Photochemical Oxidation of Substrate Water Analogs and Halides by Photosystem II

Photosystem II (PSII) is a multi-subunit protein-pigment complex with diverse redox-active cofactors, which enabled the biological availability of O-2 on Earth. The substrate specificity and the underlying redox chemistry of the Mn4CaO5 catalytic center are investigated using alternate substrates such as small molecules (ammonia and methanol) and halides (Cl-, Br-, I-) instead of the natural substrate water. Changes in the kinetic profiles of steady-state O-2 evolution and of dichlorophenolindophenol (DCIP) photochemical reduction by PSII as well as the detection of modified sites by proteomic analysis implied the possibility of alternate substrate photooxidation. Of particular interest is the role of two chlorides bound close to the putative water channels in the native system. The mutation of D2-K317 to alanine is believed to impair the binding of a catalytically relevant chloride, eliminating the chloride requirement for water oxidation catalysis. The efficiency of small molecule photooxidation by the OEC is enhanced by the mutated D2-K317A PSII complex without the competition from chloride. These results provide insight into the role of bound chloride in native PSII as a filter for enhancing the selectivity of water oxidation. The design principles for PSII may be extended to new strategies for developing highly selective catalysts.