Glutamate 189 of the D1 Polypeptide Modulates the Magnetic and Redox Properties of the Manganese Cluster and Tyrosine YZ in Photosystem II

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Abstract
Recent models for water oxidation in photosystem II postulate that the tyrosine YZ radical, YZ•, 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 YZ• requires the deprotonation of YZ by His190 of the D1 polypeptide. For YZ• to abstract both an electron and a proton from the Mn cluster, the proton abstracted from YZ 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 YZ•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 YZ•. In Mn-depleted D1-Glu189 mutant PSII particles, charge recombination between QA•- and YZ• in the mutants was accelerated, showing that the mutations alter the redox properties of YZ 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 YZ and the Mn cluster and are consistent with proposals that D1-Glu189 positions a group that accepts a proton from D1-His190.

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