Vibrational resonances and CuB displacement controlled by proton motion in cytochrome c oxidase.

Cytochrome c oxidase (CcO), found in the inner mitochondrial membranes or in many bacteria, catalyzes the four-electron reduction of molecular oxygen to water. Four protons are pumped across the inner mitochondrial membrane through CcO. In this study, quantum mechanics/molecular mechanics and molecular dynamics calculations are used to probe the spectroscopic characteristics of the ferryl intermediates in the aa(3) CcO/O(2) reaction. These highly elaborate calculations, supported by several calculations on smaller model systems, demonstrate the sensitivity of vibrational frequencies on the Coulombic field of heme a(3) and their dependence on the distance of the adjacent Cu(B) to the heme a(3)-Fe atom. This distance seems to be associated with the protonation state of the heme a(3) propionate A, and we propose that it plays a crucial role on the mechanism of action of CcO. In detail, we link proton pumping activity in CcO enzyme (a) to a multiple (1:1:2) resonance among the frequencies of Fe(IV)=O bond stretching, the breathing mode of Histidine 411, and a bending mode of the His411-Fe(IV)=O species (aa(3) from Paracoccus denitrificans numbering) and (b) to Cu(B) displacement by electrostatic interactions toward the heme a(3) iron. We find that the vibrations of the His411-Fe(IV)=O unit become highly coupled depending on the protonation state of the heme a(3) ring A propionate/Asp399 pair, and we propose a mechanism for the resonance Raman enhancement of the bending mode delta(His411-Fe(IV)=O). Calculations on model systems demonstrate that the position of Cu(B) in relation to heme a(3) iron-oxo plays a crucial role in regulating that resonance. We also discuss the origin of the coupling between bending, delta(His411-Fe(IV)=O) and nu(Fe=O) stretching modes, and the role played by such vibrational coupling interactions or Cu(B) position in controlling functional properties of the enzyme, including electron/proton coupling as well as experimental spectra.