The protonation state around Tyr_D/Tyr_D^• in photosystem II is reflected in its biphasic oxidation kinetics

The tyrosine residue D2-Tyr160 (Tyr_D) in photosystem II (PSII) can be oxidized through charge equilibrium with the oxygen evolving complex in PSII. The kinetics of the electron transfer from Tyr_D has been followed using time-resolved EPR spectroscopy after triggering the oxidation of pre-reduced Tyr_D by a short laser flash. After its oxidation Tyr_D is observed as a neutral radical (Tyr_D^•) indicating that the oxidation is coupled to a deprotonation event. The redox state of Tyr_D was reported to be determined by the two water positions identified in the crystal structure of PSII [Saito et al. (2013) Proc. Natl. Acad. Sci. USA 110, 7690]. To assess the mechanism of the proton coupled electron transfer of Tyr_D the oxidation kinetics has been followed in the presence of deuterated buffers, thereby resolving the kinetic isotope effect (KIE) of Tyr_D oxidation at different H/D concentrations. Two kinetic phases of Tyr_D oxidation – the fast phase (msec-sec time range) and the slow phase (tens of seconds time range) were resolved as was previously reported [Vass and Styring (1991) Biochemistry 30, 830]. In the presence of deuterated buffers the kinetics was significantly slower compared to normal buffers. Furthermore, although the kinetics were faster at both high pH and pD values the observed KIE was found to be similar (~ 2.4) over the whole pL range investigated. We assign the fast and slow oxidation phases to two populations of PSII centers with different water positions, proximal and distal respectively, and discuss possible deprotonation events in the vicinity of Tyr_D.