CHEM 440
Biochemistry I
J. D. Cronk   Syllabus [ Previous | Next ] Pick a lecture:
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Lecture 33. Electron transport chain

Friday 3 December 2010

The integral membrane complexes of the mitochondrial ETC.

Reading: Voet, Voet, and Pratt; Ch.18, pp.602-618.
33. Summary

Lecture 33 Summary: The integral membrane complexes of the mitochondrial electron transport chain (ETC).

The ETC is responsible for the reduction of molecular oxygen by NADH. This exergonic reaction is carried out in a precisely controlled, multistep manner that preserves much of the energy released in the form of a transmembrane electrochemical gradient. This feat is accomplished by four integral membrane protein complexes. Two mobile electron carriers shuttle electrons between these complexes, coenzyme Q (ubiquinone) and cytochrome c. Three of the four complexes pump protons across the inner mitochondrial membrane, thus generating the transmembrane electrochemical gradient. Five types of prosthetic groups are involved in electron transfer within the complexes.

NADH-Q oxidoreductase (Complex I) [EC 1.6.5.3]

Official name: NADH dehydrogenase (ubiquinone). The largest complex >900 kD in size, with 46 distinct polypeptide chains. Cofactors: FMN, iron-sulfur (Fe-S) cluster. Protons pumped: about 4 H+ per pair of electrons transferred.

 

   Ribbon structure diagram of Complex II

Succinate-Q reductase (Complex II)

140 kD, 4 subunits (pictured above). Cofactors: heme, quinone, 3 Fe-S clusters, and FAD. Does not pump H+.

Q-cytochrome c oxidoreductase (Complex III)

250 kD, 11 subunits. Cofactors: hemes (3), Fe-S. Two H+ pumped per electron pair transferred. Also called cytochrome bc1 complex.

Cytochrome c oxidase (Complex IV)

160 kD, 13 subunits. Cofactors: hemes, copper ions. About 4 H+ are pumped out of matrix per 2e- transferred.

Prosthetic groups of electron transfer

NADH, which feeds electrons at a relatively low ΔE°′ into the ETC, is a pyridine nucleotide. The five types of prosthetic groups that participate in electron transfer in the respiratory chain are flavin nucleotides (FAD and FMN), quinones, iron-sulfur (Fe-S) clusters, hemes, and copper ion.
Right: The succinate-Q reductase (Complex II) structure, with the protein not displayed in order to clearly show its prosthetic groups and their arrangement. From left to right is heme, a Q2 molecule (a ubiquinone with its nonpolar isoprenoid tail shortened to two five-carbon units), three iron-sulfur clusters, and FAD. The red dashes are meant to suggest electron transfer paths through the protein. The iron-sulfur clusters are separated by about 9 and 11 Å. The figure was drawn from pdb 1nek.   Image (small) of electron transfer prosthetic groups from complex II

 
 

Learning objectives

Page updated 07-21-10

References

  1. Hederstedt L (2003) Complex II is Complex Too. Science 299: 671-672. (Summary; Full text and enhanced content is available online to subscribers)
  2. Yankovskaya Y, et al. (2003). Architecture of Succinate Dehydrogenase and Reactive Oxygen Species Generation. Science 299: 700-704. (Abstract; Full text available online to subscribers)
  3. Xia D, et al. (1997) Crystal structure of the cytochrome bc1 complex from bovine heart mitochondria. Science 277: 60-66.
  4. Tsukihara T, et al. (1996) The whole structure of the 13-subunit oxidized cytochrome c oxidase at 2.8 Å. Science 272: 1136-1144.
  5. Michel H, et al. (1998) Cytochrome c oxidase: Structure and spectroscopy. Ann Rev Biophys Biomol Struct 27: 329-356.
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