Editing Oxidative phosphorylation (section)

=== Q-cytochrome c oxidoreductase (complex III) ===
[[File:Complex III reaction.svg|420px|thumb|right|The two electron transfer steps in complex III: [[Coenzyme Q - cytochrome c reductase|Q-cytochrome c oxidoreductase]]. After each step, Q (in the upper part of the figure) leaves the enzyme.]]

[[Coenzyme Q - cytochrome c reductase|Q-cytochrome c oxidoreductase]] is also known as ''cytochrome c reductase'', ''cytochrome bc<sub>1</sub> complex'', or simply ''complex III''.<ref>{{cite journal | vauthors = Berry EA, Guergova-Kuras M, Huang LS, Crofts AR | title = Structure and function of cytochrome bc complexes | journal = Annual Review of Biochemistry | volume = 69 | pages = 1005–1075 | year = 2000 | pmid = 10966481 | doi = 10.1146/annurev.biochem.69.1.1005 | url = http://www.life.illinois.edu/crofts/pdf_files/ARB_review.pdf | url-status = live | citeseerx = 10.1.1.319.5709 | archive-url = https://web.archive.org/web/20151228224336/http://www.life.illinois.edu/crofts/pdf_files/ARB_review.pdf | archive-date = 2015-12-28 }}</ref><ref>{{cite journal | vauthors = Crofts AR | title = The cytochrome bc1 complex: function in the context of structure | journal = Annual Review of Physiology | volume = 66 | pages = 689–733 | year = 2004 | pmid = 14977419 | doi = 10.1146/annurev.physiol.66.032102.150251 }}</ref> In mammals, this enzyme is a [[protein dimer|dimer]], with each subunit complex containing 11 protein subunits, an [2Fe-2S] iron–sulfur cluster and three [[cytochrome]]s: one [[cytochrome]] c<sub>1</sub> and two b [[cytochromes]].<ref>{{cite journal | vauthors = Iwata S, Lee JW, Okada K, Lee JK, Iwata M, Rasmussen B, Link TA, Ramaswamy S, Jap BK | title = Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex | journal = Science | volume = 281 | issue = 5373 | pages = 64–71 | date = July 1998 | pmid = 9651245 | doi = 10.1126/science.281.5373.64 | bibcode = 1998Sci...281...64I }}</ref> A cytochrome is a kind of electron-transferring protein that contains at least one [[heme]] group. The iron atoms inside complex III's heme groups alternate between a reduced ferrous (+2) and oxidized ferric (+3) state as the electrons are transferred through the protein.

The reaction catalyzed by complex III is the oxidation of one molecule of [[ubiquinol]] and the reduction of two molecules of [[cytochrome c]], a heme protein loosely associated with the mitochondrion. Unlike coenzyme Q, which carries two electrons, cytochrome c carries only one electron.

{{NumBlk|:|<chem>QH2{} + 2 Cyt\, c_{ox}{} + 2H+_{matrix} -> Q{} + 2 Cyt\, c_{red}{} + 4H+_{intermembrane}</chem>|{{EquationRef|4}}}}

As only one of the electrons can be transferred from the QH<sub>2</sub> donor to a cytochrome c acceptor at a time, the reaction mechanism of complex III is more elaborate than those of the other respiratory complexes, and occurs in two steps called the [[Q cycle]].<ref>{{cite journal | vauthors = Trumpower BL | title = The protonmotive Q cycle. Energy transduction by coupling of proton translocation to electron transfer by the cytochrome bc1 complex | journal = The Journal of Biological Chemistry | volume = 265 | issue = 20 | pages = 11409–11412 | date = July 1990 | pmid = 2164001 | doi = 10.1016/S0021-9258(19)38410-8 | url = http://www.jbc.org/cgi/reprint/265/20/11409.pdf | url-status = live | doi-access = free | archive-url = https://web.archive.org/web/20070927135240/http://www.jbc.org/cgi/reprint/265/20/11409.pdf | archive-date = 2007-09-27 }}</ref> In the first step, the enzyme binds three substrates, first, QH<sub>2</sub>, which is then oxidized, with one electron being passed to the second substrate, cytochrome c. The two protons released from QH<sub>2</sub> pass into the intermembrane space. The third substrate is Q, which accepts the second electron from the QH<sub>2</sub> and is reduced to Q<sup>.−</sup>, which is the [[semiquinone|ubisemiquinone]] [[free radical]]. The first two substrates are released, but this ubisemiquinone intermediate remains bound. In the second step, a second molecule of QH<sub>2</sub> is bound and again passes its first electron to a cytochrome c acceptor. The second electron is passed to the bound ubisemiquinone, reducing it to QH<sub>2</sub> as it gains two protons from the mitochondrial matrix. This QH<sub>2</sub> is then released from the enzyme.<ref>{{cite journal | vauthors = Hunte C, Palsdottir H, Trumpower BL | title = Protonmotive pathways and mechanisms in the cytochrome bc1 complex | journal = FEBS Letters | volume = 545 | issue = 1 | pages = 39–46 | date = June 2003 | pmid = 12788490 | doi = 10.1016/S0014-5793(03)00391-0 | s2cid = 13942619 | doi-access = free | bibcode = 2003FEBSL.545...39H }}</ref>

As coenzyme Q is reduced to ubiquinol on the inner side of the membrane and oxidized to ubiquinone on the other, a net transfer of protons across the membrane occurs, adding to the proton gradient.<ref name=Schultz/> The rather complex two-step mechanism by which this occurs is important, as it increases the efficiency of proton transfer. If, instead of the Q cycle, one molecule of QH<sub>2</sub> were used to directly reduce two molecules of cytochrome c, the efficiency would be halved, with only one proton transferred per cytochrome c reduced.<ref name=Schultz/>