Internal Electron Transfer in Cytochrome c Oxidase Is Coupled to the Protonation of a Group Close to the Bimetallic Site

Citations Over TimeTop 13% of 1994 papers

Abstract

Absorbance changes following CO dissociation by flash photolysis from mixed-valence cytochrome oxidase have been followed in the Soret and alpha regions. Apart from CO dissociation and recombination, three kinetic phases with rate constants in the range 10(5)-10(3) s-1 at pH 7.5 can be resolved in both spectral regions. The slowest one of these phases, which had earlier only been observed in the alpha region, has now been detected in the Soret region by the use of a low CO concentration to slow down the recombination reaction. This phase had been assigned to a structural change, but a kinetic difference spectrum demonstrates that it represents electron transfer from cytochrome a3 to cytochrome a. A kinetic deuterium isotope effect of 2-3 at pH 7.5 suggests that it involves proton transfer as well. The temperature dependence of the reaction gives an Arrhenius activation energy of 42 kJ.mol-1. The reaction is faster at low pH, and the equilibrium is shifted toward cytochrome a as the pH is raised. The rate and equilibrium changes can be described as involving acid-base groups with pKa values of approximately 7.7 and 8.7, respectively. The kinetic results can be simulated on the basis of a model in which one acid-base group interacts with cytochrome a3, so that its pKa drops on oxidation of this center. The group is in proton equilibrium with the solvent via a proton pathway, suggested to be a proton channel. The rate of a shift in the redox equilibrium between the two cytochromes reaches a high limit at low pH, where the channel is saturated with protons.(ABSTRACT TRUNCATED AT 250 WORDS)

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