The Role of D1-H332, a Ligand of Mn₄Ca-cluster, in the Water Oxidation Mechanism of the Oxygen-evolving Photosystem II of Thermosynechococcus elongatus

Abstract

The active site for water oxidation in Photosystem II goes through five oxidation states (S₀-S₄). It consists of a Mn₄Ca-cluster bound to 7 amino acids of the D1 and CP43 polypeptides. To study the role of one of these ligands, the D1-H332Q, we analysed purified PSII from Thermosynechococcus elongatus in which this amino acid residue was substituted by either Gln or Ser. Oxygen-evolution in both mutants under continuous light was 70-80% of that of WT*. However, the S₃ to S₀ transition seemed unaffected. The S₂Q_A^- charge recombination was a multiphasic process. The faster phases were accelerated in both mutants. The S₃Q_B^- charge recombination of H332Q, experimented by thermoluminescence, occurred at a temperature 6⁰C lower than that of the WT*. These results suggest that D1-His332 is involved in some of the S₂ and S₃ thermodynamic properties but not in the S₃Tyr_Z^. to S₀ transition.