerum DM, Hoffbuhr KC, Leary SC, Kennaway NG, Shoubridge EA.; ''Mutations in COX15 produce a defect in the mitochondrial heme biosynthetic pathway, causing early-onset fatal hypertrophic cardiomyopathy.''; PubMedEurope PMCScholia
WATSON CJ, CAMPBELL M, LOWRY PT.; ''Preferential reduction of conjugated bilirubin to urobilinogen by normal fecal flora.''; PubMedEurope PMCScholia
Cui Y, König J, Leier I, Buchholz U, Keppler D.; ''Hepatic uptake of bilirubin and its conjugates by the human organic anion transporter SLC21A6.''; PubMedEurope PMCScholia
Cunningham O, Gore MG, Mantle TJ.; ''Initial-rate kinetics of the flavin reductase reaction catalysed by human biliverdin-IXbeta reductase (BVR-B).''; PubMedEurope PMCScholia
Rigato I, Pascolo L, Fernetti C, Ostrow JD, Tiribelli C.; ''The human multidrug-resistance-associated protein MRP1 mediates ATP-dependent transport of unconjugated bilirubin.''; PubMedEurope PMCScholia
Rupe CO, Fetter MC.; ''Urinary urobilinogen determined by a mercuric chloride procedure.''; PubMedEurope PMCScholia
Astner I, Schulze JO, van den Heuvel J, Jahn D, Schubert WD, Heinz DW.; ''Crystal structure of 5-aminolevulinate synthase, the first enzyme of heme biosynthesis, and its link to XLSA in humans.''; PubMedEurope PMCScholia
Mitchell LW, Volin M, Martins J, Jaffe EK.; ''Mechanistic implications of mutations to the active site lysine of porphobilinogen synthase.''; PubMedEurope PMCScholia
Schröter W.; ''[Intracellular bilirubin transport and the membrane of the hepatic endoplasmic reticulum: new aspects in the development of transitory bilirubinemia of the newborn].''; PubMedEurope PMCScholia
Bayeva M, Khechaduri A, Wu R, Burke MA, Wasserstrom JA, Singh N, Liesa M, Shirihai OS, Langer NB, Paw BH, Ardehali H.; ''ATP-binding cassette B10 regulates early steps of heme synthesis.''; PubMedEurope PMCScholia
Kamisako T, Leier I, Cui Y, König J, Buchholz U, Hummel-Eisenbeiss J, Keppler D.; ''Transport of monoglucuronosyl and bisglucuronosyl bilirubin by recombinant human and rat multidrug resistance protein 2.''; PubMedEurope PMCScholia
Vítek L, Majer F, Muchová L, Zelenka J, Jirásková A, Branný P, Malina J, Ubik K.; ''Identification of bilirubin reduction products formed by Clostridium perfringens isolated from human neonatal fecal flora.''; PubMedEurope PMCScholia
Wakabayashi K, Nakagawa H, Tamura A, Koshiba S, Hoshijima K, Komada M, Ishikawa T.; ''Intramolecular disulfide bond is a critical check point determining degradative fates of ATP-binding cassette (ABC) transporter ABCG2 protein.''; PubMedEurope PMCScholia
Fu G, Liu H, Doerksen RJ.; ''Molecular modeling to provide insight into the substrate binding and catalytic mechanism of human biliverdin-IXα reductase.''; PubMedEurope PMCScholia
Kim DH, Jin YH, Jung EA, Han MJ, Kobashi K.; ''Purification and characterization of beta-glucuronidase from Escherichia coli HGU-3, a human intestinal bacterium.''; PubMedEurope PMCScholia
Desuzinges-Mandon E, Arnaud O, Martinez L, Huché F, Di Pietro A, Falson P.; ''ABCG2 transports and transfers heme to albumin through its large extracellular loop.''; PubMedEurope PMCScholia
Krishnamurthy PC, Du G, Fukuda Y, Sun D, Sampath J, Mercer KE, Wang J, Sosa-Pineda B, Murti KG, Schuetz JD.; ''Identification of a mammalian mitochondrial porphyrin transporter.''; PubMedEurope PMCScholia
Akagi R, Shimizu R, Furuyama K, Doss MO, Sassa S.; ''Novel molecular defects of the delta-aminolevulinate dehydratase gene in a patient with inherited acute hepatic porphyria.''; PubMedEurope PMCScholia
