LASER FLASH KINETIC-ANALYSIS OF SYNECHOCYSTIS PCC-6803 CYTOCHROME C(6) AND PLASTOCYANIN OXIDATION BY PHOTOSYSTEM-I

Laser flash absorption spectroscopy has been used to investigate the kinetics of electron transfer from reduced cytochrome c(6) and plastocyanin to photooxidized P700 in Photosystem I(PS I) particles from the cyanobacterium Synechocystis PCC 6803. Data analysis yields second-order rate constants of 1.3.10(7) M(-1) s(-1) and 1.0.10(7) M(-1) s(-1) for the heme- and copper-proteins, respectively. With the two donor proteins, the observed rate constants (k(obs)) present a linear protein-concentration dependence, thus suggesting an apparent one-step bimolecular collisional mechanism. At neutral pH, the k(obs) values monotonically increase with increasing NaCl or MgCl2 concentration, which is ascribed to the involvement of repulsive electrostatic interactions between the donor proteins and PS I. The difference in the effective concentration at which MgCl2 has its maximum effect as compared with that of NaCl is attributed to the specific role played by Mg2+ ions, which could act as electrostatic bridges between negatively charged groups. At physiological mild acid pH, cytochrome c, is a more efficient electron donor than plastocyanin. The inversion of the NaCl and MgCl2 effect at pH below 5 - that is, decreasing of k(obs) with increasing ionic strength - is interpreted as arising from the involvement of attractive ionic interactions at pH lower than the isoelectric point of the donor proteins. Some evolutive aspects on the mechanism of electron donation to PS I are discussed.