Probing the role of Valine 185 of the D1 protein in the Photosystem II oxygen evolution.

In Photosystem II (PSII), the MnCaO-cluster of the active site advances through five sequential oxidation states (S to S) before water is oxidized and O is generated. The V185 of the D1 protein has been shown to be an important amino acid in PSII function (Dilbeck et al. Biochemistry 52 (2013) 6824-6833). Here, we have studied its role by making a V185T site-directed mutant in the thermophilic cyanobacterium Thermosynechococcus elongatus. The properties of the V185T-PSII have been compared to those of the WT*3-PSII by using EPR spectroscopy, polarography, thermoluminescence and time-resolved UV-visible absorption spectroscopy. It is shown that the V185 and the chloride binding site very likely interact via the H-bond network linking Tyr and the halide. The V185 contributes to the stabilization of S into the low spin (LS), S = 1/2, configuration. Indeed, in the V185T mutant a high proportion of S exhibits a high spin (HS), S = 5/2, configuration. By using bromocresol purple as a dye, a proton release was detected in the STyr → STyr transition in the V185T mutant in contrast to the WT*3-PSII in which there is no proton release in this transition. Instead, in WT*3-PSII, a proton release kinetically much faster than the STyr → STyr transition was observed and we propose that it occurs in the STyr → STyr intermediate step before the STyr → STyr transition occurs. The dramatic slowdown of the STyr → STyr transition in the V185T mutant does not originate from a structural modification of the MnCaO cluster since the spin S = 3 S EPR signal is not modified in the mutant. More probably, it is indicative of the strong implication of V185 in the tuning of an efficient relaxation processes of the H-bond network and/or of the protein.