Kullak-Ublick GA, Ismair MG, Stieger B, Landmann L, Huber R, Pizzagalli F, Fattinger K, Meier PJ, Hagenbuch B.; ''Organic anion-transporting polypeptide B (OATP-B) and its functional comparison with three other OATPs of human liver.''; PubMedEurope PMCScholia
Cooper CL, Lash TD, Jones MA.; ''Kinetic evaluation of human cloned coproporphyrinogen oxidase using a ring isomer of the natural substrate.''; PubMedEurope PMCScholia
Simons PC, Jagt DL.; ''Bilirubin binding to human liver ligandin (glutathione S-transferase).''; PubMedEurope PMCScholia
Koníčková R, Jirásková A, Zelenka J, Lešetický L, Štícha M, Vítek L.; ''Reduction of bilirubin ditaurate by the intestinal bacterium Clostridium perfringens.''; PubMedEurope PMCScholia
Qiu W, Liesa M, Carpenter EP, Shirihai OS.; ''ATP Binding and Hydrolysis Properties of ABCB10 and Their Regulation by Glutathione.''; PubMedEurope PMCScholia
Peters WH, Jansen PL.; ''Microsomal UDP-glucuronyltransferase-catalyzed bilirubin diglucuronide formation in human liver.''; PubMedEurope PMCScholia
de Verneuil H, Sassa S, Kappas A.; ''Purification and properties of uroporphyrinogen decarboxylase from human erythrocytes. A single enzyme catalyzing the four sequential decarboxylations of uroporphyrinogens I and III.''; PubMedEurope PMCScholia
Jaffe EK, Martins J, Li J, Kervinen J, Dunbrack RL.; ''The molecular mechanism of lead inhibition of human porphobilinogen synthase.''; PubMedEurope PMCScholia
Wu CK, Dailey HA, Rose JP, Burden A, Sellers VM, Wang BC.; ''The 2.0 A structure of human ferrochelatase, the terminal enzyme of heme biosynthesis.''; PubMedEurope PMCScholia
Zhang W, Mojsilovic-Petrovic J, Andrade MF, Zhang H, Ball M, Stanimirovic DB.; ''The expression and functional characterization of ABCG2 in brain endothelial cells and vessels.''; PubMedEurope PMCScholia
Knauer MJ, Girdwood AJ, Kim RB, Tirona RG.; ''Transport function and transcriptional regulation of a liver-enriched human organic anion transporting polypeptide 2B1 transcriptional start site variant.''; PubMedEurope PMCScholia
Doyle L, Ross DD.; ''Multidrug resistance mediated by the breast cancer resistance protein BCRP (ABCG2).''; PubMedEurope PMCScholia
Lee DS, Flachsová E, Bodnárová M, Demeler B, Martásek P, Raman CS.; ''Structural basis of hereditary coproporphyria.''; PubMedEurope PMCScholia
Hostetler HA, McIntosh AL, Atshaves BP, Storey SM, Payne HR, Kier AB, Schroeder F.; ''L-FABP directly interacts with PPARalpha in cultured primary hepatocytes.''; PubMedEurope PMCScholia
Shoolingin-Jordan PM.; ''Porphobilinogen deaminase and uroporphyrinogen III synthase: structure, molecular biology, and mechanism.''; PubMedEurope PMCScholia
Moran-Jimenez MJ, Ged C, Romana M, Enriquez De Salamanca R, Taïeb A, Topi G, D'Alessandro L, de Verneuil H.; ''Uroporphyrinogen decarboxylase: complete human gene sequence and molecular study of three families with hepatoerythropoietic porphyria.''; PubMedEurope PMCScholia
Fujiwara R, Itoh T.; ''Extensive protein-protein interactions involving UDP-glucuronosyltransferase (UGT) 2B7 in human liver microsomes.''; PubMedEurope PMCScholia
König J, Cui Y, Nies AT, Keppler D.; ''A novel human organic anion transporting polypeptide localized to the basolateral hepatocyte membrane.''; PubMedEurope PMCScholia
Rowland A, Miners JO, Mackenzie PI.; ''The UDP-glucuronosyltransferases: their role in drug metabolism and detoxification.''; PubMedEurope PMCScholia
