D1-Asn-298 in photosystem II is involved in a hydrogen-bond network near the redox-active tyrosine Y for proton exit during water oxidation.

In photosynthetic water oxidation, two water molecules are converted into one oxygen molecule and four protons at the MnCaO cluster in photosystem II (PSII) via the S-state cycle. Efficient proton exit from the catalytic site to the lumen is essential for this process. However, the exit pathways of individual protons through the PSII proteins remain to be identified. In this study, we examined the involvement of a hydrogen-bond network near the redox-active tyrosine Y in proton transfer during the S-state cycle. We focused on spectroscopic analyses of a site-directed variant of D1-Asn-298, a residue involved in a hydrogen-bond network near Y We found that the D1-N298A mutant of sp. PCC 6803 exhibits an O evolution activity of ∼10% of the wild-type. D1-N298A and the wild-type D1 had very similar features of thermoluminescence glow curves and of an FTIR difference spectrum upon Y oxidation, suggesting that the hydrogen-bonded structure of Y and electron transfer from the MnCaO cluster to Y were little affected by substitution. In the D1-N298A mutant, however, the flash-number dependence of delayed luminescence showed a monotonic increase without oscillation, and FTIR difference spectra of the S-state cycle indicated partial and significant inhibition of the S → S and S → S transitions, respectively. These results suggest that the D1-N298A substitution inhibits the proton transfer processes in the S → S and S → S transitions. This in turn indicates that the hydrogen-bond network near Y can be functional as a proton transfer pathway during photosynthetic water oxidation.