Lecture 4 Summary

In this lecture, we discuss the coupling of electron transport to proton translocation, distinguishing two general modes by which such coupling can occur. Then we consider two specific examples - each illustrating one of the modes - bacteriorhodopsin and the Q cycle.

Models and mechanisms for proton translocation

The actual mechanisms of proton translocation in the electron transport chain are incompletely understood and are under active investigation. However, there are two general models that we will consider, along with specific examples of each. We will denote these models as proton pumping and redox loops. An example of proton pumping is provided by the light-driven bacteriorhodopsin integral membrane protein. A complicated, yet fairly well-characterized example of a redox loop is provided by the Q cycle, which operates when the mobile electron carrier coenzyme Q donates its electons to Complex III.

The Q cycle and Q-cytochrome c oxidoreductase (Complex III)

Each molecule of the reduced form of coenzyme Q - ubiquinol (QH₂) - carries two electrons from Complex I or Complex II to Complex III, the next H⁺-pumping complex of the electron transport chain. Complex III is also called cytochrome c reductase or cytochrome bc₁ complex. The output of Complex III is the reduced form of cytochrome c, which carries one electron.

Study questions

• Write the functional names for each of the four complexes of the electron transport chain.
• Describe and illustrate two models for coupling electron transfer to proton translocation.
• Show how the Q cycle acts as a redox loop.
• Show the how the Q cycle couples electron transport from a two-electron carrier to a one-electron carrier.

Page updated 12-27-06

References

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