Contribution of leucine 85 to the structure and function of Saccharomyces cerevisiae iso-1 cytochrome c

Cytochrome c is a small electron-transport protein whose major role is to t ransfer electrons between complex III (cytochrome reductase) and complex IV (cytochrome c oxidase) in the inner mitochondrial membrane of eukaryotes. Cytochrome c is used as a model for the examination of protein folding and structure and for the study of biological electron-transport processes. Amo ngst 96 cytochrome c sequences, residue 85 is generally conserved as either isoleucine or leucine. Spatially, the side chain is associated closely wit h that of the invariant residue Phe82, and this interaction may be importan t for optimal cytochrome c activity. The functional role of residue 85 has been examined using six site-directed mutants of Saccharomyces cerevisiae i so-1 cytochrome c, including, for the first time, kinetic data for electron transfer with the principle physiological partners. Results indicate two l ikely roles for the residue: first, heme crevice resistance to ligand excha nge, sensitive to both the hydrophobicity and volume of the side chain; sec ond, modulation of electron-transport activity through maintenance of the h ydrophobic character of the protein in the vicinity of Phe82 and the expose d heme edge, and possibly of the ability of this region to facilitate redox -linked conformational change.