Coenzyme Q – cytochrome c reductase

UCR_TM
Crystal structure of mitochondrial cytochrome bc1 complex bound with ubiquinone.[1]
Identifiers
Symbol	UCR_TM
Pfam	PF02921
InterPro	IPR004192
SCOP	1be3
SUPERFAMILY	1be3
TCDB	3.D.3
OPM superfamily	345
OPM protein	3cx5
Available protein structures: Pfam structures PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

ubiquinol—cytochrome-c reductase
Identifiers
EC number	1.10.2.2
CAS number	9027-03-6
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
Search PMC articles PubMed articles NCBI proteins

schematic illustration of complex III reactions

The coenzyme Q : cytochrome c — oxidoreductase, sometimes called the cytochrome bc₁ complex, and at other times complex III, is the third complex in the electron transport chain (EC 1.10.2.2), playing a critical role in biochemical generation of ATP (oxidative phosphorylation). Complex III is a multisubunit transmembrane protein encoded by both the mitochondrial (cytochrome b) and the nuclear genomes (all other subunits). Complex III is present in the mitochondria of all animals and all aerobic eukaryotes and the inner membranes of most eubacteria. Mutations in Complex III cause exercise intolerance as well as multisystem disorders. The bc1 complex contains 11 subunits, 3 respiratory subunits (cytochrome B, cytochrome C1, Rieske protein), 2 core proteins and 6 low-molecular weight proteins.

Ubiquinol—cytochrome-c reductase catalyzes the chemical reaction

QH₂ + 2 ferricytochrome c $\rightleftharpoons$ Q + 2 ferrocytochrome c + 2 H⁺

Thus, the two substrates of this enzyme are quinol (QH2) and ferri- (Fe³⁺) cytochrome c, whereas its 3 products are quinone (Q), ferro- (Fe²⁺) cytochrome c, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on diphenols and related substances as donor with a cytochrome as acceptor. This enzyme participates in oxidative phosphorylation. It has four cofactors: cytochrome c₁, cytochrome b-562, cytochrome b-566, and a 2-Iron ferredoxin of the Rieske type.

Nomenclature

The systematic name of this enzyme class is ubiquinol:ferricytochrome-c oxidoreductase. Other names in common use include:

coenzyme Q-cytochrome c reductase, dihydrocoenzyme Q-cytochrome c reductase, reduced ubiquinone-cytochrome c reductase, complex III, (mitochondrial electron transport), ubiquinone-cytochrome c reductase, ubiquinol-cytochrome c oxidoreductase, reduced coenzyme Q-cytochrome c reductase, ubiquinone-cytochrome c oxidoreductase, reduced ubiquinone-cytochrome c oxidoreductase,

mitochondrial electron transport complex III, ubiquinol-cytochrome c-2 oxidoreductase, ubiquinone-cytochrome b-c1 oxidoreductase, ubiquinol-cytochrome c2 reductase, ubiquinol-cytochrome c1 oxidoreductase, CoQH2-cytochrome c oxidoreductase, ubihydroquinol:cytochrome c oxidoreductase, coenzyme QH2-cytochrome c reductase, and QH2:cytochrome c oxidoreductase.

Structure

Structure of complex III

Compared to the other major proton-pumping subunits of the electron transport chain, the number of subunits found can be small, as small as three polypeptide chains. This number does increase, and eleven subunits are found in higher animals.^[2] Three subunits have prosthetic groups. The cytochrome b subunit has two b-type hemes (b_L and b_H), the cytochrome c subunit has one c-type heme (c₁), and the Rieske Iron Sulfur Protein subunit (ISP) has a two iron, two sulfur iron-sulfur cluster (2Fe•2S).

Structures of complex III: PDB: 1KYO​, PDB: 1L0L​

Composition of Complex

In vertebrates the bc₁ complex, or Complex III, contains 11 subunits: 3 respiratory subunits, 2 core proteins and 6 low-molecular weight proteins.^[3][4] Proteobacterial complexes may contain as few as three subunits.^[5]

