Oxidation of fatty acids and pyruvate in the mitochondrial matrix yield large amounts of NADH. The respiratory electron transport chain couples the re-oxidation of this NADH to NAD+ to the export of protons from the mitochonrial matrix, generating a chemiosmotic gradient across the inner mitochondrial membrane. This gradient is used to drive the synthesis of ATP; it can also be bypassed by uncoupling proteins to generate heat, a reaction in brown fat that may be important in regulation of body temperature in newborn children.
View original pathway at:Reactome.
Stanley CA, Hale DE.; ''Genetic disorders of mitochondrial fatty acid oxidation.''; PubMedEurope PMCScholia
Allan CM, Hill S, Morvaridi S, Saiki R, Johnson JS, Liau WS, Hirano K, Kawashima T, Ji Z, Loo JA, Shepherd JN, Clarke CF.; ''A conserved START domain coenzyme Q-binding polypeptide is required for efficient Q biosynthesis, respiratory electron transport, and antioxidant function in Saccharomyces cerevisiae.''; PubMedEurope PMCScholia
Yoshida S, Tsutsumi S, Muhlebach G, Sourbier C, Lee MJ, Lee S, Vartholomaiou E, Tatokoro M, Beebe K, Miyajima N, Mohney RP, Chen Y, Hasumi H, Xu W, Fukushima H, Nakamura K, Koga F, Kihara K, Trepel J, Picard D, Neckers L.; ''Molecular chaperone TRAP1 regulates a metabolic switch between mitochondrial respiration and aerobic glycolysis.''; PubMedEurope PMCScholia
Fontanesi F, Soto IC, Horn D, Barrientos A.; ''Assembly of mitochondrial cytochrome c-oxidase, a complicated and highly regulated cellular process.''; PubMedEurope PMCScholia
Boyer PD, Cross RL, Momsen W.; ''A new concept for energy coupling in oxidative phosphorylation based on a molecular explanation of the oxygen exchange reactions.''; PubMedEurope PMCScholia
Hirst J, Carroll J, Fearnley IM, Shannon RJ, Walker JE.; ''The nuclear encoded subunits of complex I from bovine heart mitochondria.''; PubMedEurope PMCScholia
Barros MH, Johnson A, Gin P, Marbois BN, Clarke CF, Tzagoloff A.; ''The Saccharomyces cerevisiae COQ10 gene encodes a START domain protein required for function of coenzyme Q in respiration.''; PubMedEurope PMCScholia
Balsa E, Marco R, Perales-Clemente E, Szklarczyk R, Calvo E, Landázuri MO, Enríquez JA.; ''NDUFA4 is a subunit of complex IV of the mammalian electron transport chain.''; PubMedEurope PMCScholia
Mitchell P.; ''Protonmotive redox mechanism of the cytochrome b-c1 complex in the respiratory chain: protonmotive ubiquinone cycle.''; PubMedEurope PMCScholia
Wikstrom MK.; ''Proton pump coupled to cytochrome c oxidase in mitochondria.''; PubMedEurope PMCScholia
Wood PA.; ''Defects in mitochondrial beta-oxidation of fatty acids.''; PubMedEurope PMCScholia
Loeffen J, Elpeleg O, Smeitink J, Smeets R, Stöckler-Ipsiroglu S, Mandel H, Sengers R, Trijbels F, van den Heuvel L.; ''Mutations in the complex I NDUFS2 gene of patients with cardiomyopathy and encephalomyopathy.''; PubMedEurope PMCScholia
Belogrudov GI, Hatefi Y.; ''Factor B and the mitochondrial ATP synthase complex.''; PubMedEurope PMCScholia
Sciacovelli M, Guzzo G, Morello V, Frezza C, Zheng L, Nannini N, Calabrese F, Laudiero G, Esposito F, Landriscina M, Defilippi P, Bernardi P, Rasola A.; ''The mitochondrial chaperone TRAP1 promotes neoplastic growth by inhibiting succinate dehydrogenase.''; PubMedEurope PMCScholia
