pH Optimum of the Photosystem II H2O Oxidation Reaction: Effects of PsbO, the Manganese-Stabilizing Protein, Cl- Retention, and Deprotonation of a Component Required for O2 Evolution Activity

Hydroxide ion inhibits Photosystem II (PSII) activity by extracting Cl- from its binding site in the O-2-evolving complex (OEC) under continuous illumination [Critchley, C., et al. (1982) Biochim. Biophys. Acta 682, 436]. The experiments reported here examine whether two subunits of PsbO, the manganese-stabilizing protein, bound to eukaryotic PSII play a role in protecting the OEC against OH- inhibition. The data show that the PSII binding properties of PsbO affect the pH optimum for O-2 evolution activity as well as the Cl- affinity of the OEC that decreases with an increasing pH. These results suggest that PsbO functions as a barrier against inhibition of the OEC by OH-. Through facilitation of efficient retention of Cl- in PSII [Popelkova, H., et al. (2008) Biochemistry 47, 12593], PsbO influences the ability of Cl- to resist OH--induced release from its site in the OEC. Preventing inhibition by OH- allows for normal (short) lifetimes of the S-2 and S-3 states in darkness [Roose, J. L., et al. (2011) Biochemistry 50, 5988] and for maximal steady-state activity by PSII. The data presented here indicate that activation of H2O oxidation occurs with a pK(a) of similar to 6.5, which could be a function of deprotonation of one or more amino acid residues that reside near the OEC active site on the D1 and CP43 intrinsic subunits of the PSII reaction center.