Glutamate 189 of the D1 polypeptide modulates the magnetic and redox properties of the manganese cluster and tyrosine Y(Z) in photosystem II

Recent models for water oxidation in photosystem II postulate that the tyrosine Y(Z) radical, Y(Z)·, abstracts both an electron and a proton from the Mn cluster during one or more steps in the catalytic cycle. This coupling of proton- and electron-transfer events is postulated to provide the necessary driving force for oxidizing the Mn cluster in its higher oxidation states. The formation of Y(Z)· requires the deprotonation of Y(Z) by His190 of the D1 polypeptide. For Y(Z)· to abstract both an electron and a proton from the Mn cluster, the proton abstracted from Y(Z) must be transferred rapidly from D1-His190 to the lumenal surface via one or more proton-transfer pathways. The proton acceptor for D1-His190 has been proposed to be either Glu189 of the D1 polypeptide or a group positioned by this residue. To further define the role of D1-Glu189, 17 D1-Glu189 mutations were constructed in the cyanobacterium Synechocystis sp. PCC 6803. Several of these mutants are of particular interest because they appear to assemble Mn clusters in 70- 80% of reaction centers in vivo, but evolve no O₂. The EPR and electron- transfer properties of PSII particles isolated from the D1-E189Q, D1-E189L, D1-E189D, D1-E189N, D1-E189H, D1-E189G, and D1-E189S mutants were examined. Intact PSII particles isolated from mutants that evolved no O₂ also exhibited no S₁ or S₂ state multiline EPR signals and were unable to advance beyond an altered Y(Z)·S₂ state, as shown by the accumulation of narrow 'split' EPR signals under multiple turnover conditions. In the D1- E189G and D1-E189S mutants, the quantum yield for oxidizing the S₁ state Mn cluster was very low, corresponding to a ≥1400-fold slowing of the rate of Mn oxidation by Y(Z)·. In Mn-depleted D1-Glu189 mutant PSII particles, charge recombination between Q(A)·^- and Y(Z)· in the mutants was accelerated, showing that the mutations alter the redox properties of Y(Z) in addition to those of the Mn cluster. These results are consistent with D1- Glu189 participating in a network of hydrogen bonds that modulates the properties of both Y(Z) and the Mn cluster and are consistent with proposals that D1-Glu189 positions a group that accepts a proton from D1-His190.