Trumpower BL, Gennis RB.; ''Energy transduction by cytochrome complexes in mitochondrial and bacterial respiration: the enzymology of coupling electron transfer reactions to transmembrane proton translocation.''; PubMedEurope PMCScholia
Stiburek L, Hansikova H, Tesarova M, Cerna L, Zeman J.; ''Biogenesis of eukaryotic cytochrome c oxidase.''; PubMedEurope PMCScholia
Schultz BE, Chan SI.; ''Structures and proton-pumping strategies of mitochondrial respiratory enzymes.''; PubMedEurope PMCScholia
Garlid KD, Jaburek M, Jezek P.; ''Mechanism of uncoupling protein action.''; PubMedEurope PMCScholia
Belogrudov GI.; ''Factor B is essential for ATP synthesis by mitochondria.''; PubMedEurope PMCScholia
Roe CR, Roe DS.; ''Recent developments in the investigation of inherited metabolic disorders using cultured human cells.''; PubMedEurope PMCScholia
Loeffen JL, Triepels RH, van den Heuvel LP, Schuelke M, Buskens CA, Smeets RJ, Trijbels JM, Smeitink JA.; ''cDNA of eight nuclear encoded subunits of NADH:ubiquinone oxidoreductase: human complex I cDNA characterization completed.''; PubMedEurope PMCScholia
Estornell E, Fato R, Castelluccio C, Cavazzoni M, Parenti Castelli G, Lenaz G.; ''Saturation kinetics of coenzyme Q in NADH and succinate oxidation in beef heart mitochondria.''; PubMedEurope PMCScholia
Soto IC, Fontanesi F, Liu J, Barrientos A.; ''Biogenesis and assembly of eukaryotic cytochrome c oxidase catalytic core.''; PubMedEurope PMCScholia
Kevelam SH, Rodenburg RJ, Wolf NI, Ferreira P, Lunsing RJ, Nijtmans LG, Mitchell A, Arroyo HA, Rating D, Vanderver A, van Berkel CG, Abbink TE, Heutink P, van der Knaap MS.; ''NUBPL mutations in patients with complex I deficiency and a distinct MRI pattern.''; PubMedEurope PMCScholia
Schuelke M, Loeffen J, Mariman E, Smeitink J, van den Heuvel L.; ''Cloning of the human mitochondrial 51 kDa subunit (NDUFV1) reveals a 100% antisense homology of its 3'UTR with the 5'UTR of the gamma-interferon inducible protein (IP-30) precursor: is this a link between mitochondrial myopathy and inflammation?''; PubMedEurope PMCScholia
Garlid KD, Orosz DE, Modrianský M, Vassanelli S, Jezek P.; ''On the mechanism of fatty acid-induced proton transport by mitochondrial uncoupling protein.''; PubMedEurope PMCScholia
Pitceathly RD, Rahman S, Wedatilake Y, Polke JM, Cirak S, Foley AR, Sailer A, Hurles ME, Stalker J, Hargreaves I, Woodward CE, Sweeney MG, Muntoni F, Houlden H, Taanman JW, Hanna MG, UK10K Consortium.; ''NDUFA4 mutations underlie dysfunction of a cytochrome c oxidase subunit linked to human neurological disease.''; PubMedEurope PMCScholia
Smeitink J, Sengers R, Trijbels F, van den Heuvel L.; ''Human NADH:ubiquinone oxidoreductase.''; PubMedEurope PMCScholia
Mckenzie M, Ryan MT.; ''Assembly factors of human mitochondrial complex I and their defects in disease.''; PubMedEurope PMCScholia
Andrews B, Carroll J, Ding S, Fearnley IM, Walker JE.; ''Assembly factors for the membrane arm of human complex I.''; PubMedEurope PMCScholia
Jezek P, Hanus J, Semrad C, Garlid KD.; ''Photoactivated azido fatty acid irreversibly inhibits anion and proton transport through the mitochondrial uncoupling protein.''; PubMedEurope PMCScholia
Bourges I, Ramus C, Mousson de Camaret B, Beugnot R, Remacle C, Cardol P, Hofhaus G, Issartel JP.; ''Structural organization of mitochondrial human complex I: role of the ND4 and ND5 mitochondria-encoded subunits and interaction with prohibitin.''; PubMedEurope PMCScholia
