# Coenzyme Q – cytochrome c reductase

The coenzyme Q : cytochrome c – oxidoreductase, sometimes called the cytochrome bc$_1$ 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 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
    QH$_2$ + 2 ferricytochrome c $\rightleftharpoons$ Q + 2 ferrocytochrome c + 2 H$^+$
Thus, the two substrates of this enzyme are quinol (QH$_2$) and ferri- (Fe$^{3+}$) cytochrome c, whereas its 3 products are quinone (Q), ferro- (Fe$^{2+}$) 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 c$_1$, 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

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. 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 (c$_1$), 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 bc$_1$ complex, or Complex III, contains 11 subunits: 3 respiratory subunits, 2 core proteins and 6 low-molecular weight proteins. Proteobacterial complexes may contain as few as three subunits.

### 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 | Pfam PF13631 |
| 2 | CYC1 / Cyt c1 | CYC1 | Cytochrome c1, heme protein, mitochondrial | Pfam PF02167 |
| 3 | Rieske / UCR1 | UQCRFS1 | Cytochrome b-c1 complex subunit Rieske, mitochondrial EC 1.10.2.2 | Pfam PF02921 , Pfam PF00355 |
| Core protein subunits |||||
| 4 | QCR1 / SU1 | UQCRC1 | Cytochrome b-c1 complex subunit 1, mitochondrial | Pfam PF00675, Pfam PF05193 |
| 5 | QCR2 / SU2 | UQCRC2 | Cytochrome b-c1 complex subunit 2, mitochondrial | Pfam PF00675, Pfam PF05193 |
| Low-molecular weight protein subunits |||||
| 6 | QCR6 / SU6 | UQCRH | Cytochrome b-c1 complex subunit 6, mitochondrial | Pfam PF02320 |
| 7 | QCR7 / SU7 | UQCRB | Cytochrome b-c1 complex subunit 7 | Pfam PF02271 |
| 8 | QCR8 / SU8 | UQCRQ | Cytochrome b-c1 complex subunit 8 | Pfam PF02939 |
| 9 | QCR9 / SU9 | UQCRFS1$^a$ | (N-terminal of Rieske, no separate entry) | Pfam PF09165 |
| 10 | QCR10 / SU10 | UQCR10 | Cytochrome b-c1 complex subunit 9 | Pfam PF05365 |
| 11 | QCR11 / SU11 | UQCR11 | Cytochrome b-c1 complex subunit 10 | 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:
    QH$_2$ + 2 cytochrome c (Fe$^{III}$) + 2 H$^+$ $_{in}$ → Q + 2 cytochrome c (Fe$^{II}$) + 4 H$^+$ $_{out}$
In the process called Q cycle, 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 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 QH$_2$ oxidised to Q
* 1 x Q reduced to QH$_2$
* 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:
* Cytochrome b binds a ubiquinol and a ubiquinone.
* The 2Fe/2S center and B$_L$ heme each pull an electron off the bound ubiquinol, releasing two protons into the intermembrane space.
* One electron is transferred to cytochrome c$_1$ from the 2Fe/2S centre, whilst another is transferred from the B$_L$ heme to the B$_H$ Heme.
* Cytochrome c$_1$ 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.
* Cytochrome c diffuses. The first ubiquinol (now oxidised to ubiquinone) is released, whilst the semiquinone remains bound.
Round 2:
* A second ubiquinol is bound by cytochrome b.
* The 2Fe/2S center and B$_L$ heme each pull an electron off the bound ubiquinol, releasing two protons into the intermembrane space.
* One electron is transferred to cytochrome c$_1$ from the 2Fe/2S centre, whilst another is transferred from the B$_L$ heme to the B$_H$ Heme.
* Cytochrome c$_1$ 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.
* The second ubiquinol (now oxidised to ubiquinone), along with the newly formed ubiquinol are released.

## 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 QH$_2$ 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).
Also propylhexedrine inhibits cytochrome c reductase.

## 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). 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. Superoxide produced at the Qo site can be released both into the mitochondrial matrix and into the intermembrane space, where it can then reach the cytosol. This could be explained by the fact that Complex III might produce superoxide as membrane permeable HOO$^•$ rather than as membrane impermeable O$^{−.}$ $_2$.

## 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.

## Mutations in complex III genes in human disease

Mutations in complex III-related genes typically manifest as exercise intolerance. Other mutations have been reported to cause septo-optic dysplasia and multisystem disorders. 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.
The extent to which these various pathologies are due to bioenergetic deficits or overproduction of superoxide is presently unknown.