Ionic strength and pH dependence of the reaction between plastocyanin and Photosystem 1. Evidence of a rate-limiting conformational change

The electron-transfer reaction between spinach wild-type plastocyanin (Pc(WT)) two site-directed mutants, Pc(Thr79His) and Pc(Lys81His), and spinach Photosystem 1 particles, has been studied as a function of protein concentration, ionic strength and pH by using laser-flash absorption spectroscopy. The kinetic data are interpreted using the simplest possible three-step model, involving a rate-limiting conformational change preceding intracomplex electron transfer. The three proteins show similar concentration, pH and ionic strength dependencies. The effects of ionic strength and pH on the reaction indicate a strong influence of complementary charges on complex formation and stabilization. Studies with apoprotein support the opinion that the hydrophobic patch is critical for an productive interaction with the reaction center of Photosystem 1. Together with earlier site-directed mutagenesis studies, the absence of a detectable Photosystem 1 reaction in the presence of reduced azurin, stellacyanin, cytochrome c and cytochrome c₅₅₁, demonstrates the existence of a high level of specificity in the protein-protein interface in the formation of an efficient electron-transfer complex.