Alterations in carboxylate ligation at the active site of photosystem II

Photosystem II (PSII) is the photosynthetic enzyme catalyzing the oxidation of water and reduction of plastoquinone (Q). This reaction occurs at a cat alytic site containing four manganese atoms and cycling among five oxidatio n states, the S-n states, where n refers to the number of oxidizing equival ents stored. Biochemical and spectroscopic techniques have been used previo usly to conclude that aspartate 170 in the D1 subunit influences the struct ure and function of the PSII active site (Boerner, R. J., Nguyen, A. P., Ba rry, B. A., and Debus, R, J. (1992) Biochemistry 31, 6660-6672). Substituti on of glutamate for aspartate 170 resulted in an assembled manganese cluste r, which was capable of enzymatic turnover, but at lower steady-state oxyge n evolution rates. Here, we obtained the difference (light-minus-dark) Four ier transform IR spectrum associated with the S(2)Q(-)-minus-S(1)Q transiti on by illumination of oxygen-evolving wild-type and DE170D1 PSII preparatio ns at 200 K. These spectra are known to be dominated by contributions from carboxylic acid and carboxylate residues that are close to or ligating the manganese cluster. Substitution of glutamate for aspartate 170 results in a lterations in the S(2)Q(-)-minus-S(1)Q spectrum; the alterations are consis tent with a change in carboxylate coordination to manganese or calcium. In particular, the spectra are consistent with a shift from bridging/bidentate carboxylates in wild-type PSII to unidentate carboxylate ligation in DE170 D1 PSII.