CARBOXYMETHYLATION OF THE METHIONINE RESIDUES OF CYTOCHROMEc

Abstract

Treatment of cytochrome c with iodoacetate at pH = 3.0 leads to loss of activity in the succinate oxidase system, alteration of spectra, and loss of the specific hemochrome properties characteristic of the native protein. The change in properties is paralleled by the incorporation of carboxymethyl groups (up to a maximum of two) and the destruction of methionine. The chymotryptic peptide maps also suggest that the loci of modification are the methionine residues at positions 65 and 80. Both residues are equally amenable to attack at about the same rate as free methionine, suggesting that they are located on the surface of the molecule. Certain features of the kinetics of carboxymethylation suggest the possible existence of a monocarboxymethyl derivative with unmodified properties, but modification of both methionines clearly leads to marked loss of function.