Mimaki M, Wang X, McKenzie M, Thorburn DR, Ryan MT.; ''Understanding mitochondrial complex I assembly in health and disease.''; PubMedEurope PMCScholia
Sass JO.; ''Inborn errors of ketogenesis and ketone body utilization.''; PubMedEurope PMCScholia
Sheftel AD, Stehling O, Pierik AJ, Netz DJ, Kerscher S, Elsässer HP, Wittig I, Balk J, Brandt U, Lill R.; ''Human ind1, an iron-sulfur cluster assembly factor for respiratory complex I.''; PubMedEurope PMCScholia
Carilla-Latorre S, Gallardo ME, Annesley SJ, Calvo-Garrido J, Graña O, Accari SL, Smith PK, Valencia A, Garesse R, Fisher PR, Escalante R.; ''MidA is a putative methyltransferase that is required for mitochondrial complex I function.''; PubMedEurope PMCScholia
MacLennan DH, Lenaz G, Szarkowska L.; ''Studies on the mechanims of oxidative phosphorylation. IX. Effect of cytochrome c on energy-linked processes.''; PubMedEurope PMCScholia
Mitchell P.; ''Possible molecular mechanisms of the protonmotive function of cytochrome systems.''; PubMedEurope PMCScholia
Echtay KS, Murphy MP, Smith RA, Talbot DA, Brand MD.; ''Superoxide activates mitochondrial uncoupling protein 2 from the matrix side. Studies using targeted antioxidants.''; PubMedEurope PMCScholia
Friedrich T, Böttcher B.; ''The gross structure of the respiratory complex I: a Lego System.''; PubMedEurope PMCScholia
Guzzo G, Sciacovelli M, Bernardi P, Rasola A.; ''Inhibition of succinate dehydrogenase by the mitochondrial chaperone TRAP1 has anti-oxidant and anti-apoptotic effects on tumor cells.''; PubMedEurope PMCScholia
Mitochondrial ATP synthase subunit s (ATP5S) appears to be an essential subunit necessary for H+ conduction of ATP synthase (Belogrudov & Hatefi 2002, Belogrudov 2002).
Acetoacetate, beta-hydroxybutyrate, and acetone collectively are called ketone bodies. The first two are synthesized from acetyl-CoA, in the mitochondria of liver cells; acetone is formed by spontaneous decarboxylation of acetoacetate. Ketone body synthesis in liver is effectively irreversible because the enzyme that catalyzes the conversion of acetoacetate to acetoacetyl-CoA is not present in liver cells.
Ketone bodies, unlike fatty acids and triglycerides, are water-soluble. They are exported from the liver, and are taken up by other tissues, notably brain and skeletal and cardiac muscle. There, they are broken down to acetyl-CoA which is oxidized via the TCA cycle to yield energy. In a normal person, this pathway of ketone body synthesis and utilization is most active during extended periods of fasting. Under these conditions, mobilization and breakdown of stored fatty acids generates abundant acetyl-CoA acetyl-CoA in liver cells for synthesis of ketone bodies, and their utilization in other tissues minimizes the demand of these tissues for glucose (Sass 2011).
Beta-oxidation begins once fatty acids have been imported into the mitochondrial matrix by carnitine acyltransferases. The beta-oxidation spiral of fatty acids metabolism involves the repetitive removal of two carbon units from the fatty acyl chain. There are four steps to this process: oxidation, hydration, a second oxidation, and finally thiolysis. The last step releases the two-carbon acetyl-CoA and a ready primed acyl-CoA that takes another turn down the spiral. In total each turn of the beta-oxidation spiral produces one NADH, one FADH2, and one acetyl-CoA.
