Coenzyme Q – cytochrome c reductase

Template:Infobox enzyme 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 (Template:EC number), 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 bacteria. 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 <math>\rightleftharpoons</math> 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

File:Cytochrome bc1 complex.png

Structure of complex III (click to enlarge)

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.<ref name="pmid9651245">Template:Cite journal</ref> 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: Template:PDB, Template:PDB

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.<ref name="pmid9565029">Template:Cite journal</ref><ref name="pmid22690928">Template:Cite journal</ref> Proteobacterial complexes may contain as few as three subunits.<ref name=yang1984>Template:Cite journal</ref>

Table of subunit composition of complex III

No.	Subunit name	Human gene symbol	Protein description from UniProt	Pfam family with Human protein
Respiratory subunit proteins
1	MT-CYB / Cyt b	MT-CYB	Cytochrome b	Template:Pfam
2	CYC1 / Cyt c1	CYC1	Cytochrome c1, heme protein, mitochondrial	Template:Pfam
3	Rieske / UCR1	UQCRFS1	Cytochrome b-c1 complex subunit Rieske, mitochondrial Template:EC number	Template:Pfam, Template:Pfam
Core protein subunits
4	QCR1 / SU1	UQCRC1	Cytochrome b-c1 complex subunit 1, mitochondrial	Template:Pfam, Template:Pfam
5	QCR2 / SU2	UQCRC2	Cytochrome b-c1 complex subunit 2, mitochondrial	Template:Pfam, Template:Pfam
Low-molecular weight protein subunits
6	QCR6 / SU6	UQCRH	Cytochrome b-c1 complex subunit 6, mitochondrial	Template:Pfam
7	QCR7 / SU7	UQCRB	Cytochrome b-c1 complex subunit 7	Template:Pfam
8	QCR8 / SU8	UQCRQ	Cytochrome b-c1 complex subunit 8	Template:Pfam
9	QCR9 / SU9	UQCRFS1a	(N-terminal of Rieske, no separate entry)	Template:Pfam
10	QCR10 / SU10	UQCR10	Cytochrome b-c1 complex subunit 9	Template:Pfam
11	QCR11 / SU11	UQCR11	Cytochrome b-c1 complex subunit 10	Template:Pfam

• ^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

File:Complex III.png

schematic illustration of complex III reactions

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 HTemplate:Su → Q + 2 cytochrome c (Fe^II) + 4 HTemplate:Su

In the process called Q cycle,<ref name="pmid15047094">Template:Cite book</ref><ref name="pmid14977419">Template:Cite journal</ref> 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

File:Theqcycle.gif

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 c 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 protons 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 protons 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 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.<ref name="isbn0-12-518121-3">Template:Cite book</ref>

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). Some Q_o site inhibitors have been commercialized as insecticides (IRAC group 20).<ref name=":02">Template:Cite book</ref>

Also propylhexedrine inhibits cytochrome c reductase.<ref name="pmid241101">Template:Cite journal</ref>

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).<ref name="pmid17640558">Template:Cite journal</ref> 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.<ref name="pmid8870073">Template:Cite journal</ref> Superoxide produced at the Qo site can be released both into the mitochondrial matrix<ref name="Muller, F. 2000">Template:Cite journal</ref><ref name="Muller, F. L. 2004">Template:Cite journal</ref> and into the intermembrane space, where it can then reach the cytosol.<ref name="Muller, F. 2000" /><ref name="pmid11139407">Template:Cite journal</ref> This could be explained by the fact that Complex III might produce superoxide as membrane permeable HOO^• rather than as membrane impermeable [[superoxide|OTemplate:Su]].<ref name="Muller, F. L. 2004" />

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. Helps remove the N-terminal fragment of UQCRFS1, which would otherwise interfere with complex III function.<ref>Template:Cite journal</ref>

Mutations in complex III genes in human disease

Mutations in complex III-related genes typically manifest as exercise intolerance.<ref name="pmid17053512">Template:Cite journal</ref><ref name="pmid17484047">Template:Cite journal</ref> Other mutations have been reported to cause septo-optic dysplasia<ref name="pmid11891837">Template:Cite journal</ref> and multisystem disorders.<ref name="pmid11601507">Template:Cite journal</ref> 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.<ref name="pmid14718526">Template:Cite journal</ref>

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

• Mitochondrial electron transport chain (annotated diagram).svg

  ETC

Further reading

• Template:Cite journal
• Template:Cite journal
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External links

• Template:Webarchive at lbl.gov
• cytochrome bc₁ complex site (Antony R. Crofts) Template:Webarchive at uiuc.edu
• Template:Webarchive at scripps.edu
• Template:Webarchive (Requires MDL Chime)
• Template:UMichOPM - Calculated positions of bc1 and related complexes in membranes
• Template:MeshName
• #124000 MITOCHONDRIAL COMPLEX III DEFICIENCY, NUCLEAR TYPE 1; MC3DN1 on OMIM; lists all other types of complex III deficiency