Griffiths WC, Diamond I, Dextraze P.; ''The albumin binding of unconjugated bilirubin in serum.''; PubMedEurope PMCScholia
Bugiani M, Tiranti V, Farina L, Uziel G, Zeviani M.; ''Novel mutations in COX15 in a long surviving Leigh syndrome patient with cytochrome c oxidase deficiency.''; PubMedEurope PMCScholia
Gardner LC, Smith SJ, Cox TM.; ''Biosynthesis of delta-aminolevulinic acid and the regulation of heme formation by immature erythroid cells in man.''; PubMedEurope PMCScholia
Shoolingin-Jordan PM, Al-Dbass A, McNeill LA, Sarwar M, Butler D.; ''Human porphobilinogen deaminase mutations in the investigation of the mechanism of dipyrromethane cofactor assembly and tetrapyrrole formation.''; PubMedEurope PMCScholia
Petruzzella V, Tiranti V, Fernandez P, Ianna P, Carrozzo R, Zeviani M.; ''Identification and characterization of human cDNAs specific to BCS1, PET112, SCO1, COX15, and COX11, five genes involved in the formation and function of the mitochondrial respiratory chain.''; PubMedEurope PMCScholia
Dailey TA, Dailey HA.; ''Human protoporphyrinogen oxidase: expression, purification, and characterization of the cloned enzyme.''; PubMedEurope PMCScholia
van de Steeg E, Stránecký V, Hartmannová H, Nosková L, Hřebíček M, Wagenaar E, van Esch A, de Waart DR, Oude Elferink RP, Kenworthy KE, Sticová E, al-Edreesi M, Knisely AS, Kmoch S, Jirsa M, Schinkel AH.; ''Complete OATP1B1 and OATP1B3 deficiency causes human Rotor syndrome by interrupting conjugated bilirubin reuptake into the liver.''; PubMedEurope PMCScholia
Wolfrum C, Borrmann CM, Borchers T, Spener F.; ''Fatty acids and hypolipidemic drugs regulate peroxisome proliferator-activated receptors alpha - and gamma-mediated gene expression via liver fatty acid binding protein: a signaling path to the nucleus.''; PubMedEurope PMCScholia
Gordon ER, Sommerer U, Goresky CA.; ''The hepatic microsomal formation of bilirubin diglucuronide.''; PubMedEurope PMCScholia
Fevery J, Van Damme B, Michiels R, De Groote J, Heirwegh KP.; ''Bilirubin conjugates in bile of man and rat in the normal state and in liver disease.''; PubMedEurope PMCScholia
Bosma PJ, Seppen J, Goldhoorn B, Bakker C, Oude Elferink RP, Chowdhury JR, Chowdhury NR, Jansen PL.; ''Bilirubin UDP-glucuronosyltransferase 1 is the only relevant bilirubin glucuronidating isoform in man.''; PubMedEurope PMCScholia
Chowdhury JR, Chowdhury NR, Wu G, Shouval R, Arias IM.; ''Bilirubin mono- and diglucuronide formation by human liver in vitro: assay by high-pressure liquid chromatography.''; PubMedEurope PMCScholia
Levi AJ, Gatmaitan Z, Arias IM.; ''Two hepatic cytoplasmic protein fractions, Y and Z, and their possible role in the hepatic uptake of bilirubin, sulfobromophthalein, and other anions.''; PubMedEurope PMCScholia
Elder GH, Evans JO.; ''Evidence that the coproporphyrinogen oxidase activity of rat liver is situated in the intermembrane space of mitochondria.''; PubMedEurope PMCScholia
Glerum DM, Tzagoloff A.; ''Isolation of a human cDNA for heme A:farnesyltransferase by functional complementation of a yeast cox10 mutant.''; PubMedEurope PMCScholia
Heme O and heme A are specifically synthesised for the heme-copper respiratory oxidases. Mitochondrial protoheme IX farnesyltransferase (COX10) mediates the transformation of protoheme IX (heme) and farnesyl diphosphate (FAPP) to heme O (Glerum & Tzagoloff 1994). COX10 is highly expressed in muscle, heart and brain (Murakami et al. 1997).
