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Originally published as Biophys J. BioFAST on March 2, 2006.
doi:10.1529/biophysj.105.076984
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Biophysical Journal 90:3886-3896 (2006)
© 2006 The Biophysical Society

Function of Redox-Active Tyrosine in Photosystem II

Hiroshi Ishikita and Ernst-Walter Knapp

Institute of Chemistry and Biochemistry, Free University of Berlin, Berlin, Germany

Correspondence: Address reprint requests to E. W. Knapp, Tel.: 49-30-83-85-43-87; E-mail: knapp{at}chemie.fu-berlin.de.

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 Formula (but not Formula) and sensitive to the protonation state of D2-Arg180. This points to an electrostatic link between YD and PD2.




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