Mn2+ reduces Yz + in manganese-depleted Photosystem II preparations

Manganese in the oxygen-evolving complex is a physiological electron donor to Photosystem II. PS II depleted of manganese may oxidize exogenous reductants including benzidine and Mn2+. Using flash photolysis with electron spin resonance detection, we examined the room-temperature reaction kinetics of these reductants with Yz +, the tyrosine radical formed in PS II membranes under illumination. Kinetics were measured with membranes that did or did not contain the 33 kDa extrinsic polypeptide of PS II, whose presence had no effect on the reaction kinetics with either reductant. The rate of Yz + reduction by benzidine was a linear function of benzidine concentration. The rate of Yz + reduction by Mn2+ at pH 6 increased linearly at low Mn2+ concentrations and reached a maximum at the Mn2+ concentrations equal to several times the reaction center concentration. The rate was inhibited by K+, Ca2+ and Mg2+. These data are described by a model in which negative charge on the membrane causes a local increase in the cation concentration. The rate of Yz + reduction at pH 7.5 was biphasic with a fast 400 μs phase that suggests binding of Mn2+ near Yz + at a site that may be one of the native manganese binding sites.