PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Dual action of chloride ion on hydroxylamine inhibition of Photosystem II: Activation and attenuation. Thomas Kuntzleman^*,1, Charles Yocum^{1, 2}, ¹ Department of Chemistry, Ann Arbor, MI, USA² Department of Molecular, Cellular and Developmental Biology, Ann Arbor, MI, USA

ABSTRACT- Oxygen evolution by PSII is inhibited by ammonia binding to two sites, only one of which is sensitive to the presence of chloride ion; it has been shown by ESEEM that ammonia binds directly to the PSII manganese cluster at the chloride insensitive site. Larger amines (methylamine, amino-2-ethyl propanediol), however, inhibit primarily at the chloride site. It has been suggested that ammonia is a water analogue when it binds at the chloride insensitive site. Nitrogen evolution from hydroxylamine treated chloroplasts upon illumination is consistent with the proposal that it is also a water substrate analogue. However, because methylated hydroxylamines inhibit PSII only at the chloride site, it has also been proposed that none of the hydroxylamines should be considered to be substrate analogues. This proposal does not account for the discovery that the reaction between hydroxylamine and PSII is not slowed by chloride. A more extensive investigation between hydroxylamine and PSII shows that chloride and the ions that can replace it to activate water oxidation will also act as activators of the hydroxylamine reaction with PSII. Fluoride ion, which inhibits water oxidation, was observed to slightly attenuate hydroxylamine inhibition. Kinetic analyses of data from these reactions suggest that hydroxylamine can react with PSII at two sites. At one site, chloride activates the reaction between hydroxylamine and PSII manganese. At the second site, chloride ion interferes with the reaction, apparently by blocking access of hydroxylamine to that site, as in the case with larger amine ligands and reductants. These results can be rationalized in terms of a model in which chloride prevents hydroxylamines from binding to non-substrate, redox-active sites on the PSII Mn cluster. Reactivity of hydroxylamine at the substrate sites is regulated by chloride and surrogate ions, perhaps in a manner similar to that by which these anions affect the ability of PSII to oxidize water.

KEY WORDS: hydroxylamine, substrate, photosystem II, chloride