Complex 3

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In enzymology, an ubiquinol---cytochrome-c reductase (EC 1.10.2.2) is an enzyme that catalyzes the chemical reaction

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

Thus, the two substrates of this enzyme are QH2 and ferricytochrome c, whereas its 3 products are Q, ferrocytochrome 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. 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. This enzyme participates in oxidative phosphorylation. It has 4 cofactors: cytochrome c1, Cytochrome b-562, Cytochrome b-566, and 2-Iron ferredoxin.

 

Contents

Structural studies

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 [1]. Three subunits have prosthetic groups. The cytochrome b subunit has two b-type hemes (bL and bH), 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).

concerted As of late 2007, 35 structures have been solved for this class of enzymes, with PDB accession codes 1BCC, 1BE3, 1BGY, 1EZV, 1KB9, 1KYO, 1L0L, 1L0N, 1NTK, 1NTM, 1NTZ, 1NU1, 1P84, 1PP9, 1PPJ, 1QCR, 1RIE, 1SQB, 1SQP, 1SQQ, 1SQV, 1SQX, 1ZRT, 2A06, 2BCC, 2FYN, 2FYU, 2IBZ, 2NUK, 2NUM, 2NVE, 2NVF, 2NVG, 2NWF, and 3BCC.

 

Reaction Mechanism

The Q cycle

The reaction mechanism for complex III (Cytochrome bc1 , Coenzyme Q: Cytochrome C Oxidoreductase) is named the Q cycle or the ubiquinone cycle. In this cycle four protons get released into the P or Positive side (inter membrane space) but only two protons get taken up from the N or Negative side (matrix), see animation to the right. As a result a proton gradient is formed across the membrane. Also, two ubiquinols get oxidized to ubiquinones and one ubiquinone gets reduced to ubiquinol! All this is accomplished by the transfer of two electrons from two ubiquinols to two cytochrome c's as well as two electrons from the same two ubiquinols to a ubiquinone. The reaction goes as follows.

1. Ubiquinol binds to cytochrome b.
2. The 2Fe/2S center and BL Heme each pull an electron off the bound ubiquinone and two hydrogens are released into the intermembrane space.
3. The 2Fe/2S center transfers its electron to cytochrome c1 and the BL Heme transfers its electron to the BL Heme.
4. Cytocrome c1 gives its then transfers its electron to a water soluble cytochrome c (not to be confused with cytochrome c1 which is membrane bound)and the BL Heme transfers its electron to a nearby ubiquinone turning the ubiquinone into a ubisemiquinone.
5. Cytochrome c diffuses and the fully oxidized ubiquinol is released. 6. Another ubiquinol binds to cytochrome b.
7. The 2Fe/2S center and BL Heme each pull an electron off the bound ubiquinone and two hydrogens are released into the intermembrane space.
8. The 2Fe/2S center transfers its electron to cytochrome c1 and the BL Heme transfers its electron to the BH Heme.
9. Cytocrome c1 gives its then transfers its electron to a water soluble cytochrome c and the BH Heme transfers its electron as well as two hydrogens from the matrix to the nearby ubisemiquinone turning the ubisemiquinone into a ubiquinol.
10.The fully oxidized to ubiquinone and ubiquinol is released.[2]


 

 

References

  1. ^ Iwata S., Lee J.W., Okada K., Lee J.K., Iwata M., Rasmussen B., Link T.A., Ramaswamy S., Jap B.K. (1998) Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex. Science 281: 64-71
  2. ^ Nicholls, David and Stuart Ferguson. Bioenergetics3. Acociate Press: San Diego, California 2002. pg 114-117
  • IUBMB entry for 1.10.2.2
  • BRENDA references for 1.10.2.2 (Recommended.)
  • PubMed references for 1.10.2.2
  • PubMed Central references for 1.10.2.2
  • Google Scholar references for 1.10.2.2
  • Marres CM, Slater EC (1977). "Polypeptide composition of purified QH2:cytochrome c oxidoreductase from beef-heart mitochondria". Biochim. Biophys. Acta. 462: 531–48. PMID 597492. 
  • Rieske JS (1976). "Composition, structure, and function of complex III of the respiratory chain". Biochim. Biophys. Acta. 456: 195–247. PMID 788795. 
  • Wikstrom M, Krab K, Saraste M (1981). "Proton-translocating cytochrome complexes". Annu. Rev. Biochem. 50: 623–55. doi:10.1146/annurev.bi.50.070181.003203. PMID 6267990. 

 

External links

The CAS registry number for this enzyme class is 9027-03-6.

  • IUBMB entry for 1.10.2.2
  • KEGG entry for 1.10.2.2
  • BRENDA entry for 1.10.2.2
  • NiceZyme view of 1.10.2.2
  • EC2PDB: PDB structures for 1.10.2.2
  • PRIAM entry for 1.10.2.2
  • PUMA2 entry for 1.10.2.2
  • IntEnz: Integrated Enzyme entry for 1.10.2.2
  • MetaCyc entry for 1.10.2.2
  • Atomic-resolution structures of enzymes belonging to this class


 

 

Gene Ontology (GO) codes

  • EGO entry for GO code 0008121: ubiquinol-cytochrome-c reductase
  • AMIGO entry for GO code 0008121: ubiquinol-cytochrome-c reductase
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