Lecture 3 Summary

Overview of the 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. 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.

The four membrane-associated protein complexes

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

Official name: NADH dehydrogenase (ubiquinone). The largest complex - 880 kD in size, with at least 38 distinct polypeptide chains. Cofactors: FMN, iron-sulfur (Fe-S) cluster. Protons pumped: about 4 H⁺ per pair of electrons transferred.

Succinate-Q reductase (Complex II)

140 kD, 4 subunits. Cofactors: FAD, Fe-S. Does not pump H⁺.

Q-cytochrome c oxidoreductase (Complex III)

250 kD, 10 subunits. Cofactors: hemes (3), Fe-S. Two H⁺ pumped per electron pair transferred.

Cytochrome c oxidase (Complex IV)

160 kD, 10 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 DE^o' 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.

Study questions

• Write the functional names for each of the four complexes of the electron transport chain.
• Name and describe the characteristics of the two mobile electron carriers of the ETC.
• Name and describe the types prosthetic groups participating in electron transfer

Page updated 12-27-06

References

1. Berg, Tymoczko, and Stryer. Biochemistry (BTS): 6th edition (2007, Freeman) Ch.18, pp.509-520.