Kinetic studies on [methionine sulphoxide] cytochrome c

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Abstract
A cytochrome c heme ligand, methionine-80, was photo-oxidized to methionine sulfoxide and the subsequent changes in redox properties and ligand binding were monitored kinetically. Isoelectric focusing of the product showed the presence of a single oxidized species, capable of binding CO when reduced. The binding of CO to the reduced protein was followed in stopped-flow experiments, which revealed the presence of 2 binding processes, at neutral pH, with rate constants of k+1 = 3.4 .times. 103 M-1 .cntdot. S-1 and k+2 = 5.80 .times. 102M-1 .cntdot. S-1. When CO was photolytically dissociated from the reduced protein 2 recombination processes were observed with rates almost identical with those observed in the stopped-flow experiments (k+1 = 3.3 .times. 103M-1 .cntdot. S-1 and k+2 = 6.0 .times. 102M-1 .cntdot. S-1). These findings provide evidence of 2 reduced forms of the protein. The reduction of [methionine sulfoxide]cytochrome c by Cr2+ at neutral pH in stopped-flow experiments showed the presence of a single second-order reduction process (k = 7.2 .times. 103M-1 .cntdot. S-1, activation energy = 44kJ/mol) and one first-order process. This protein was compared with some other chemically modified cytochromes.