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The replacement of Phe82 in yeast iso-1-cytochrome c by a glycine residue
substantially alters both the tertiary structure and electron transfer
properties of this protein. The largest structural change involves a polypeptide
chain refolding of residues 79 through 85. Refolding places glycines 82, 83 and
84 immediately adjacent to the plane of the heme group in a spatial positioning
comparable to that of the phenyl ring of Phe82 in the wild-type protein. Despite
this perturbation in structure, solvent accessibility computations show that
heme solvent exposure has not increased in the Gly82 variant protein. However,
refolding does result in the introduction of a number of polar groups into the
hydrophobic heme pocket. This appears to be responsible for the decreased
reduction potential of the heme in this protein. The present study, along with
that of the Ser82 variant protein (Louie et al., 1988b), clearly establishes the
link between dielectric constant within the heme crevice and reduction
potential. The further anomalously low electron transfer activity of the Gly82
variant protein would appear to arise from two factors. First, the polypeptide
chain medium now adjacent to the heme is unable to facilitate electron transfer
in a manner similar to that of the aromatic side-chain of Phe82. Second,
polypeptide chain refolding significantly alters the surface contour of the
Gly82 protein rendering it less suitable to interact with the corresponding
complementary surfaces of redox partners. Our data support the conclusion that
Phe82 plays a number of roles in the electron transfer process mediated by yeast
iso-1-cytochrome c. These include the maintenance of the heme environment,
provision of an optimal medium along the path of electron transfer and formation
of interactions at the contact interface in complexes with redox partners.
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