A Comparative Kinetic Analysis of the Flavin-Photosensitized Oxidation and Reduction of Plastocyanin and Cytochrome c6 from Different Organisms

Abstract— Laser flash photolysis spectroscopy has been used to investigate the kinetics of oxidation and reduction of pho-tosynthetic plastocyanin (Pc) and cytochrome c₆ (Cyt) from the cyanobacteria Anabaena PCC 7119 and Syne-chocystis PCC 6803 by lumiflavin, riboflavin and flavin mononucleotide (FMN). For both redox reactions similar steric and electrostatic effects were observed with Syne-chocystis proteins and Anabaena Cyt, thus suggesting that the same (or closely adjacent) sites are being used for electron entry and removal. In the case of Anabaena Pc, however, both the steric and electrostatic effects suggest that FMN-dependent protein oxidation and reduction may occur at different sites, that is oxidation at the hydrophobic patch and reduction at the hydrophilic one. Kinetic pK_a values of 5.8 and 6.2 have been determined for the lumiflavin-dependent reduction of Anabaena and Monoraphidium Pc, respectively, whereas the oxidation reactions appear to be pH independent. Not only the reduction kinetics but also protein tyrosine fluorescence quenching by iodide ions suggest the occurrence of pH-induced conformational changes in Pc. In conclusion, kinetic and fluorescent studies indicate that there are considerable quantitative similarities between Cyt and Pc when isolated from the same organism, consistent with the identical physiological role that these two proteins play inside the cells.