Structural influences on reduction potential, electron transfer and stability in cytochrome c

The work described in this thesis was directed toward understanding the contributions of Phe82, Leu85 and associated residues to the structure and function of cytochrome c. One focus of these studies concerned the roles of these residues at the interactive face formed during electron transfer reactions involving cytochrome c and its redox partners. Phe82 was found to make an important contribution to this process, while the adjacent Leu85 does not have a significant impact. However, both residues indirectly affect complex formation by influencing the placement of the side chain of Arg13. In related studies, Leu85 was found to contribute to the formation of the heme binding pocket as a participant in a cluster of conserved leucine residues. This leucine cluster, as well as an adjacent internal hydrophobic cavity, were shown to provide a measure of conformational flexibility when mutations are made in cytochrome c and may facilitate the structural transition of this protein between oxidation states. It was also determined that replacement of Leu94, which is also a member of this leucine cluster, did not alter the helix-helix interactions of the N- and C-terminal helices of cytochrome c as had been suggested by earlier studies. Also apparent from further studies was the important functional role played by Phe82 in the regulation of heme reduction potential which could be attributed to its contribution to the nonpolar environment of the heme pocket. Related to this, it was found that the effects of polar groups on reduction potential were mitigated when these groups were shielded from the heme macrocycle by intervening heme substituents. An additional factor in the regulation of reduction potential was characterized as part of the study of Phe82 and distal residues involved in interactions with heme propionate A. Here it was shown that the mechanisms by which Arg38 and Asn52 influence reduction potential differ from that of Phe82. Furthermore, for Arg38 and Asn52, these effects overlap and synergistic effects are observed when they are replaced in concert. In contrast, the structural implications of either single or multiple substitutions at Arg38, Asn52 and Phe82 were determined to be independent as was the impact on structural stability as a whole. Overall it is clear from these studies that Phe82, Leu85, and likely many other residues in cytochrome c, have multiple structural and functional roles and that the interplay between the roles of these residues are correspondingly complex and difficult to predict.Medicine, Faculty ofBiochemistry and Molecular Biology, Department ofGraduat