In biological water oxidation, a redox-active tyrosine residue (D1-Tyr161 or Y-Z) mediates electron transfer between the Mn4CaO5 cluster of the 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 Mn4CaO5 cluster, Yz 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 Yz and the plastoquinone radical Q(A)(center dot-) allows trapping of an S-2 variant, the so-called S-2(trapped) state (S-2(t)), that is capable of forming Y-z(center dot) at cryogenic temperature. To identify the differences between the S-2 and S-2(t) states, we used the (S2Yz center dot)-Y-t intermediate as a probe for the S-2(t) state and followed the (S2Yz center dot)-Y-t/Q(A)(center dot-) recombination kinetics at 10 K using time-resolved electron paramagnetic resonance spectroscopy in H2O and D2O. The results show that while (S2Yz center dot)-Y-t/Q(A)(center dot-) 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 521 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.