Experimental Studies of Electron Transfer between Wild Type and Mutagenic Cyanobacterial Cytochrome c₅₅₃ and Photosystem I

A mutant form of the protein complex cytochrome c₅₅₃ (cyt c₅₅₃) has been constructed by site-directed mutagenesis in Thermosynechococcus elongatus (T. elongates) to elucidate the binding and electron transfer properties between cyt c₅₅₃ and photosystem I (PSI). The electron-transfer between wild type T. elongatus cyt c₅₅₃ and a mutant form of cyt c₅₅₃ and T. elongatus and Chlamydomonas reinhardtii (C. reinhardtii) PSI, has been studied as a function of cyt c₅₅₃ concentration, ionic strength, pH and the detergents used to stabilize the protein. The effects of each of these variables were measured by an oxygen uptake assay. The mutated T. elongatus cyt c₅₅₃ shows a higher electron transfer rate to C. reinhardtii PSI indicating that the insertion of acidic residues to the protein has facilitated the electrostatic interactions between cyt c₅₅₃ and PSI. The effects of ionic strength and pH on the reaction indicate a strong influence of complementary charges on complex formation and stabilization.