Maskiewicz R, Bruice T C
Proc Natl Acad Sci U S A. 1977 Dec;74(12):5231-4. doi: 10.1073/pnas.74.12.5231.
The influence of oxidation state on the pH dependence of the dissolution of the Fe(4)S(4) clusters of Chromatium vinosum ferredoxin and high-potential iron protein (HIPIP) has been studied. The first-order rate constants (k(obs)) for dissolution of both the Fe(4)S(4)(S-Cys)(4) (2-) and Fe(4)S(4)(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 Fe(4)S(4) cluster of the HIPIP accompanies its change in oxidation state. Whereas the values of k(obs) for dissolution of HIPIP containing the Fe(4)S(4)(S-Cys)(4) (2-) cluster are strictly second order to [H(+)] and [HO(-)], the pH dependence for dissolution of the HIPIP Fe(4)S(4)(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.
研究了氧化态对嗜酒色杆菌铁氧化还原蛋白和高电位铁蛋白(HIPIP)中Fe(4)S(4)簇溶解的pH依赖性的影响。铁氧化还原蛋白的Fe(4)S(4)(S-Cys)(4) (2-)和Fe(4)S(4)(S-Cys)(4) (3-)簇溶解的一级速率常数(k(obs))遵循相同的总体动力学方程,但具有不同的比速率和平衡常数。速率和平衡常数对氧化态的依赖性可以基于簇对H(+)和HO(-)静电亲和力的伴随变化来解释。HIPIP的Fe(4)S(4)簇溶解动力学的pH依赖性随其氧化态的变化而发生更剧烈的变化。含有Fe(4)S(4)(S-Cys)(4) (2-)簇的HIPIP溶解的k(obs)值对[H(+)]和[HO(-)]严格为二级,而HIPIP Fe(4)S(4)(S-Cys)(4) (1-)簇溶解的pH依赖性表明对[H(+)]为一级依赖性,对[HO(-)]为二级依赖性,以及自发或水的速率。这些反应性差异可能与簇电荷密度的变化有关。溶解机制包括在酸性pH下的预平衡质子化和在碱性pH下的预平衡配体交换。