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 tyro sine 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 o f proton- and electron-transfer events is postulated to provide the necessa ry driving force for oxidizing the Mn cluster in its higher oxidation state s. The formation of Y-Z(.) requires the deprotonation of Y-Z by His190 of t he D1 polypeptide. For Y-Z(.) to abstract both an electron and a proton fro m the Mn cluster, the proton abstracted from Y-Z must be transferred rapidl y from D1-His190 to the lumenal surface via one or more proton-transfer pat hways, The proton acceptor for D1-His190 has been proposed to be either Glu 189 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 t he 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 O-2, The EPR and electrontrans fer properties of PSII particles isolated from the D1-E189Q, D1-E189L, D1-E 189D, D1-E189N, D1-E189H, D1-EI89G, and D1-E189S mutants were examined. Int act PSII particles isolated from mutants that evolved no Oz also exhibited no S-1 or S-2 State multiline EPR signals and were unable to advance beyond an altered (YZS2)-S-. state, as shown by the accumulation of narrow "split " EPR signals under multiple turnover conditions. In the D1-E189G and D1-E1 89S mutants, the quantum yield for oxidizing the S1 state Mn cluster was ve ry low, corresponding to a greater than or equal to 1400-fold slowing of th e rate of Mn oxidation by Y-Z(.). In Mn-depleted D1-Glu189 mutant PSII part icles, charge recombination between QA(.-) and Y-Z(.) in the mutants was ac celerated, 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-G lu189 participating in a network of hydrogen bonds that modulates the prope rties 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.