Long-Range Electron Transfer in Ruthenium-Modified Cytochromes c. σ-Tunneling Pathways through Aromatic Residues

Abstract

Studies of intramolecular electron transfer (ET) in chemically-modified metalloproteins have suggested that the long-range donor-acceptor coupling is determined by the structure of the bridging polypeptide medium. One issue that remains largely unclear is whether aromatic side chains in the intervening medium enhance these electronic couplings to a significant extent To address this issue, we have prepared site-directed mutants of Saccharomyces cerevisiae iso-1-cytchrome c, where in each case a surface histidine for ruthenium labeling is introduced at either position 58 or 66. Based on the crystal structure of the protein, a tryptophan (at position 59) or a tyrosine (67) is found along the ET path between the Ru-modified histidine and the heme. In order to probe the role of the bridging Tyr67 in the His66 variant, this internal residue has been replaced with a phenylalanine.