Temperature Dependence of the Reduction Potential of Cu_A in Carbon Monoxide Inhibited Cytochrome c Oxidase

The temperature dependence of the reduction potential of the Cu_A site in carbon monoxide inhibited cytochrome c oxidase has been measured with a spectroelectrochemical method adapted to the relatively weak near-infrared absorption of this copper ion. These measurements, together with parallel measurements on the 604-nm absorption due to Fe_a, indicate that an interaction between Cu_A and Fe_a causes the reduction potential for one of these sites to be decreased by approximately 40 mV upon reduction of the other. The temperature dependence of the Cu_A reduction potential indicates a relatively large and negative standard entropy of reduction of Cu_A(ΔS°' = -48.7 ± 2.3 eu). Possible implications of the intersite redox interaction and the large standard entropy of reduction of the Cu_A site are discussed.