Heme A is the prosthetic group of cytochrome c oxidase, the terminal enzyme in the respiratory chain. It is formed by the action of cytochrome c oxidase assembly protein COX15 homolog (COX15) on heme O (Petruzzella et al. 1998, Antonicka et al. 2003). Defects in COX15 cause of mitochondrial complex IV deficiency (MT-C4D; MIM:220110), also called cytochrome c oxidase deficiency resulting in a disorder of the mitochondrial respiratory chain seen as heterogeneous clinical manifestations, ranging from isolated myopathy to severe multisystem disease affecting several tissues and organs (Antonicka et al. 2003). Defects in COX15 also cause Leigh syndrome (LS; MIM:256000), an early-onset progressive neurodegenerative disorder characterised by the presence of focal, bilateral lesions in one or more areas of the central nervous system (Oquendo et al. 2004, Bugiani et al. 2005).
Heme oxygenase (HO) cleaves the heme ring at the alpha-methene bridge to form bilverdin. This reaction forms the only endogenous source of carbon monoxide. HO-1 is inducible and is thought to have an antioxidant role as it's activated in virtually all cell types and by many types of "oxidative stress" (Poss and Tonegawa, 1997). HO-2 is non-inducible.
Bilirubin is the breakdown product of heme, causing death if allowed to accumulate in the blood. It is highly lipophilic thus requires conjugation to become more water soluble to aid excretion.
Bilirubin is a breakdown product of heme, causing death if allowed to accumulate in the blood. It is highly lipophilic and thus requires conjugation to become more water soluble to aid excretion. UGT1A1 is the only enzyme that converts bilirubin to either a monoglucuronide or diglucuronide. Mutations of the UGT1A1 gene cause complete loss or partial activity for bilirubin glucuronidation.
The principal conjugate of bilirubin in bile is bilirubin diglucuronide. The monmeric form of UGT1A1 (Bilirubin UDP-glucuronyltransferase) only conjugates the first step of bilirubin conjugation to form the monoglucuronide. A tetrameric form of UGT1A1 can convert bilirubin to both the monoglucuronide and the diglucuronide.
Protoporphyrin IX (PRIN9) is transported into the mitochondrial matrix where it becomes available for the last step in the heme biosynthetic pathway. The transporter that mediates this event is unknown.
Six electrons are oxidized in protophorphyrinogen IX (PPGEN9) to form the planar macrocycle protoporphyrin IX (PRIN9). This reaction is performed by the enzyme protoporphyrinogen oxidase (PPO). PPO functions as a homodimer containing one non-covalently-bound FAD. The protein resides on the outer surface of the inner mitochondrial membrane. PPO deficiency is associated with variegate porphyria in vivo.
O2-dependent coproporpyrinogen oxidase (CPO) catalyzes the conversion of coproporphyrinogen III (COPRO3) to protoporphyrinogen IX (PPGEN9). The localization of the human enzyme to the mitochondrial intermembrane space is inferred from studies of the homologous rat enzyme (Elder and Evans 1978). The human enzyme functions as a homodimer (Lee et al. 2005). Enzyme deficiency is associated with hereditary coproporphyria in vivo.
Cytosolic uroporphyrinogen decarboxylase (UROD) catalyzes the sequntial removal of four carboxylic groups from the acetic acid side chains of uroporphyrinogen III (URO3) to form coproporphyrinogen III (COPRO3) (de Verneuil et al. 1983). Human UROD is a dimer (Whitby et al. 1998). Heterogenous and homogenous deficiencies of UROD are associated with porphyria cutanea tarda and hepatoerythropoietic porphyria respectively in vivo (Moran-Jiminez et al. 1996).
5-Aminolevulinic acid dehydratase (ALAD aka porphobilinogen synthase, PBGS), catalyzes the asymmetric condensation of two molecules of ALA to form porphobilinogen (PBG). The substrate that becomes the acetyl side chain-containing half of PBG is called A-side ALA; the half that becomes the propionyl side chains and the pyrrole nitrogen is called P-ALA (Jaffe 2004). PBG is the first pyrrole formed, the precursor to all tetrapyrrole pigments such as heme and chlorophyll. There are at least eight bonds that are made or broken during this reaction. The active form of the ALAD enzyme is an octamer complexed with eight Zn2+ ions, four that are strongly bound and four that are weakly bound. The four weakly bound ones are dispensible for enzyme activity in vitro (Bevan et al. 1980; Mitchell et al. 2001). Deficiencies of ALAD enzyme in vivo are associated with 5-aminolevulinate dehydratase-deficient porphyria (e.g., Akagi et al. 2000).
