title: 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.
abstract: 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 H(2)O oxidation occurs with a pK(a) of ∼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.
authors.affiliation: Department of Molecular, Cellular and Developmental Biology, The University of Michigan, Ann Arbor, Michigan 48109, USA.
authors.forename: Alan
authors.initials: A
authors.lastname: Commet
authors.forename: Nicholas
authors.initials: N
authors.lastname: Boswell
authors.forename: Charles F
authors.initials: CF
authors.lastname: Yocum
authors.forename: Hana
authors.initials: H
authors.lastname: Popelka
mesh_terms: D002712:Chlorides / Q000378:metabolism*; D006878:Hydroxides / Q000494:pharmacology; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D018724:Spinacia oleracea
chemical_list: D002712:Chlorides; D006878:Hydroxides; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; C052389:oxygen-evolving enhancer protein 1, plant; C031356:hydroxide ion; D010100:Oxygen