Contribution of backbone dynamics to entropy changes occurring on oxidation of cytochrome b5.: Can redox linked changes in hydrogen bond networks modulate reduction potentials?

Changes in backbone dynamics occurring upon oxidation of rat cytochrome bs have been examined through model free analyses of N-15-relaxation rates of both oxidation states of the protein. Based on the observed changes, an upper bound for the contribution of backbone dynamics to the entropy change associated with oxidation has been calculated. The magnitude of this backbone contribution, 70 +/- 7 J/K.mol, is strikingly similar to the total entropy change associated with oxidation of the protein determined through an analysis of the temperature dependence of the reduction potential. Origins of the differences in dynamic behavior of the oxidized and reduced proteins can be attributed to redox linked changes in hydrogen bond strengths based on large-scale differences in amide proton exchange rates observed between the oxidation states. Based on these observations the magnitude and possible significance of entropic contributions to the electromotive force are discussed. Analysis of the N-15-relaxation rates included modeling of anisotropic diffusional behavior which was expected based on the distinct physical asymmetry of the protein. An axially symmetric diffusion tensor model was found to fit the rotational reorientational properties of the protein in both oxidation states. The contribution of paramagnetic relaxation to the N-15-relaxation rates of the oxidized protein was calculated based on a set of modified Solomon-Bloembergen equations. The determination of the electronic correlation time of the paramagnetic center was based on fits to the proton relaxation rate enhancements of protons in close proximity to the paramagnetic center. Analyses of the dynamic properties of the oxidized cytochrome bs were based on multiple field (i.e., 500 and 750 MHz) NMR measurements of N-15 T-1 and T-2 relaxation times.