Cytosolic uroporphyrinogen decarboxylase (UROD) catalyzes the sequential removal of four carboxylic groups from the acetic acid side chains of uroporphyrinogen I (URO1) to form coproporphyrinogen I (COPRO1). UROD catalyzes this reaction less efficiently than the decarboxylation of uroporphyrinogen III (de Verneuil et al. 1983).
Lead binds to ALAD enzyme displacing half the zinc ions essential for its catalytic activity and inactivating it. Lead is a major environmental toxin and this enzyme is one of its principal molecular targets (Jaffe et al. 2001).
Cytosolic porphobilinogen deaminase catalyzes the polymerization of four molecules of porphobilinogen (PBG) to generate hydroxymethylbilane (HMB), an unstable tetrapyrrole. This reaction is the first step in the formation of the tetrapyrrole macrocycle. Two isoforms of porphobilinogen deaminase are generated by alternative splicing, one expresssed in erythroid tissues and one ubiquitously expressed in the body. Deficiencies of both forms of PBG deaminase are associated with acute intermittent porphyria.
5-aminolevulinate is transported from the mitochondrial matrix to the cytosol. The transporter that enables it to cross the inner mitochondrial membrane is unknown.
Ferrochelatase (FECH) catalyzes the insertion of ferrous iron into protoporphyrin IX (PRIN9) to form heme. FECH is localized on the matrix surface of the inner mitochondrial membrane and this reaction takes place within the mitochondrial matrix. The enzyme functions as a homodimer with each monomer containing a nitric oxide-sensitive 2Fe-2S cluster. Enzyme deficiency is associated with erythropoietic protoporphyria in vivo, and inhibition of ferrochelatase activity is a clinically important consequence of lead poisoning (Piomelli et al. 1987).
The committed step for porphyrin synthesis is the formation of 5-aminolevulinate (ALA) by condensation of glycine (from the general amino acid pool) and succinyl-CoA (from the TCA cycle), in the mitochondrial matrix. The reaction is catalyzed by two different ALA synthases, one expressed ubiquitously (ALAS1) and the other only expressed in erythroid precursors (ALAS2). Both enzymes are expressed as homodimers and require pyridoxal 5-phosphate as a cofactor. No disease-causing mutations of ALAS1 have been recognised in humans. Mutations in ALAS2 cause X-linked sideroblastic anaemia (XLSA), characterised by a microcytic hypochromic anaemia.
Cytosolic uroporphyrinogen III synthase (URO3S) catalyzes the conversion of HMB (hydroxymethylbilane) to uroporphyrinogen III, a reaction involving ring closure and intramolecular rearrangement. Uroporphyrinogen III represents a branch point for the pathways leading to formation of heme, chlorophyll and corrins. HMB is rapidly converted from a linear tetrapyrrole to the cyclic form. Deficiencies of URO3S in vivo are associated with congenital erythropoietic porphyria.
Coproporpyrinogen III (COPRO3) enters the mitochondrial intermembrane space from the cytosol. It is not known whether this process is facilitated by a transporter.
Porphyrins readily combine with metals by coordinating them in the central cavity. Iron (heme) and magnesium (chlorophyll) are two well known examples although zinc, copper, nickel and cobalt form other known metal-containing phorphyrins. A porphyrin which has no metal in the cavity is called a free base.
Some iron-containing porphyrins are called hemes (heme-containing proteins or hemoproteins) and these are found extensively in nature ie. hemoglobin. Hemoglobin is quantitatively the most important hemoprotein. The hemoglobin iron is the transfer site of oxygen and carries it in the blood all round the body for cell respiration. Other examples are cytochromes present in mitochondria and endoplasmic reticulum which takes part in electron transfer events, catalase and peroxidase whic protect the body against the oxidant hydrogen peroxide and tryptophan oxygenase which is present in intermediary metabolism. Hemoproteins are synthesized in all mammalian cells and the major sites are erythropoietic tissue and the liver.
The processes by which heme is synthesized, transported, and metabolized are a critical part of human iron metabolism (Severance and Hamze 2009); here the core processes of heme biosynthesis and catabolism have been annotated.
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Pb2+
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Pb2+
Zn2+Deficiencies of ALAD enzyme in vivo are associated with 5-aminolevulinate dehydratase-deficient porphyria (e.g., Akagi et al. 2000).
No disease-causing mutations of ALAS1 have been recognised in humans. Mutations in ALAS2 cause X-linked sideroblastic anaemia (XLSA), characterised by a microcytic hypochromic anaemia.