The study of 1-electron equivalent oxidation-reduction reactions by fast pulse generation of reagents. Cytochrome c/ferri-ferrocyanide system

Abstract

The method of pulse radiolysis was used to generate reagents in situ in times (500 ns to 1.5 mus) short compared with the rates of the observed biochemical processes. This "instant" mixing technique is compared with rapid stopped flow measurements (limited in rates and concentrations) and T-jump measurements (limited to relaxation in the neighborhood of equilibrium) for the ferro-ferricytochrome c (C(II)-C(III))/ferro-ferricyanide (FCN(II)-FCN(III)) system. The reagents generated in situ were C(II) or FCN(III). Kinetically indistinguishable binding sites exist on C(II) and C(III) for hexacyanide anions. Reductive electron transfer to the protein proceeds within the FCN(II)-C(III) complex, with a rate of 400 s-1. The binding of FCN(II) on C(II) slows down the oxidation of C(II) by FCN(III). The sites of interaction on C(II) or C(III) with FCN(III) show effective charges of approximately +2. The association constant per binding site derived from the kinetics of electron transfer is greater than or equal to 10(4) M-1 for FCN(II)-C(II) and less than or equal to 10(4) M-1 for FCN(III)-C(III). Specific clusters of amino acids in the model of cytochrome C are suggested as binding sites. The oxidation-reduction reactions of FCN appear to involve electron equivalent transfer to and from such somewhat remote binding sites on the protein. Anions such as phosphate or sulphate also bind to these, less strongly than hexacyanides. In the presence of perchlorate the kinetics show the resolution of the pK=9.3 of C(III) into two parts: (a) optical changes at 695 nm due to ligand interchange on the heme-iron, unaffected by perchlorate and (b), a kinetic change leading to biphasic oxidation of C(II), with pK=7.4. This is attributed to the effect of perchlorate on water structure in the close environment of the binding sites. The high rate of oxidation of relaxed C(II) by FCN(III), (2 X 10(8) M-1 S-1 at mu=0) is not in agreement with an outer sphere Marcus mechanism. Nonrelaxed C(II) having a structure closer to C(III) transfers electron to FCN(III) even faster (k=3 X 10(9) M-1 S-1 at mu=0).