A spectroelectrochemical study of carboxymethylated cytochrome- c

Abstract

Direct electrochemical reduction of dicarboxymethylated cytochrome c [(Cm)-cyt c] has been carried out to investigate the effect of the displacement of methionine 80 (Met 80) from the sixth coordination position on the redox potential of heme iron. Differential pulse and cyclic voltammetry were employed in the present study. At gold microelectrodes or gold-coated RVC electrodes, the reduction process occurs in the absence of mediators. The rate of heterogeneous charge transfer for (Cm)-cyt c appears to be greater than that operative in native cytochrome c. A value of −0.218 versus s.h.e. at neutral pH was determined for the formal standard potential ( U′ 0,7) from spectroelectrochemical measurements. The large change in U′ 0 of the modified cytochrome c with respect to the native protein (Δ U ⋍0.5 V) is in agreement with rearrangement of the tertiary structure induced by rupture of the Met 80-heme iron bond, which leads to a high degree of heme exposure to the solvent. In addition, the results support the important role of Met 80 in assuring stability, through its bond with the heme iron, to the close crevice structure.