Dioxygen reduction by cytochrome oxidase: a proton transfer limited reaction

Abstract

The kinetic constraints that are imposed on cytochrome oxidase in its dual function as the terminal oxidant in the respiratory process and as a redox-linked proton pump provide a unique opportunity to investigate the molecular details of biological O2 activation. By using flow/flash techniques, it is possible to visualize individual steps in the O2-binding and reduction process, and results from a number of spectroscopic investigations on the oxidation of reduced cytochrome oxidase by O2 are now available. In this article, we use these results to synthesize a reaction mechanism for O2 activation in the enzyme and to simulate time-concentration profiles for a number of intermediates that have been observed experimentally. Kinetic manifestations of the consequences of coupling exergonic electron transfer to endergonic proton translocation emerge from this analysis. An important conclusion is that, in achieving efficiency in this redox-coupled proton translocation mechanism, rate limitation in dioxygen activation in cytochrome oxidase is transferred to protonation reactions that occur late in the reduction reaction. As a consequence, potentially toxic intermediate oxidation states of dioxygen accumulate to substantial concentration during the reduction reaction, which allows us to detect and characterize these species.