Function of redox-active tyrosine in photosystem II

¹ Free University of Berlin
² Freie Universitat Berlin

^* To whom correspondence should be addressed. E-mail: knapp{at}chemie.fu-berlin.de.

Submitted on November 1, 2005
Revised on December 4, 2005
Accepted on 2 February 2006

	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-His190/D1-Asn298) and Y_D (Y_D/D2-His189/D2-Arg294/CP47-Glu364) are crucial for E_m(Y_Z/D). When D1-His190 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-Arg357 upon S state transition, which may suggest its involvement in the proton exit pathway. E_m(Y_D) was affected by formation of P_D2+ (but not P_D1+) and sensitive to the protonation state of D2-Arg180. This points to an electrostatic link between Y_D and P_D2.

Key Words: CP43-Arg357, Poisson-Boltzmann, YZ/YD redox potential, electron transfer, pKa computation, proton exit pathway

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