THE RATE OF REDUCTION OF OXIDIZED REDOX-ACTIVE TYROSINE, Z+, BY EXOGENOUS MN2+ IS SLOWED IN A SITE-DIRECTED MUTANT, AT ASPARTATE 170 OF POLYPEPTIDE D1 OF PHOTOSYSTEM-II, INACTIVE FOR PHOTOSYNTHETIC OXYGEN EVOLUTION

Eleven different site-directed mutants at Asp-170 of Photosystem II polypeptide D1 of Synechocystis PCC 6803 show varying degrees of activity for light-driven water oxidation depending on the nature of the substitution (Nixon, P.J. and Diner, B.A. (1992) Biochemistry 31, 942-948). These range from the most active, glutamate, with near wild-type rates of oxygen evolution to the totally inactive serine and alanine. Photosystem II core complexes lacking the tetranuclear Mn cluster responsible for water oxidation and isolated from wild-type and D1-Asp170Ser, were compared with respect to the kinetics of reduction by Mn2+ of the photooxidized redox-active tyrosine, Z+. The dependence of the rate of reduction of Z+ on the Mn2+ concentration shows mixed first and second-order behavior in the case of the wild-type and second-order behavior alone in the case of the mutant. The second-order rate constants for the wild-type and mutant complexes were 3.1.10(7) M-1 s-1 and 5.5.10(5) M-1 s-1, respectively. The ratio of these rate constants is consistent with the K(m) values determined earlier for the blockage of charge recombination by Mn2+. D1-Aspartate 170 is therefore implicated in a facilitated pathway for the oxidation of Mn2+, which most likely includes participation in a high-affinity metal binding site (Mn2+ and/or Mn3+) in the Photosystem II reaction center.