Function of Redox-Active Tyrosine in Photosystem II

Abstract

Water oxidation at photosystem II Mn-cluster is mediated by the redox-active tyrosine Y Z. We calculated the redox potential ( E m) of Y Z and its symmetrical counterpart Y D, by solving the linearized Poisson-Boltzmann equation. The calculated E m(Y /Y −) were +926 mV/+694 mV for Y Z/Y D with the Mn-cluster in S2 state. Together with the asymmetric position of the Mn-cluster relative to Y Z/D, differences in H-bond network between Y Z (Y Z/D1-His 190/D1-Asn 298) and Y D (Y D/D2-His 189/D2-Arg 294/CP47-Glu 364) are crucial for E m(Y Z/D). When D1-His 190 is protonated, corresponding to a thermally activated state, the calculated E m(Y Z) was +1216 mV, which is as high as the E m for P D1/D2. We observed deprotonation at CP43-Arg 357 upon S-state transition, which may suggest its involvement in the proton exit pathway. E m(Y D) was affected by formation of P D 2 + (but not P D 1 + ) and sensitive to the protonation state of D2-Arg 180. This points to an electrostatic link between Y D and P D2.