Further oxidation of acetyl-CoA via the tricarboxylic acid cycle generates additional FADH2 and NADH. All reduced cofactors are used by the mitochondrial electron transport chain to form ATP. The complete oxidation of a fatty acid molecule produces numerous ATP molecules. Palmitate, used as the model here, produces 129 ATPs.
Beta-oxidation pathways differ for saturated and unsaturated fatty acids. The beta-oxidation of saturated fatty acids requires four different enzymatic steps. Beta-oxidation produces and consumes intermediates with a trans configuration; unsaturated fatty acids that have bonds in the cis configuration require three separate enzymatic steps to prepare these molecules for the beta-oxidation pathway.
This event is deduced on the basis of bovine experimental data. Complex II (succinate dehydrogenase) transfers electrons from the TCA cycle to ubiquinone. The 6th step in the TCA cycle is where succinate is dehydrogenated to fumarate with subsequent reduction of FAD to FADH2. FADH2 provides the electrons for the transport chain. Succinate dehydrogenase belongs to subclass 1 of the SQR family (succinate:quinone reductase) (classified by Hagerhall, C and Hederstedt, L [1996]). It consists of 4 subunits (referred to as A, B, C and D), all nuclear-encoded and is located on the matrix side of the inner mitochondrial membrane. Subunits A and B are hydrophilic whereas subunits C and D are integral proteins of the inner membrane. SQRs usually contain 3 Fe-S clusters bound by the B subunit. Succinate dehydrogenase contains one [2Fe-2S] cluster, one [4Fe-4S] cluster and one [3Fe-4S] cluster. Additionally, the A subunit has a covalently-bound FAD group. Reduced complex II has this FAD converted to FADH2. The electrons from complex II are transferred to ubiquinone (also called Q, Coenzyme Q or CoQ), a small mobile carrier of electrons located within the inner membrane. Ubiquinone is reduced to ubiquinol during this process.
The mitochondrial heat shock protein 75 kDa (TRAP1) inhibits Complex II of the respiratory chain which elicits respiratory downregulation, leading to a pseudohypoxic state. This state is caused by succinate-dependent HIF1-alpha stabilisation which, in turn, can promote tumorigenesis (Sciacovelli et al. 2013, Yoshida et al. 2013, Guzzo et al. 2014).
Complex IV (COX, cytochrome c oxidase) contains the hemeprotein cytochrome a and a3. It also contains copper atoms which undergo a transition from Cu+ to Cu2+ during the transfer of electrons through the complex to molecular oxygen. A bimetallic centre containing a copper atom and a heme-linked iron protein binds oxygen after 4 electrons have been picked up. Water, the final product of oxygen reduction, is then released. Oxygen is the final electron acceptor in the respiratory chain. The overall reaction can be summed as
4Cyt c (red.) + 12H+ (in) + O2 = 4Cyt c (ox.) + 2H2O + 8H+ (out)
Four protons are taken up from the matrix side of the membrane to form the water (scalar protons). Wikstrom (1977) suggests 4 protons are additionally transferred out from the matrix to the intermembrane space.
COX ancillary proteins mediate membrane insertion, catalytic core processing, copper transport and insertion into core subunits and heme A biosynthesis (Stilburek et al. 2006, Fontanesi et al. 2006, Soto et al. 2012). To date, all Mendelian disorders presenting COX deficiency have been assigned to mutations in ancillary factors, with the exception of an infantile encephalomyopathy caused by a defective COX6B1 and an exocrine pancreatic insufficiency caused by a defective COX4I2 gene (Soto et al. 2012). Balsa et al have shown that NDUFA4, formerly considered to be a constituent of NADH dehydrogenase (Complex I), is instead a component of the cytochrome c oxidase (CIV) (Balsa et al. 2012). Patients with NDUFA4 mutations display COX deficiencies (Pitceathly et al. 2013).
