The D1-V185N mutation alters substrate water exchange by stabilizing alternative structures of the Mn4Ca-cluster in photosystem II

In photosynthesis, the oxygen-evolving complex (OEC) of pigment-protein photosystem II (PSII) orchestrates oxidation water. Introduction V185N mutation into D1 protein was previously reported to drastically slow O2-release and strongly perturb water network surrounding Mn4Ca cluster. Employing time-resolved membrane inlet mass spectrometry, we measured here H218O/H216O-exchange kinetics fast (Wf) (Ws) exchanging substrate waters bound in S1, S2 S3 states cluster PSII core complexes isolated from wild type D1-V185N strains Synechocystis sp. PCC 6803. We found that rate exchange for Ws increased S1 states, while both Wf rates were decreased state. Additionally, used EPR spectroscopy characterize its interaction with redox active D1-Tyr161 (YZ). state, observed a greatly diminished multiline signal V185N-PSII could be recovered by addition ammonia. The split state not affected, absent mutant. These findings are rationalized proposal N185 residue stabilizes binding an additional water-derived ligand at Mn1 site via hydrogen bonding. Implications sites discussed.