The O2-evolving center of photosystem II (PSII) contains a Mn tetramer complex that serves as the catalyst for the H2O-oxidation reaction. In a ligand-substitution reaction thought to be analogous to the substrate-binding reaction, NH3 coordinates to the Mn complex only after formation of the S2 state, which then exhibits an altered S2-state multiline electron paramagnetic resonance (EPR) signal. Taking into account this result, Brudvig and Crabtree proposed a model for the mechanism of photosynthetic O2 evolution, which suggests that coordination of ligands to the Mn complex is governed by the relative electron deficiency of the Mn complex and the Lewis basicity of the attacking ligand. However, the interaction of hydroxylamine with the O2-evolving center in the S1 state is not consistent with this mechanism. When spinach PSII membranes are incubated in darkness in the presence of hydroxylamine or N-methyl-substituted analogues, the yield of the S2-state multiline EPR signal obtained after illumination at 210 K steadily decreases. The reaction rate is first-order with respect to the hydroxylamine concentration, depends inversely on the Cl- concentration, and increases when the pH is raised. These results indicate that the free-base form of hydroxylamine binds to a Cl--binding site prior to reacting with the Mn complex; subsequently, hydroxylamine probably reduces the Mn complex from the S1 state to the S-1 state through an outer-sphere electron-transfer mechanism. The rate of hydroxylamine's reaction with the Mn complex is reduced about 17-fold with each added N-methyl substituent, showing that the binding site is sterically selective for small ligands. NH3 also binds to the O2-evolving center in the S1 state in a manner inversely dependent on the Cl- concentration, causing the Mn complex to exhibit a g = 4.1 EPR signal in lieu of the multiline EPR signal in the S2 state. The results suggest that primary amines and hydroxylamines bind to the O2-evolving center in the S1 state at the same Cl--binding site. Thus, two ligand-binding sites exist in the O2-evolving center. One site is located on the Mn complex and is restricted to binding NH3 and presumably H2O. This site is assigned to the substrate-binding site. A second site is identified as the Cl--binding site; primary amines and hydroxylamines compete with Cl- for coordination to this site. In view of the finding that hydroxylamines bind to the Cl- binding site, we conclude that hydroxylamines should not be considered as substrate analogues for the H2O-oxidation reaction catalyzed by PSII.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1988|
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