Complex I (NADH:ubiquinone oxidoreductase or NADH dehydrogenase) utilizes NADH formed from glycolysis and the TCA cycle to pump protons out of the mitochondrial matrix. It is the largest enzyme complex in the electron transport chain, containing 45 subunits. Seven subunits (ND1-6, ND4L) are encoded by mitochondrial DNA (Loeffen et al [1998]), the remainder are encoded in the nucleus. The enzyme has a FMN prosthetic group and 8 Iron-Sulfur (Fe-S) clusters. The electrons from NADH oxidation pass through the flavin (FMN) and Fe-S clusters to ubiquinone (CoQ). This electron transfer is coupled with the translocation of protons from the mitochondrial matrix to the intermembrane space. For each electron transferred, 2 protons can be pumped out of the matrix. As there are 2 electrons transferred, 4 protons can be pumped out. Complex I is made up of 3 sub-complexes - Iron-Sulfur protein fraction (IP), Flavoprotein fraction (FP) and the Hydrophobic protein fraction (HP), probably arranged in an L-shaped structure with the IP and FP fractions protruding into the mitochondrial matrix and the HP arm lying within the inner mitochondrial membrane. The overall reaction can be summed as below: NADH + Ubiquinone + 5H+ (mito. matrix) = NAD+ + Ubiquinol + 4H+ (intermemb. space) The electrons from complex I are transferred to ubiquinone (Coenzyme Q, CoQ), a small mobile carrier of electrons located within the inner membrane. Ubiquinone is reduced to ubiquinol (QH2) during this process.
Mitochondrial coenzyme Q-binding protein COQ10 homologs A and B (COQ10A and B) are thought to be required for correct coenzyme CoQ in the respiratory chain. Their function in humans is unknown but the yeast model suggests functions in facilitating de novo CoQ biosynthesis and in delivering it to one or more complexes of the respiratory electron transport chain (Barros et al. 2005, Allan et al. 2013).
The protonmotive Q cycle is the mechanism by which complex III transfers electrons from ubiquinol to cytochrome c, linking this process to translocation of protons across the membrane. This cycle is complicated by the fact that both ubiquinol is oxidised and ubiquinone is reduced during this process. Through a complex series of electron transfers, Complex III consumes two molecules of ubiquinol (QH2) and two molecules of oxidized cytochrome c, generates one molecule of ubiquinone (Q) and two molecules of reduced cytochrome c, regenerates one molecule of ubiquinol (QH2), and mediates the translocation of two protons from the mitochondrial matrix to the mitochondrial intermembrane space. The overall reaction can be summed up as
In the tight configuration, the beta subunit catalyzes the reaction of ADP + Pi to ATP + water. ATP is still tightly bound to the subunit at this stage.
In the last step, the beta subunit is converted to the open form and ATP is released. Passage of protons through the Fo part causes a ring of approximately 10 subunits to rotate. This rotation in turn drives the rotation of the gamma subunits, which forms part of one of the stalks. The gamma subunit moves between the three beta subunits which are held in place by the second stalk which can be regarded as a stator. The polypeptide called OSCP connects the stator stalk to the assembly of alpha and beta subunits. It is this step that is coupled to proton translocation as energy is required to break the strong bond between ATP and the protein.
The beta subunit has 3 conformations; tight, open and loose. ADP and Pi bind to the subunit in the loose form. On binding, this subunit is converted to the tight configuration.
At the beginning of this reaction, 1 molecule of 'Fatty Acid "head-out"' is present. At the end of this reaction, 1 molecule of 'Fatty Acid "head-in"' is present.
This reaction takes place in the 'mitochondrial inner membrane'.
At the beginning of this reaction, 1 molecule of 'H+', and 1 molecule of 'Fatty Acid anion "head-out"' are present. At the end of this reaction, 1 molecule of 'Fatty Acid "head-out"' is present.
This reaction takes place in the 'mitochondrial inner membrane'.
A FA anion diffuses laterally within the membrane towards UCP. The membrane potential drives the FA anion to an energy well halfway up on UCP. The electric field created by the redox-linked proton ejection drives the head group to the energy well.
At the beginning of this reaction, 1 molecule of 'Fatty Acid "head-in"' is present. At the end of this reaction, 1 molecule of 'H+', and 1 molecule of 'Fatty Acid anion "head-in"' are present.
