Electron transport by C-type cytochromes

Abstract

The kinetics of reduction of horse heartcytochrome c have been investigated using the reductants sodium dithionite and potassium ferrocyanide. Sodium dithionite reduction at pH 7.0 yields rate constants of 2.8 X 10(8)M(-1)sec-1 for SO2 AND 6 X 10(5) M-1 sec-1 for S2O4 at infinite dilution. Moreover, the data presented demonstrates the participation of positively charged amino acid side chains at the site of electron transfer. The effect of pH on the reduction of ferricytochrome c requires a minimum of two pK values for description (pK1 = 7.0 +/- 0.4, pK2 = 9.3 +/- 0.3). Based on the pK values determined, one or more lysines and a residues(s) with a low pK are implicated as the positively charged residues participating in electron transfer. From a comparison of the rates of reduction of various denatured forms of cytochrome c we feel that the most viable conclusion is that electron transfer takes place at the exposed heme edge in the vicinity of the amino acid side chains indicated above. Ferrocyanide reduction of ferri-horse heart cytochrome c takes place in a kinetically complex manner. A mechanism is described which includes complexes of ferrocyanide and ferricytochrome c and ferricyanide and ferrocytochrome c. As was found for dithionite reduction a positively charged region of the cytochrome c participates in electron transfer. Combining our results with ferrocyanide and dithionite we conclude that avaible data is compatible with a single mechanism of electron transfer. It is suggested that the kinetic distinction between different reductants lies in the lifetime of the transient complex formed, with the order ferrocyanide greater than S2O4 greater than SO2.