| Abstract: | The cytochrome a and a3 sites in uninhibited, detergent-solubilized cytochrome c oxidase have been studied under a wide range of conditions using thin-layer spectroelectrochemistry. The observed absorbance changes at the alpha and Soret absorbance maxima have been used together to estimate the extents of reduction of cytochromes a and a3, using the absorbance properties of these cytochromes deduced from previous measurements employing ligand inhibition of cytochrome a3. The resulting Nernst plots, combined with the results of parallel studies on the carbon monoxide-inhibited enzyme (Ellis, W. R., Jr., Wang, H., Blair, D. F., Gray, H. B., and Chan, S. I. (1986) Biochemistry 25, 161-167; Wang, H., Blair, D. F., Ellis, W. R., Jr., Gray, H. B., and Chan, S. I. (1986) Biochemistry 25, 167-171), indicate that the cytochrome a site participates in anticooperative thermodynamic interactions which involve all three of the other metal sites in the protein. Using an analysis which resolves the intrinsic thermodynamic properties of the cytochromes from the effects of the intersite interactions, the pH, temperature, and ionic strength dependences of the cytochrome reduction potentials have been measured. The standard entropy of reduction of cytochrome a in the native enzyme is large and negative, in agreement with measurements on the carbon monoxide-inhibited enzyme. The reduction potential of cytochrome a is only moderately (less than -30 mV/pH unit) dependent upon pH, which implies that its reduction is linked to the uptake, on the average, of only about 0.5 protons at pH 7.0, and significantly less at the higher pH values relevant to the mitochondrial matrix. The thermodynamic properties of cytochrome a3 were found to be different in the two enzyme batches studied: in one batch, the cytochrome a3 reduction potential decreased steeply (about -56 mV/pH unit) with increasing pH, indicating stoichiometric (1 H+/e-) coupling of protonation to reduction. In the other batch, the cytochrome a3 potential was insensitive to pH below pH 7.5 and decreased at higher pH values in a manner suggesting coupling to an ionizable group with pKa near 7.8. The temperature dependence of the cytochrome a3 reduction potential indicates that its standard entropy of reduction is more positive than that of myoglobin, another high-spin metalloprotein heme, and significantly more positive than that of cytochrome a.(ABSTRACT TRUNCATED AT 400 WORDS) |
