Fast electron transfer involving cytochrome

The establishment of fast electron transfer over these distances of up to 25 A is important, as similar distances are involved in certain natural redox partners. Thus cytochrome c and cytochrome c peroxidase form a complex in which the heme edge-to-edge separation is about 16 A. 

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. 

Azurin is involved in the electron-transfer chain to cytochrome oxidase in bacterial respiration. Reduction of oxidized azurin by cytochrome c551 is very fast,946 947 but the reverse process is complicated through the existence of a slow conformational equilibrium. 

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