Electrostatic effects on the kinetics of oxidation-reduction reactions of c-type cytochromes

The kinetics of the oxidation-reduction reactions between horse heart cytochrome c, Euglena gracilis cytochrome c552, and ions (ascorbate, ferricyanide, and ferrocyanide) was investigated as a function of ionic strength at pH 7, 25 degrees C. The ionic strength was varied between 0.002 and 0.02 M. Data were analyzed according to four different functions of ionic strength. Results showed that the Kirkwood-Tanford smeared charge model holds well for the calculation of the activity coefficients and that the whole charges of these proteins are reflected in the rates of their reactions. Chemical modifications or changes in the pH that altered the charge of the proteins affected the primary salt effects as predicted by the smeared charge model.