Table of Subunit composition of Complex III

No.	Subunit name	Human protein	Protein description from UniProt	Pfam family with Human protein
Respiratory subunit proteins
1	MT-CYB / Cyt b	CYB_HUMAN	Cytochrome b	Pfam PF13631
2	CYC1 / Cyt c1	CY1_HUMAN	Cytochrome c1, heme protein, mitochondrial	Pfam PF02167
3	Rieske / UCR1	UCRI_HUMAN	Cytochrome b-c1 complex subunit Rieske, mitochondrial EC 1.10.2.2	Pfam PF02921 , Pfam PF00355
Core protein subunits
4	QCR1 / SU1	QCR1_HUMAN	Cytochrome b-c1 complex subunit 1, mitochondrial	Pfam PF00675, Pfam PF05193
5	QCR2 / SU2	QCR2_HUMAN	Cytochrome b-c1 complex subunit 2, mitochondrial	Pfam PF00675, Pfam PF05193
Low-molecular weight protein subunits
6	QCR6 / SU6	QCR6_HUMAN	Cytochrome b-c1 complex subunit 6, mitochondrial	Pfam PF02320
7	QCR7 / SU7	QCR7_HUMAN	Cytochrome b-c1 complex subunit 7	Pfam PF02271
8	QCR8 / SU8	QCR8_HUMAN	Cytochrome b-c1 complex subunit 8	Pfam PF02939
9	QCR9 / SU9 / UCRC	QCR9_HUMAN^a	Cytochrome b-c1 complex subunit 9	Pfam PF09165
10	QCR10 / SU10	QCR10_HUMAN	Cytochrome b-c1 complex subunit 10	Pfam PF05365
11	QCR11 / SU11	QCR11_HUMAN	Cytochrome b-c1 complex subunit 11	Pfam PF08997

• ^a In vertebrates, a cleavage product of 8 kDa from the N-terminus of the Rieske protein (Signal peptide) is retained in the complex as subunit 9. Thus subunits 10 and 11 correspond to fungal QCR9p and QCR10p.

Reaction

It catalyzes the reduction of cytochrome c by oxidation of coenzyme Q (CoQ) and the concomitant pumping of 4 protons from the mitochondrial matrix to the intermembrane space:

QH₂ + 2 cytochrome c (Fe^III) + 2 H+
in → Q + 2 cytochrome c (Fe^II) + 4 H+
out

In the process called Q cycle,^[6][7] two protons are consumed from the matrix (M), four protons are released into the inter membrane space (IM) and two electrons are passed to cytochrome c.

Reaction mechanism

The Q cycle

The reaction mechanism for complex III (cytochrome bc1, coenzyme Q: cytochrome C oxidoreductase) is known as the ubiquinone ("Q") cycle. In this cycle four protons get released into the positive "P" side (inter membrane space), but only two protons get taken up from the negative "N" side (matrix). As a result, a proton gradient is formed across the membrane. In the overall reaction, two ubiquinols are oxidized to ubiquinones and one ubiquinone is reduced to ubiquinol. In the complete mechanism, two electrons are transferred from ubiquinol to ubiquinone, via two cytochrome c intermediates.

Overall:

• 2 x QH₂ oxidised to Q
• 1 x Q reduced to QH₂
• 2 x Cyt c₁ reduced
• 4 x H+ released into intermembrane space
• 2 x H⁺ picked up from matrix

The reaction proceeds according to the following steps:

Round 1:

1. Cytochrome b binds a ubiquinol and a ubiquinone.
2. The 2Fe/2S center and B_L heme each pull an electron off the bound ubiquinol, releasing two hydrogens into the intermembrane space.
3. One electron is transferred to cytochrome c₁ from the 2Fe/2S centre, whilst another is transferred from the B_L heme to the B_H Heme.
4. Cytochrome c₁ transfers its electron to cytochrome c (not to be confused with cytochrome c1), and the B_H Heme transfers its electron to a nearby ubiquinone, resulting in the formation of a ubisemiquinone.
5. Cytochrome c diffuses. The first ubiquinol (now oxidised to ubiquinone) is released, whilst the semiquinone remains bound.

Round 2:

1. A second ubiquinol is bound by cytochrome b.
2. The 2Fe/2S center and B_L heme each pull an electron off the bound ubiquinol, releasing two hydrogens into the intermembrane space.
3. One electron is transferred to cytochrome c₁ from the 2Fe/2S centre, whilst another is transferred from the B_L heme to the B_H Heme.
4. Cytocrome c₁ then transfers its electron to cytochrome c, whilst the nearby semiquinone produced from round 1 picks up a second electron from the B_H heme, along with two protons from the matrix.
5. The second ubiquinol (now oxidised to ubiquinone), along with the newly formed ubiquinol are released.^[8]

Inhibitors of complex III

There are three distinct groups of Complex III inhibitors.

• Antimycin A binds to the Q_i site and inhibits the transfer of electrons in Complex III from heme b_H to oxidized Q (Qi site inhibitor).
• Myxothiazol and stigmatellin binds to the Q_o site and inhibits the transfer of electrons from reduced QH₂ to the Rieske Iron sulfur protein. Myxothiazol and stigmatellin bind to distinct but overlapping pockets within the Q_o site.
  • Myxothiazol binds nearer to cytochrome bL (hence termed a "proximal" inhibitor).
  • Stigmatellin binds farther from heme bL and nearer the Rieske Iron sulfur protein, with which it strongly interacts.

