How the Reorganization Free Energy Affects the Reduction Potential of Structurally Homologous Cytochromes

Differences in the reduction potential E0 among structurally similar metalloproteins cannot always be fully explained on the basis of their 3-D structures. We investigate the molecular determinants to E0 using the mixed quantum mechanics/molecular mechanics approach named perturbed matrix method (PMM); after comparison with experimental values to assess the reliability of our calculations, we investigate the relationship between the change in free energy upon reduction ΔA0 and the reorganization energy. We find that the reduction potential of cytochromes can be regarded as the result of the sum of two terms, one being mostly dependent on the energy fluctuations within a limited range around the mean transition energy and the second being mostly dependent linearly on the difference Δλ = λred – λox of the reorganization free energies for the ox → red (λred) and for the red → ox (λox) relaxations.