Electron Transfer Reactions between Cytochrome ƒ and Plastocyanin from Brassica komatsuna

Abstract

The protein-protein electron transfer reactions between cytochrome ƒ and plastocyanin, both purified from Brassica komatsuna (Brassica rapa L. var. perviridis Bailey), have been studied as a function of pH, ionic strength, and temperature. The second-order rate constant for the oxidation of ferrocytochrome ƒ by plastocyanin was found to be k=4.5×10⁷ M^-1•s^-1 at pH 7.0, μ 0.2M, and 20°C, with activation parameters ΔH_??_=8.4 kcal/mol and ΔS_??_=4.9 cal/mol•deg. Respective rate constant and activation parameters obtained for the reduction of ferricytochrome ƒ by plastocyanin were k=1.9×10⁷M^-1•s^-1, ΔH_??_=8.6 kcal/mol, and ΔS_??_=3.9 cal/mol•deg. The high rate constants for these reactions are attributable not to a low activation enthalpy but to a positive activation entropy term. The rate constants both for the oxidation and the reduction of cytochrome f by plastocyanin drastically decreased with increasing ionic strength, indicating the importance of electrostatic interactions. Divalent cations are more effective than monovalent cations in reducing the rates of these reactions. The rate constants for the oxidation of cytochrome ƒ by plastocyanin are constant between pH 6.0 and 9.0 but decrease markedly above pH 9.0 and below pH 6.0. In the case of the reduction of cytochrome ƒ by plastocyanin, an optimum pH around 7.0 was obtained and a biphasic feature was observed at alkaline pH. The results are discussed in relation to photosynthetic electron transport systems.