This reaction takes place in the 'mitochondrial inner membrane'.
At the beginning of this reaction, 1 molecule of 'FA anion:UCP dimer "head-out" complex' is present. At the end of this reaction, 1 molecule of 'UCP dimer', and 1 molecule of 'Fatty Acid anion "head-out"' are present.
This reaction takes place in the 'mitochondrial inner membrane' (Garlid et al. 1996).
Electron transfer flavoprotein (ETF) is a 63kDa heterodimer composed of alpha and beta subunits and binds one FAD and one AMP per dimer. ETF resides on the matrix face of the mitochondrial inner membrane. Reducing equivalents from the beta-oxidation of fatty acyl CoAs are transferred to ETF, reducing the ETF-bound FAD to FADH2 (Wood 1999).
ETF-ubiquinone oxidoreductase (ETFDH), catalyses the re-oxidation of reduced ETF, with ubiquinone (CoQ) as the electron acceptor being reduced to ubiquinol (QH2) (Estornell et al. 1992, MacLennan et al. 1997).
In this reaction, 1 molecule of 'H+' is translocated from mitochondrial intermembrane space to mitochondrial matrix.
This reaction takes place in the 'mitochondrial inner membrane' and is mediated by the 'hydrogen ion transporter activity' of 'UCP dimer' (Echtay et al. 2002a, Echtay et al. 2002b).
The iron-sulfur protein NUBPL is thought to bind the cofactor [4Fe-4S] cluster and deliver it to complex I (NADH dehydrogenase) subunits during its biogenesis. The exact mechanism of transfer is unknown but defects in NUBPL are shown to cause mitochondrial complex I deficiency (MT-C1D) with a distinct MRI pattern (Sheftel et al. 2009, Kevelam et al. 2013).
In total, eight iron-sulfur (4Fe-4S) clusters are incorporated into six subunits (mitochondrial matrix-located NDUFS1, S2, S7, S8 and mitochondrial membrane-located V1 and V2) (Andrews et al. 2013). Incorporation into NDUFV1 and V2 (located on the mitochondrial membrane) is shown in a separate reaction. The mechanism of transfer in all cases is unknown.
In total, eight iron-sulfur (4Fe-4S) clusters are incorporated into six subunits (mitochondrial matrix-located NDUFS1, S2, S7, S8 and mitochondrial membrane-located V1 and V2) (Andrews et al. 2013). Incorporation into NDUFS1, S2, S7 and S8 is shown in a separate reaction. The mechanism of transfer is unknown. NDUFV1 also binds FMN (Schuelke et al. 1998).
The hydrophobic protein fraction (HP) is assembled with NDUFA3, 8, 9 and 13 amongst many others and anchored to the inner mitochondrial membrane by Intermediate 1 assembly factors NDUFAF3 (C3orf60), NDUFAF4 (C6orf66) and TIMMDC1 (C3orf1) to form Intermediate 2 (Mckenzie & Ryan 2010, Andrews et al. 2013).
Subunits NDUFA12, NDUFS1, 4, 6, NDUFV1, 2 and 3 with the assembly factor NDUFAF2 comprises the peripheral arm, called the flavoprotein (FP) subcomplex. In addition, remaining subunits such as NDUFS5 join here (Mimaki et al. 2012).
A complex I intermediate of 315kDa (reestimated from the original 400kDa) is formed centred around the core subunits NADH dehydrogenase [ubiquinone] iron-sulfur proteins 2 and 3 (NDUFS2 and NDUFS3) with other complex I subunits and assembly factor subunits (forming IP and HP subcomplexes). The IP subcomplex is anchored to the inner mitochondrial membrane by NADH-ubiquinone oxidoreductase chain 1 (MT-ND1) (together with NDUFAF5 and/or 6) (Mckenzie & Ryan 2010, Andrews et al. 2013).
