Contributions to cytochromecinner- And outer-sphere reorganization energy

Cytochromescare small water-soluble proteins that catalyze electron transfer in metabolism and energy conversion processes.Hydrogenobacter thermophiluscytochromec₅₅₂presents a curious case in displaying fluxionality of its heme axial methionine ligand; this behavior is altered by single point mutation of the Q64 residue to N64 or V64, which fixes the ligand in a single configuration. The reorganization energy (λ) of these cytochromec₅₅₂variants is experimentally determined using a combination of rotating disc electrochemistry, chronoamperometry and cyclic voltammetry. The differences between theλdetermined from these complementary techniques helps to deconvolute the contribution of the active site and its immediate environment to the overallλ(λ_Total). The experimentally determinedλvalues in conjunction with DFT calculations indicate that the differences inλamong the protein variants are mainly due to the differences in contributions from the protein environment and not just inner-sphereλ. DFT calculations indicate that the position of residue 64, responsible for the orientation of the axial methionine, determines the geometric relaxation of the redox active molecular orbital (RAMO). The orientation of the RAMO with respect to the heme is key to determining electron transfer coupling (H_AB) which results in higher ET rates in the wild-type protein relative to the Q64V mutant despite a 150 mV higherλ_Totalin the former.