Electron transfer after flash photolysis of mixed-valence carboxycytochrome c oxidase

Abstract

The light-induced difference spectra of the fully reduced ( a 3+ a 2+ 3-CO) complex and the mixed-valence carboxycytochrome c oxidase ( a 3+ a 2+ 3-CO) during steady-state illumination and after flash photolysis showed marked differences. The differences appear to be due to electron transfer between the redox centres in the enzyme. The product of the absorbance coefficient and the quantum yield was found to be equal in both enzyme species, both when determined from the rates of photolysis and from the values of the dissociation constants of the cytochrome a 2+ 3-CO complex. This would confirm that the spectral properties of cytochrome a 3 are not affected by the redox state of cytochrome a and Cu A. When the absorbance changes after photolysis of cytochrome a 2+ 3-CO with a laser flash were followed on a time scale from 1 μs to 1 s in the fully reduced carboxycytochrome c oxidase, only the CO recombination reaction was observed. However, in the mixed-valence enzyme an additional fast absorbance change ( k = 7·10 3s −1) was detected. The kinetic difference spectrum of this fast change showed a peak at 415 nm and a trough at 445 nm, corresponding to oxidation of cytochrome a 3. Concomitantly, a decrease of the 830 nm band was observed due to reduction of Cu A. This demonstrates that in the partially reduced enzyme a pathway is present between Cu A and the cytochrome a 3-Cu B pair, via which electrons are transferred rapidly.