Proton Translocation via Tautomerization of Asn298 During the S2–S3 State Transition in the Oxygen-Evolving Complex of Photosystem II

In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or Y Z ) mediates electron transfer between the Mn 4 CaO 5 cluster of the oxygen-evolving complex and the charge-separation site of photosystem II (PSII), driving the cluster through progressively higher oxidation states S i ( i = 0-4). In contrast to lower S-states (S 0 , S 1 ), in higher S-states (S 2 , S 3 ) of the Mn 4 CaO 5 cluster, Y Z cannot be oxidized at cryogenic temperatures due to the accumulation of positive charge in the S 1 → S 2 transition. However, oxidation of Y Z by illumination of S 2 at 77-190 K followed by rapid freezing and charge recombination between Y Z • and the plastoquinone radical Q A •- allows trapping of an S 2 variant, the so-called S 2 trapped state (S 2 t ), that is capable of forming Y Z • at cryogenic temperature. To identify the differences between the S 2 and S 2 t states, we used the S 2 t Y Z • intermediate as a probe for the S 2 t state and followed the S 2 t Y Z • /Q A •- recombination kinetics at 10 K using time-resolved electron paramagnetic resonance spectroscopy in H 2 O and D 2 O. The results show that while S 2 t Y Z • /Q A •- recombination can be described as pure electron transfer occurring in the Marcus inverted region, the S 2 t → S 2 reversion depends on proton rearrangement and exhibits a strong kinetic isotope effect. This suggests that Y Z oxidation in the S 2 t state is facilitated by favorable proton redistribution in the vicinity of Y Z , most likely within the hydrogen-bonded Y Z -His190-Asn298 triad. Computational models show that tautomerization of Asn298 to its imidic acid form enables proton translocation to an adjacent asparagine-rich cavity of water molecules that functions as a proton reservoir and can further participate in proton egress to the lumen.