In the last step, the MCIA complex and it is assumed all of the assembly factors (NDUFAF2-7, TIMMDC1) dissociate from the 980kDa complex to leave mature Complex I (Mckenzie & Ryan 2010, Andrews et al. 2013).
Distal components of the membrane arm MT-ND4 and 5 associate with the 550kDa complex to form the 815kDa complex (Mckenzie & Ryan 2010, Andrews et al. 2013).
Membrane arm subunits MT-ND2, 3 and 6 and NDUFB6 associate with the assembly factors TMEM126B, NDUFAF1, ECSIT and ACAD9 (which form the MCIA complex) forming a 370kDa subcomplex (Mckenzie & Ryan 2010, Andrews et al. 2013).
Complex I assembly begins with the formation of a 315kDa subcomplex, centred around the core subunits NADH dehydrogenase [ubiquinone] iron-sulfur proteins 2 and 3 (NDUFS2 and NDUFS3) (Mckenzie & Ryan 2010, Mimaki et al. 2012, Andrews et al. 2013). NDUFS2 is thought to be bound to NDUFAF7 (Carilla-Latorre et al. 2010). Defects in NDUFS2 can cause mitochondrial complex I deficiency (MT-C1D; OMIM:252010), causing a wide range of clinical disorders, ranging from lethal neonatal disease to adult-onset neurodegenerative disorders (Loeffen et al. 2001). As an initial part of the 315kDa subcomplex, the subunits NDUFS7, S8 and A9, together with NDUFS2 and S3, form an evolutionarily conserved hydrogenase module as part of the Iron-Sulfur protein fraction (IP) subcomplex (termed Intermediate 1 here) (Mckenzie & Ryan 2010, Andrews et al. 2013).
The subunits NDUFS7, S8 and A9, together with NDUFS2 and S3, form an evolutionarily conserved hydrogenase module as part of the Iron-Sulfur protein fraction (IP) subcomplex (Mckenzie & Ryan 2010, Andrews et al. 2013).
Subunits NDUFA12, NDUFS1, 4, 6, NDUFV1, 2 and 3 with the assembly factor NDUFAF2 comprises the peripheral arm, called the flavoprotein (FP) subcomplex (Mimaki et al. 2012).
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Ketone bodies, unlike fatty acids and triglycerides, are water-soluble. They are exported from the liver, and are taken up by other tissues, notably brain and skeletal and cardiac muscle. There, they are broken down to acetyl-CoA which is oxidized via the TCA cycle to yield energy. In a normal person, this pathway of ketone body synthesis and utilization is most active during extended periods of fasting. Under these conditions, mobilization and breakdown of stored fatty acids generates abundant acetyl-CoA acetyl-CoA in liver cells for synthesis of ketone bodies, and their utilization in other tissues minimizes the demand of these tissues for glucose (Sass 2011).
Further oxidation of acetyl-CoA via the tricarboxylic acid cycle generates additional FADH2 and NADH. All reduced cofactors are used by the mitochondrial electron transport chain to form ATP. The complete oxidation of a fatty acid molecule produces numerous ATP molecules. Palmitate, used as the model here, produces 129 ATPs.
Beta-oxidation pathways differ for saturated and unsaturated fatty acids. The beta-oxidation of saturated fatty acids requires four different enzymatic steps. Beta-oxidation produces and consumes intermediates with a trans configuration; unsaturated fatty acids that have bonds in the cis configuration require three separate enzymatic steps to prepare these molecules for the beta-oxidation pathway.
dehydrogenase
complex (oxidised)dehydrogenase
complex (reduced)Annotated Interactions
Complex II (succinate dehydrogenase) transfers electrons from the TCA cycle to ubiquinone. The 6th step in the TCA cycle is where succinate is dehydrogenated to fumarate with subsequent reduction of FAD to FADH2. FADH2 provides the electrons for the transport chain. Succinate dehydrogenase belongs to subclass 1 of the SQR family (succinate:quinone reductase) (classified by Hagerhall, C and Hederstedt, L [1996]).
