Molecular origin of the pH dependence of tyrosine D oxidation kinetics and radical stability in photosystem II.

A role for redox-active tyrosines has been demonstrated in many important biological processes, including water oxidation carried out by photosystem II (PSII) of oxygenic photosynthesis. The rates of tyrosine oxidation and reduction and the Tyr/Tyr reduction potential are undoubtedly controlled by the immediate environment of the tyrosine, with the coupling of electron and proton transfer, a critical component of the kinetic and redox behavior. It has been demonstrated by Faller et al. that the rate of oxidation of tyrosine D (TyrD) at room temperature and the extent of TyrD oxidation at cryogenic temperatures, following flash excitation, dramatically increase as a function of pH with a pKa of ≈7.6 [Faller et al. 2001 Proc. Natl. Acad. Sci. USA 98, 14368–14373; Faller et al. 2001 Biochemistry 41, 12914–12920]. In this work, we investigated, using FTIR difference spectroscopy, the mechanistic reasons behind this large pH dependence. These studies were carried out on Mn-depleted PSII core complexes isolated from Synechocystis sp. PCC 6803, WT unlabeled and labeled with 13C6-, or 13C1(4)-labeled tyrosine, as well as on the D2-Gln164Glu mutant. The main conclusions of this work are that the pH-induced changes involve the reduced TyrD state and not the oxidized TyrD state and that TyrD does not exist in the tyrosinate form between pH 6 and 10. We can also exclude a change in the protonation state of D2-His189 as being responsible for the large pH dependence of TyrD oxidation. Indeed, our data are consistent with D2-His189 being neutral both in the TyrD and TyrD states in the whole pH6-10 range. We show that the interactions between reduced TyrD and D2-His189 are modulated by the pH. At pH greater than 7.5, the ν(CO) mode frequency of TyrD indicates that TyrD is involved in a strong hydrogen bond, as a hydrogen bond donor only, in a fraction of the PSII centers. At pH below 7.5, the hydrogen-bonding interaction formed by TyrD is weaker and TyrD could be also involved as a hydrogen bond acceptor, according to calculations performed by Takahashi and Noguchi [J. Phys. Chem. B 2007 111, 13833–13844]. The involvement of TyrD in this strong hydrogen-bonding interaction correlates with the ability to oxidize TyrD at cryogenic temperatures and rapidly at room temperature. A strong hydrogen-bonding interaction is also observed at pH 6 in the D2-Gln164Glu mutant, showing that the residue at position D2-164 regulates the properties of TyrD. The IR data point to the role of a protonatable group(s) (with a pKa of ≈7) other than D2-His189 and TyrD, in modifying the characteristics of the TyrD hydrogen-bonding interactions, and hence its oxidation properties. It remains to be determined whether the strong hydrogen-bonding interaction involves D2-His189 and if TyrD oxidation involves the same proton transfer route at low and at high pH.