Glycerol binding at the narrow channel of photosystem II stabilizes the low-spin S state of the oxygen-evolving complex.

The oxygen-evolving complex (OEC) of photosystem II (PSII) cycles through redox intermediate states S (i = 0-4) during the photochemical oxidation of water. The S state involves an equilibrium of two isomers including the low-spin S (LS-S) state with its characteristic electron paramagnetic resonance (EPR) multiline signal centered at g = 2.0, and a high-spin S (HS-S) state with its g = 4.1 EPR signal. The relative intensities of the two EPR signals change under experimental conditions that shift the HS-S/LS-S state equilibrium. Here, we analyze the effect of glycerol on the relative stability of the LS-S and HS-S states when bound at the narrow channel of PSII, as reported in an X-ray crystal structure of cyanobacterial PSII. Our quantum mechanics/molecular mechanics (QM/MM) hybrid models of cyanobacterial PSII show that the glycerol molecule perturbs the hydrogen-bond network in the narrow channel, increasing the pK of D1-Asp61 and stabilizing the LS-S state relative to the HS-S state. The reported results are consistent with the absence of the HS-S state EPR signal in native cyanobacterial PSII EPR spectra and suggest that the narrow water channel hydrogen-bond network regulates the relative stability of OEC catalytic intermediates during water oxidation.