Function of Redox-Active Tyrosine in Photosystem II

Water oxidation at photosystem II Mn-cluster is mediated by the redox-active tyrosine YZ. We calculated the redox potential (Em) of YZ and its symmetrical counterpart YD, by solving the linearized Poisson-Boltzmann equation. The calculated Em(Y/Y−) were +926 mV/+694 mV for YZ/YD with the Mn-cluster in S2 state. Together with the asymmetric position of the Mn-cluster relative to YZ/D, differences in H-bond network between YZ (YZ/D1-His190/D1-Asn298) and YD (YD/D2-His189/D2-Arg294/CP47-Glu364) are crucial for Em(YZ/D). When D1-His190 is protonated, corresponding to a thermally activated state, the calculated Em(YZ) was +1216 mV, which is as high as the Em for PD1/D2. We observed deprotonation at CP43-Arg357 upon S-state transition, which may suggest its involvement in the proton exit pathway. Em(YD) was affected by formation of PD2+ (but not PD1+) and sensitive to the protonation state of D2-Arg180. This points to an electrostatic link between YD and PD2.