Hydrogen-Bonding of Tyrosine-Z in Photosystem II: Probably Absent Without Calcium and Without Manganese at Acid ph but Strong in Oxygen Evolving Centers

Abstract

Photosystem II (PSII) evolves O₂ from two bound water molecules at a site that contains manganese (Mn₄), a further cofactor (X), one atom of Ca²⁺ and Cl⁻, and a redox active tyrosine (D1Tyr161, Y_Z). Each absorbed quantum of light induces electron transfer from the chlorophyll(s) of P₆₈₀ to the quinone Q_A. P ⁺₆₈₀ is reduced by Mn₄X via Y_Z. The further function of Y_Z is under debate. Crucial are its hydrogen-bonding properties [1]. By studying the optical absorption difference spectrum Y ^OX_Z -Y_Z we have found [1] that Y_Z, when reduced, resembles a hydrogen-bonded tyrosinate (Y ^(−)…_Z H⁽⁺⁾B). in O₂-evolving PSII. The hydroxyl proton is still trapped inside the protein upon oxidation (Y ^•…_Z H⁺B). In contrast to O₂-evolving PSII where the pK of the hydrogen bonded network (B) is low, in Mn-depleted centers it is shifted to 7 [2]. At more acid pH the hydrogen bond(s) to the reduced Y_Z are weakened or eliminated (Y_ZH H⁺B) so that the Y_Z oxidation requires the release of a proton into the bulk (Y ^•_Z H⁺B+H ⁺_buik ). The charged species, Y ^•…_Z H⁺, presumably acts as an electrostatical promotor of water oxidation [2]. Calcium is an essential cofactor of O₂ evolution. Ca²⁺-depleted PSII has been used as a model for the interaction between Y_Z and Mn in the O₂-evolving system [3]. In the absence of Ca²⁺ only two electrons can be extracted from Mn₄XY_Z, P ⁺₆₈₀ reduction is drastically retarded [4], and even the redox potential of Q_A is increased [5]. We studied the properties of Y_Z in Ca²⁺-depleted PSII core particles by optical spectroscopy. The spectrum Y ^OX_Z -Y_Z differed from O₂-evolving PSII but resembled the one in Mn-depleted MI at acid pH, indicative of a weakly or not hydrogen-bonded Y_Z [1]. We conclude: Ca²⁺ modulates the features of the hydrogen bonded network that interacts with Y_Z.