MECHANISM OF MN-II OXIDATION BY THE EXTRINSIC 33 KDA PROTEIN OF PHOTOSYSTEM-II

The isolated extrinsic 33 kDa of PSII catalyses the oxidation of Mn-II in the presence of Cl- and a thiol reagent such as beta-mercaptoethanol. The chloride requirement was specific and the K-m for Cl- was 20 mM. The known inhibitors of O-2 evolution such as NH2OH and NH4+ were strong inhibitors of this oxidase activity. The reduced form of the 33 kDa protein was active as the oxidase. Addition of P-ME abruptly inhibited O, evolution in PSII particles initiating an O-2 uptake, suggesting that the disulfide form of MSP was essential for O-2 evolution activity. While the reduction of the 33 kDa protein by P-ME was shown to be essential for the induction of Mn-II oxidation to Mn-III associated with O-2 uptake, there was 1:2 stoichiometry between O-2 uptake and Mn-III formed and SH disappearance. The geometry of the catalytic sulfhydryl site has been probed by fluorometry using O-phthalaldehyde (OPA). A characteristic 337 nm absorption band observed in OPA-bound protein indicated the formation of thioisoindole derivative at the Mn-II bonding site. The CD spectrum and the fluorescence emission and excitation spectra suggest conformational changes in the oxidized and reduced form of this protein. The results of these studies suggest that tryptophan 241 and two lysine residues of this protein are in close proximity to functional cysteine residues at the active site. In the proposed reaction mechanism for Mn-II oxidase activity, the reduced 33 kDa protein in the presence of Cl- ions catalyses the oxidation of Mn-II to Mn-III associated with oxygen uptake.