Protein matrix and dielectric effect in cytochrome c

The effect of the protein matrix on the standard potential of a buried redo x center has been investigated by using a selection of mutants and chemical derivatives in Saccharomyces cerevisiae cytochrome c isoform 1. Assuming o nly local structural perturbation and no alteration of the iron-ligation ch emistry, DeltaE(m)(0,) can be regarded as a measure of the difference in po lypeptide solvation of the heme charge, which reflects the dielectric prope rties of the protein. The evaluation of an apparent dielectric constant (U- exp/U-theo) yields variable, and sometimes even negative, values if U-exp = DeltaG(0)redox. However, some consistent result are observed if U-exp = De ltaH(0)redox with a measured epsilon (Delta Delta Hredox) = 19 +/- 6. The v ariability is thus attributed to an entropic factor (epsilon (Delta Delta S redox)) that is investigated using a series of substitutions of Asn(52) and /or Tyr(67). In double mutants Y67F/N521 Y67F/ N52V, where most of the hydr ogen bond network in the heme crevice is eliminated, DeltaS(redox). compare s to the wild type. This indicates that a fully consistent hydrogen bond ne twork has a similar polarizability as an apolar matrix. We therefore argue that the variability in net dielectric susceptibility arises from conformat ional polarizability, a factor that is not a function of atomic properties and coordinates and is therefore hard to predict using conventional physica l relationships.