Roles of dipolar effects and local charge in the ionic strength dependence of redox reactions between c-type cytochromes.

Redox reactions between different c-type cytochromes were monitored by stopped-flow spectroscopy. Second-order rate constants were determined at different ionic strengths for the reactions of Paracoccus denitrificans cytochrome c-551i with its physiologic redox partner cytochrome c-550, and with the nonphysiologic partner horse heart cytochrome c. The latter two cytochromes are structurally quite similar and exhibit identical redox potentials but bear net charges of -7 and +7, respectively. Despite these opposite overall charges, the ionic strength dependencies for the reaction of each with the acidic cytochrome c-551i were very similar. The observed decrease in reaction rate with increasing ionic strength that was observed with cytochromes c-550 and c-551i, the latter of which bears a net charge of -20, cannot be explained simply on the basis of monopole-monopole interactions. These data were analyzed by two different methods: one which treats proteins as both monopoles and dipoles and considers the net charge; and another which neglects dipolar effects and considers only the local charge of the reactive site of the protein rather than net charge. The applicability of each method to the analysis of these data and to protein electrostatic interactions in general is discussed.