The oxidation-reduction kinetics of the reaction of cytochrome c1 with non-physiological redox agents.

The kinetics of the oxidation-reduction reactions of cytochrome c1 with ascorbate, ferricyanide, triphenanthrolinecobalt(III) and N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) have been examined using the stopped-flow technique. The reduction of ferricytochrome c1 by ascorbic acid is investigated as a function of pH. It is shown that at neutral and alkaline pH the reduction of the protein is mainly performed by the doubly deprotonated form of ascorbate. From the ionic-strength-dependence studies of the reactions of cytochrome c1 with ascorbate, ferricyanide and triphenanthrolinecobalt(III), it is demonstrated that the reactions rate is governed by electrostatic interactions. The second-order rate constants for the reaction of cytochrome c1 with ascorbate, ferricyanide, TMPD and triphenanthrolinecobalt(III) are 1.4 . 10(4), 3.2 . 10(3), 3.8 . 10(4) and 1.3 . 10(8) M-1 . s-1 (pH 7.9, I = 0, 10 degrees C), respectively. Application of the Debye-Hückel theory to the data of the ionic-strength-dependence studies of these redox reactions of cytochrome c1 yielded for ferrocytochrome c1 and ferricytochrome c1 a net charge of --5 and --4, respectively. The latter value is close to that of --3 for the oxidized enzyme, calculated from the amino acid sequence of the protein. This implies that not a local charge on the surface of the protein, but the overall net charge of cytochrome c1 governs the reaction rate with small redox molecules.