Dependence of the rates of dissolution of the Fe4S4 clusters of Chromatium vinosum high-potential iron protein and ferredoxin on cluster oxidation state

摘要

The influence of oxidation state on the pH dependence of the dissolution of the Fe4S4 clusters of Chromatium vinosum ferredoxin and high-potential iron protein (HIPIP) has been studied. The first-order rate constants (kobs) for dissolution of both the Fe4S4(S-Cys)4^2- and Fe4S4(S-Cys)4^3- clusters of the ferredoxin follow the same overall kinetic equation but with differing specific rate and equilibrium constants. The dependence of rate and equilibrium constants upon oxidation state may be rationalized on the basis of the accompanying change in electrostatic affinity of a cluster toward H⁺ and HO^-. A more drastic change in the pH dependence of the kinetics of dissolution of the Fe4S4 cluster of the HIPIP accompanies its change in oxidation state. Whereas the values of kobs for dissolution of HIPIP containing the Fe4S4(S-Cys)4^2- cluster are strictly second order to [H⁺] and [HO^-], the pH dependence for dissolution of the HIPIP Fe4S₄(S-Cys)₄^1- cluster indicates a first-order dependence upon [H⁺], a second-order dependence upon [HO^-], and a spontaneous or water rate. These reactivity differences may be related to changes in cluster charge density. Mechanisms of dissolution involve preequilibrium protonation at acidic pH and preequilibrium ligand exchange at basic pH.