Acetate binding at the photosystem II oxygen evolving complex:: An S2-state multiline signal ESEEM study

Previously, using acetate deuterated in the methyl hydrogen positions, we showed that acetate binds in close proximity to the Mn cluster/Y-Z(.) tyrosine dual spin complex in acetate-inhibited photosystem II (PSII) preparations exhibiting the split EPR signal arising from the S-2-Y-Z(.) interaction [Force, D. A.; Randall, D. W.; Britt, R. D. Biochemistry 1997, 36, 12062-12070]. By using paramagnetic NO to quench the paramagnetism of Y-Z(.), we are able to observe the ESEEM spectrum of deuterated acetate interacting with only the Mn cluster. A good fit of the ESEEM data indicates two H-2 dipolar hyperfine couplings of 0.097 MHz and one of 0.190 MHz. Modeling of these dipolar interactions, using our dangler 3 + 1 model for the S-2-state of the Mn cluster, reveals distances consistent with direct ligation of acetate to the Mn cluster. As acetate inhibition is competitive with the essential cofactor Cl-, this suggests that Cl- ligates directly to the Mn cluster. The effect of acetate binding on the structure of the Mn cluster is investigated by comparing the Mn-histidine coupling in NO/acetate-treated PSII and untreated PSII using ESEEM. We find that the addition of acetate and NO does not affect the histidine ligation to the Mn cluster. We also investigate the ability of acetate to access Y-Z(.) in Mn-depleted PSII, a PSII preparation expected to be more solvent accessible than intact PSII. We detect no coupling between Y-Z(.) and acetate. We have previously shown that small alcohols such as methanol can ligate to the Mn cluster with ease, while larger alcohols such as 2-propanol, as well as DMSO, are excluded [Force, D. A.; Randall, D. W.; Lorigan, G. A.; Clemens, K. L; Britt, R. D. J. Am. Chem. Soc. 1998, 120, 13321-13333]. We probe the effect of acetate binding on the ability of methanol and DMSO to bind to the Mn cluster. We find that methanol is able to bind to the Mn cluster in the presence of acetate. We detect no DMSO binding in the presence of acetate. Thus, acetate binding does not increase the affinity or accessibility for DMSO binding at the Mn cluster. We also explore the possibility that the acetate binding site is also a binding site for substrate water. By comparing the ratioed three-pulse ESEEM spectra of a control, untreated PSII sample in 50% D2O to an NO/acetate-treated PSII sample in 50% D2O, we find that the binding of acetate to the oxygen evolving complex of photosystem II displaces deuterons bound very closely to the Mn cluster.