Abstract
Electron transport through biomolecules and in biological transport networks is of great importance to bioenergetics and biocatalysis. More generally, it is of crucial importance to understand how the pathways that connect buried metallocofactors to other cofactors, and to protein surfaces, affect the biological chemistry of metalloproteins. In terms of electron transfer (ET), the strongest coupling pathways usually comprise covalent and hydrogen bonded networks, with a limited number of through-space contacts. Herein, we set out to determine the relative roles of hydrogen bonds involved in ET via an established heme-to-surface tunneling pathway in cytochrome (cyt) c (i.e., heme-W59-D60-E61-N62). A series of cyt c variants were produced where a ruthenium tris(diimine) photooxidant was placed at position 62 via covalent modification of the N62C residue. Surprisingly, variants where the H-bonding residues W59 and D60 were replaced (i.e., W59F and D60A) showed no change in ET rate from the ferrous heme to Ru(III). In contrast, changing the composition of an alternative tunneling pathway (i.e., heme-M64-N63-C62) with the M64L substitution shows a factor of 2 decrease in the rate of heme-to-Ru ET. This pathway involves a through-space tunneling step between the heme and M64 residue, and such steps are usually disfavored. To rationalize why the heme-M64-N63-C62 is preferred, molecular dynamics (MD) simulations and Pathways analysis were employed. These simulations show that the change in heme-Ru ET rates is attributed to different conformations with compressed donor-acceptor distances, by ∼2 Å in pathway distance, in the M64-containing protein as compared to the M64L protein. The change in distance is correlated with changes in the electronic coupling that are in accord with the experimentally observed heme-Ru ET rates. Remarkably, the M64L variation at the core of the protein translates to changes in cofactor dynamics at the protein surface. The surface changes identified by MD simulations include dynamic anion-π and dipole-dipole interactions. These interactions influence the strength of tunneling pathways and ET rates by facilitating decreases in through-space tunneling distances in key coupling pathways.