Role of D1-His190 in the Proton-Coupled Oxidation of Tyrosine YZ in Manganese-Depleted Photosystem II

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Abstract
To further characterize the role of D1-His190 in the oxidation of tyrosine YZ in photosystem II, the pH dependence of P680•+ reduction was measured in H190A and Mn-depleted wild-type* PSII particles isolated from the cyanobacterium, Synechocystis sp. PCC 6803. Measurements were conducted in the presence and absence of imidazole and other small organic bases. In H190A PSII particles, rapid reduction of P680•+ attributed to electron transfer from YZ increased dramatically above pH 9, with an apparent pKA of ∼10.3. In the presence of ethanolamine and imidazole, this dramatic increase occurred at lower pH values, with the efficiency of YZ oxidation correlating with the solution pKA value of the added base. We conclude that the pKA of YZ is ∼10.3 in D1-H190A PSII particles. In Mn-depleted wild-type* PSII particles, P680•+ reduction was accelerated by all exogenous bases examined (substituted imidazoles, histidine, Tris, and 1,4-diazabicyclo[2.2.2]octane). We conclude that YZ is solvent accessible in Mn-depleted wild-type* PSII particles and that its pKA is near that of tyrosine in solution. In Mn-depleted wild-type* PSII particles, over 80% of the kinetics of P680•+ reduction after a flash could be described by three kinetic components. The individual rate constants of these components varied slightly with pH, but their relative proportions varied dramatically with pH, showing apparent pKA values of 7.5 and 6.25 (6.9 and 5.8 in the presence of Ca2+ and Mg2+ ions). An additional pKA value (pKA < 4.5) may also be present. To describe these data, we propose (1) the pKA of His190 is 6.9−7.5, depending on buffer ions, (2) the deprotonation of YZ is facilitated by the transient formation of a either a hydrogen bond or a hydrogen-bonded water bridge between YZ and D1-His190, and (3) when protonated, D1-His190 interacts with nearby residues having pKA values near 6 and 4. Because YZ and D1-His190 are located near the Mn cluster, these residues may interact with the Mn cluster in the intact system.