The reduction of carboxymethyl-cytochrome c by chromous ions

The reduction of ferricytochrome c and ferricytochrome c carboxymethylated at the haem-linked methionine (residue 80) by Cr(2+) ions was studied by stopped-flow techniques. At pH6.2 the kinetics of reduction of ferricytochrome c are simple and correspond to a second-order rate constant of 1.21x10(3)m(-1).s(-1). Under identical conditions the kinetics of reduction of the carboxymethyl derivative, carboxymethyl-cytochrome c, are complex; two Cr(2+)-concentration-dependent processes (1.5x10(4)m(-1).s(-1) and 1.3x10(3)m(-1).s(-1)) lead to the formation of an intermediate which decays in monomolecular fashion (0.15s(-1)) to form the normal fully reduced material. The kinetic difference spectrum for the overall process corresponds to that found statically, whereas the kinetic difference spectrum of the intermediate minus the oxidized form resembles that of the low-spin ferrous form of carboxymethyl-cytochrome c minus oxidized carboxymethyl-cytochrome c. A model is proposed in which the reduction of low-spin ferric carboxymethyl-cytochrome c to high-spin ferrous carboxymethyl-cytochrome c involves a low-spin ferrous intermediate. The monomolecular step involving the decay of this low-spin ferrous intermediate is associated with an activation energy of approx. 126kJ.mol(-1) and is thought to involve both a change of spin state and a protein-conformational event. Although carboxymethyl-cytochrome c represents a mixture of species separable on a charge basis, the above observations were independent of which species was chosen for study.