Some have been commercialized as fungicides (the strobilurin derivatives, best known of which is azoxystrobin; QoI inhibitors) and as anti-malaria agents (atovaquone).

Also propylhexedrine inhibits cytochrome c reductase.^[9]

Oxygen free radicals

A small fraction of electrons leave the electron transport chain before reaching complex IV. Premature electron leakage to oxygen results in the formation of superoxide. The relevance of this otherwise minor side reaction is that superoxide and other reactive oxygen species are highly toxic and are thought to play a role in several pathologies, as well as aging (the free radical theory of aging).^[10] Electron leakage occurs mainly at the Q_o site and is stimulated by antimycin A. Antimycin A locks the b hemes in the reduced state by preventing their re-oxidation at the Q_i site, which, in turn, causes the steady-state concentrations of the Q_o semiquinone to rise, the latter species reacting with oxygen to form superoxide. The effect of high membrane potential is thought to have a similar effect.^[11] Superoxide produced at the Qo site can be released both into the mitochondrial matrix^[12][13] and into the intermembrane space, where it can then reach the cytosol.^[12][14] This could be explained by the fact that Complex III might produce superoxide as membrane permeable HOO^• rather than as membrane impermeable O−.
2.^[13]

Human gene names

MT-CYB: mtDNA encoded cytochrome b; mutations associated with exercise intolerance

CYC1:cytochrome c1

CYCS: cytochrome c

UQCRFS1: Rieske iron sulfur protein

UQCRB: Ubiquinone binding protein, mutation linked with mitochondrial complex III deficiency nuclear type 3

UQCRH: hinge protein

UQCRC2: Core 2, mutations linked to mitochondrial complex III deficiency, nuclear type 5

UQCRC1: Core 1

UQCR: 6.4KD subunit

UQCR10: 7.2KD subunit

TTC19: Newly identified subunit, mutations linked to complex III deficiency nuclear type 2

Mutations in Complex III genes in human disease

Mutations in Complex III-related genes typically manifest as exercise intolerance.^[15][16] Other mutations have been reported to cause septo-optic dysplasia^[17] and multisystem disorders.^[18] However, mutations in BCS1L, a gene responsible for proper maturation of Complex III, can result in Björnstad syndrome and the GRACILE syndrome, which in neonates are lethal conditions that have multisystem and neurologic manifestations typifying severe mitochondrial disorders. The pathogenicity of several mutations has been verified in model systems such as yeast.^[19]

The extent to which these various pathologies are due to bioenergetic deficits or overproduction of superoxide is presently unknown.

