期刊
BIOCHEMISTRY
卷 39, 期 21, 页码 6275-6287出版社
AMER CHEMICAL SOC
DOI: 10.1021/bi992749w
关键词
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资金
- NIGMS NIH HHS [GM43496, GM48242] Funding Source: Medline
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 O-2, The EPR and electrontransfer properties of PSII particles isolated from the D1-E189Q, D1-E189L, D1-E189D, D1-E189N, D1-E189H, D1-EI89G, and D1-E189S mutants were examined. Intact 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-E189S mutants, the quantum yield for oxidizing the S1 state Mn cluster was very low, corresponding to a greater than or equal to 1400-fold slowing of the rate of Mn oxidation by Y-Z(.). In Mn-depleted D1-Glu189 mutant PSII particles, charge recombination between QA(.-) 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.
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