Investigating the mechanism of electron transfer to the binuclear center in Cu-heme oxidases

Novel experimental evidence is presented further supporting the hypothesis that, starting with resting oxidized cytochrome c oxidase, the internal electron transfer to the oxygen binding site is kinetically controlled. The reduction of the enzyme was followed spectroscopically and in the presence of NO or CO, used as trapping ligands for reduced cytochrome a3; ruthenium hexamine was used as a spectroscopically silent electron donor. Consistent with the high combination rate constant for reduced cytochrome a3, NO proved to be a very efficient trapping ligand, while CO did not. The results are discussed in view of two alternative (thermodynamic and kinetic) hypotheses of control of electron transfer to the binuclear (cyt.a3-Cu(B)) center. Fulfilling the prediction of the kinetic control hypothesis: i) the reduction of cytochrome a3 and ligation are synchronous and proceed at the intrinsic rate of cytochrome a3 reduction, ii) the measured rate of formation of the nitrosyl derivative is independent of the concentration of both the reductant and NO.