See also

• Cellular respiration
• Photosynthetic reaction centre

Additional images

• 

  ETC

References

1. PDB: 1ntz​; Gao X, Wen X, Esser L, Quinn B, Yu L, Yu CA, Xia D (August 2003). "Structural basis for the quinone reduction in the bc1 complex: a comparative analysis of crystal structures of mitochondrial cytochrome bc1 with bound substrate and inhibitors at the Qi site". Biochemistry. 42 (30): 9067–80. doi:10.1021/bi0341814. PMID 12885240.
2. Iwata S, Lee JW, Okada K, Lee JK, Iwata M, Rasmussen B, Link TA, Ramaswamy S, Jap BK (July 1998). "Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex". Science. 281 (5373): 64–71. doi:10.1126/science.281.5373.64. PMID 9651245.
3. Zhang Z, Huang L, Shulmeister VM, Chi YI, Kim KK, Hung LW, et al. (1998). "Electron transfer by domain movement in cytochrome bc1.". Nature. 392 (6677): 677–84. doi:10.1038/33612. PMID 9565029.
4. Hao GF, Wang F, Li H, Zhu XL, Yang WC, Huang LS, et al. (2012). "Computational discovery of picomolar Q(o) site inhibitors of cytochrome bc1 complex.". J Am Chem Soc. 134 (27): 11168–76. doi:10.1021/ja3001908. PMID 22690928.
5. Yang XH, Trumpower BL (1986). "Purification of a three-subunit ubiquinol-cytochrome c oxidoreductase complex from Paracoccus denitrificans". J Biol Chem. 261: 12282–9. PMID 3017970.
6. Kramer DM, Roberts AG, Muller F, Cape J, Bowman MK (2004). "Q-cycle bypass reactions at the Qo site of the cytochrome bc1 (and related) complexes". Meth. Enzymol. Methods in Enzymology. 382: 21–45. doi:10.1016/S0076-6879(04)82002-0. ISBN 978-0-12-182786-1. PMID 15047094.
7. Crofts AR (2004). "The cytochrome bc1 complex: function in the context of structure". Annu. Rev. Physiol. 66: 689–733. doi:10.1146/annurev.physiol.66.032102.150251. PMID 14977419.
8. Ferguson SJ, Nicholls D, Ferguson S (2002). Bioenergetics (3rd ed.). San Diego: Academic. pp. 114–117. ISBN 0-12-518121-3.
9. Holmes, J. H.; Sapeika, N; Zwarenstein, H (1975). "Inhibitory effect of anti-obesity drugs on NADH dehydrogenase of mouse heart homogenates". Research communications in chemical pathology and pharmacology. 11 (4): 645–6. PMID 241101.
10. Muller, F. L.; Lustgarten, M. S.; Jang, Y.; Richardson, A. & Van Remmen, H. (2007). "Trends in oxidative aging theories". Free Radic. Biol. Med. 43 (4): 477–503. doi:10.1016/j.freeradbiomed.2007.03.034. PMID 17640558.
11. Skulachev VP (May 1996). "Role of uncoupled and non-coupled oxidations in maintenance of safely low levels of oxygen and its one-electron reductants". Q. Rev. Biophys. 29 (2): 169–202. doi:10.1017/s0033583500005795. PMID 8870073.
12. Muller F (2000). "The nature and mechanism of superoxide production by the electron transport chain: Its relevance to aging". AGE. 23 (4): 227–253. doi:10.1007/s11357-000-0022-9. PMID 23604868.
13. Muller FL, Liu Y, Van Remmen H (November 2004). "Complex III releases superoxide to both sides of the inner mitochondrial membrane". J. Biol. Chem. 279 (47): 49064–73. doi:10.1074/jbc.M407715200. PMID 15317809.
14. Han D, Williams E, Cadenas E (January 2001). "Mitochondrial respiratory chain-dependent generation of superoxide anion and its release into the intermembrane space". Biochem. J. 353 (Pt 2): 411–6. doi:10.1042/0264-6021:3530411. PMC 1221585. PMID 11139407.
15. DiMauro S (November 2006). "Mitochondrial myopathies". Curr Opin Rheumatol. 18 (6): 636–41. doi:10.1097/01.bor.0000245729.17759.f2. PMID 17053512.
16. DiMauro S (June 2007). "Mitochondrial DNA medicine". Biosci. Rep. 27 (1–3): 5–9. doi:10.1007/s10540-007-9032-5. PMID 17484047.
17. Schuelke M, Krude H, Finckh B, Mayatepek E, Janssen A, Schmelz M, Trefz F, Trijbels F, Smeitink J (March 2002). "Septo-optic dysplasia associated with a new mitochondrial cytochrome b mutation". Ann. Neurol. 51 (3): 388–92. doi:10.1002/ana.10151. PMID 11891837.
18. Wibrand F, Ravn K, Schwartz M, Rosenberg T, Horn N, Vissing J (October 2001). "Multisystem disorder associated with a missense mutation in the mitochondrial cytochrome b gene". Ann. Neurol. 50 (4): 540–3. doi:10.1002/ana.1224. PMID 11601507.
19. Fisher N, Castleden CK, Bourges I, Brasseur G, Dujardin G, Meunier B (March 2004). "Human disease-related mutations in cytochrome b studied in yeast". J. Biol. Chem. 279 (13): 12951–8. doi:10.1074/jbc.M313866200. PMID 14718526.

Further reading

• Marres CM, Slater EC (1977). "Polypeptide composition of purified QH2:cytochrome c oxidoreductase from beef-heart mitochondria". Biochim. Biophys. Acta. 462 (3): 531–548. doi:10.1016/0005-2728(77)90099-8. PMID 597492.
• Rieske JS (1976). "Composition, structure, and function of complex III of the respiratory chain". Biochim. Biophys. Acta. 456 (2): 195–247. doi:10.1016/0304-4173(76)90012-4. PMID 788795.
• Wikstrom M, Krab K, Saraste M (1981). "Proton-translocating cytochrome complexes". Annu. Rev. Biochem. 50: 623–655. doi:10.1146/annurev.bi.50.070181.003203. PMID 6267990.

External links

• cytochrome bc₁ complex site (Edward A. Berry) at lbl.gov
• cytochrome bc₁ complex site (Antony R. Crofts) at uiuc.edu
• PROMISE Database: cytochrome bc₁ complex at scripps.edu
• Interactive Molecular Model of Complex III (Requires MDL Chime)
• UMich Orientation of Proteins in Membranes families/superfamily-3 - Calculated positions of bc1 and related complexes in membranes
• Coenzyme Q-Cytochrome-c Reductase at the US National Library of Medicine Medical Subject Headings (MeSH)