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Protein Matrix and Dielectric Effect in Cytochromec *

C. Blouin,C. Wallace

Published 2001 in Journal of Biological Chemistry

ABSTRACT

The effect of the protein matrix on the standard potential of a buried redox center has been investigated by using a selection of mutants and chemical derivatives in Saccharomyces cerevisiae cytochrome c isoform 1. Assuming only local structural perturbation and no alteration of the iron-ligation chemistry, ΔE m 0′ can be regarded as a measure of the difference in polypeptide solvation of the heme charge, which reflects the dielectric properties of the protein. The evaluation of an apparent dielectric constant (U exp/U theo) yields variable, and sometimes even negative, values ifU exp = ΔG 0 redox. However, some consistent result are observed if U exp = ΔH 0 redox, with a measured εΔΔ H redox = 19 ± 6. The variability is thus attributed to an entropic factor (εΔΔ S redox) that is investigated using a series of substitutions of Asn52 and/or Tyr67. In double mutants Y67F/N52I Y67F/N52V, where most of the hydrogen bond network in the heme crevice is eliminated, ΔS redox compares to the wild type. This indicates that a fully consistent hydrogen bond network has a similar polarizability as an apolar matrix. We therefore argue that the variability in net dielectric susceptibility arises from conformational polarizability, a factor that is not a function of atomic properties and coordinates and is therefore hard to predict using conventional physical relationships.

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