Protein matrix and dielectric effect in cytochrome c.
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, Delta E(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 if U(exp) = Delta G(0)redox. However, some consistent result are observed if U(exp) = Delta H(0)redox, with a measured epsilon(Delta Delta)(H)(redox) = 19 +/- 6. The variability 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/N52I Y67F/N52V, where most of the hydrogen bond network in the heme crevice is eliminated, Delta S(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.
Blouin
C
C
Department of Biochemistry and Molecular Biology, Dalhousie University, Nova Scotia, Canada B3H 4H7. cblouin@is2.dal.ca
Wallace
C J
CJ
D019943:Amino Acid Substitution; D002942:Circular Dichroism; D003196:Computer Graphics; D003574:Cytochrome c Group / Q000737:chemistry* / Q000378:metabolism; D019277:Entropy; D007501:Iron; D007700:Kinetics; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D017354:Point Mutation; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D055672:Static Electricity; D013816:Thermodynamics
D003574:Cytochrome c Group; D011994:Recombinant Proteins; D007501:Iron