It consists of 4 subunits (referred to as A, B, C and D), all nuclear-encoded and is located on the matrix side of the inner mitochondrial membrane. Subunits A and B are hydrophilic whereas subunits C and D are integral proteins of the inner membrane. SQRs usually contain 3 Fe-S clusters bound by the B subunit. Succinate dehydrogenase contains one [2Fe-2S] cluster, one [4Fe-4S] cluster and one [3Fe-4S] cluster. Additionally, the A subunit has a covalently-bound FAD group. Reduced complex II has this FAD converted to FADH2. The electrons from complex II are transferred to ubiquinone (also called Q, Coenzyme Q or CoQ), a small mobile carrier of electrons located within the inner membrane. Ubiquinone is reduced to ubiquinol during this process.
The mitochondrial heat shock protein 75 kDa (TRAP1) inhibits Complex II of the respiratory chain which elicits respiratory downregulation, leading to a pseudohypoxic state. This state is caused by succinate-dependent HIF1-alpha stabilisation which, in turn, can promote tumorigenesis (Sciacovelli et al. 2013, Yoshida et al. 2013, Guzzo et al. 2014).
4Cyt c (red.) + 12H+ (in) + O2 = 4Cyt c (ox.) + 2H2O + 8H+ (out)
Four protons are taken up from the matrix side of the membrane to form the water (scalar protons). Wikstrom (1977) suggests 4 protons are additionally transferred out from the matrix to the intermembrane space.
COX ancillary proteins mediate membrane insertion, catalytic core processing, copper transport and insertion into core subunits and heme A biosynthesis (Stilburek et al. 2006, Fontanesi et al. 2006, Soto et al. 2012). To date, all Mendelian disorders presenting COX deficiency have been assigned to mutations in ancillary factors, with the exception of an infantile encephalomyopathy caused by a defective COX6B1 and an exocrine pancreatic insufficiency caused by a defective COX4I2 gene (Soto et al. 2012). Balsa et al have shown that NDUFA4, formerly considered to be a constituent of NADH dehydrogenase (Complex I), is instead a component of the cytochrome c oxidase (CIV) (Balsa et al. 2012). Patients with NDUFA4 mutations display COX deficiencies (Pitceathly et al. 2013).
Complex I is made up of 3 sub-complexes - Iron-Sulfur protein fraction (IP), Flavoprotein fraction (FP) and the Hydrophobic protein fraction (HP), probably arranged in an L-shaped structure with the IP and FP fractions protruding into the mitochondrial matrix and the HP arm lying within the inner mitochondrial membrane. The overall reaction can be summed as below:
NADH + Ubiquinone + 5H+ (mito. matrix) = NAD+ + Ubiquinol + 4H+ (intermemb. space)
The electrons from complex I are transferred to ubiquinone (Coenzyme Q, CoQ), a small mobile carrier of electrons located within the inner membrane. Ubiquinone is reduced to ubiquinol (QH2) during this process.
Mitochondrial coenzyme Q-binding protein COQ10 homologs A and B (COQ10A and B) are thought to be required for correct coenzyme CoQ in the respiratory chain. Their function in humans is unknown but the yeast model suggests functions in facilitating de novo CoQ biosynthesis and in delivering it to one or more complexes of the respiratory electron transport chain (Barros et al. 2005, Allan et al. 2013).
2QH2 + 2cyt c (ox.) + Q + 2H+ (matrix) = 2Q + 2cyt c (red.) + QH2 + 4H+ (intermemb. space)
This reaction takes place in the 'mitochondrial inner membrane'.
This reaction takes place in the 'mitochondrial inner membrane'.
This reaction takes place in the 'mitochondrial inner membrane'.
This reaction takes place in the 'mitochondrial inner membrane' (Garlid et al. 1996).
This reaction takes place in the 'mitochondrial inner membrane' and is mediated by the 'hydrogen ion transporter activity' of 'UCP dimer' (Echtay et al. 2002a, Echtay et al. 2002b).
dehydrogenase
complex (oxidised)dehydrogenase
complex (reduced)dehydrogenase
complex (reduced)