Steady state kinetics ionic equilibria

The present view is that cytochrome a is the acceptor of electrons from cytochrome c, but that a simple linear electron-transfer sequence from cytochrome a to Cua and then to the cytochrome 03/Cub centre is unlikely. Instead the sequence shown in equation (63) holds, where cytochrome a is in rapid equilibrium with Cua. These views depend largely upon pre-steady-state kinetics of the redox half reactions of the enzyme with its two substrates, ferrocytochrome c and O2. However, these conclusions are not in accord with kinetic studies under conditions when both substrates are bound to the enzyme, and which show maximal rates of electron transfer from cytochrome c to O2. In particular some of the cytochrome c is oxidized at a faster rate than a metal centre in the oxidase. In contrast, at high ionic strength conditions, where the cytochrome c and the cytochrome oxidase are mainly dissociated, oxidation of cytochrome c occurs only slowly following the complete oxidation of the oxidase. These results for the fast oxidation of cytochrome c have been interpreted in terms of direct electron transfer from cytochrome c to the bridged peroxo intermediate involving 03 and Cub, or to a two-electron transfer to O2 from cytochromes a and 03 during the initial phase of the reaction. 

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