How does cytochrome oxidase pump protons?

Abstract

We are now in an exciting era of molecular bioenergetics. High-resolution x-ray structures have been determined for several of the key components of bioenergetics systems, including the respiratory enzymes cytochrome oxidase (1–4) and the bc1 complex (5, 6), the F1 component of the F1Fo-ATPase (7), the light-driven proton pump bacteriorhodopsin (8, 9), and the bacterial photosynthetic reaction center (10–12). Each of these membrane proteins generates (or can use) a protonmotive force across the membrane by using unique mechanisms. In cytochrome oxidase the complex oxygen chemistry is somehow linked at different stages to driving protons about 50 Å across the membrane against an electrochemical gradient. In this issue of the Proceedings, Hartmut Michel (45) offers a new model of how this process might work. He presents a challenge to a paradigm that has been generally accepted by researchers in the field for the past decade. To be sure, his proposal will spark healthy debate and a new round of experiments designed to test the predictions of the new model that distinguish it from other models that have been offered. Cytochrome oxidase is a magnificent enzyme. A wide variety of biophysical techniques in combination with site-directed mutagenesis and the recent x-ray crystallography have resulted in a remarkable increase in our understanding of this enzyme in the past few years (for review see ref. 13). The x-ray structures have been determined for the 13-subunit bovine heart mitochondrial oxidase (3, 4) as well as the four-subunit oxidase from the soil bacterium Paracoccus denitrificans (1, 2). The x-ray structures show that subunits I, II, and III are very similar in both systems. The Paracoccusoxidase also has been isolated in a two-subunit form (subunits I and II) that is fully …