Does Aspartate 170 of the D1 Polypeptide Ligate the Manganese Cluster in Photosystem II? An EPR and ESEEM Study

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Abstract
Aspartate 170 of the D1 polypeptide provides part of the high-affinity binding site for the first Mn(II) ion that is photooxidized during the light-driven assembly of the (Mn)4 cluster in photosystem II [Campbell, K. A., Force, D. A., Nixon, P. J., Dole, F., Diner, B. A., and Britt, R. D. (2000) J. Am. Chem. Soc. 122, 3754−3761]. However, despite a wealth of data on D1-Asp170 mutants accumulated over the past decade, there is no consensus about whether this residue ligates the assembled (Mn)4 cluster. To address this issue, we have conducted an EPR and ESEEM (electron spin-echo envelope modulation) study of D1-D170H PSII particles purified from the cyanobacterium Synechocystis sp. PCC 6803. The line shapes of the S1 and S2 state multiline EPR signals of D1-D170H PSII particles are unchanged from those of wild-type PSII particles, and the signal amplitudes correlate approximately with the lower O2 evolving activity of the mutant PSII particles (40−60% compared to that of the wild type). These data provide further evidence that the assembled (Mn)4 clusters in D1-D170H cells function normally, even though the assembly of the (Mn)4 cluster is inefficient in this mutant. In the two-pulse frequency domain ESEEM spectrum of the 9.2 GHz S2 state multiline EPR signal of D1-D170H PSII particles, the histidyl nitrogen modulation observed at 4−5 MHz is unchanged from that of wild-type PSII particles and no significant new modulation is observed. Three scenarios are presented to explain this result. (1) D1-Asp170 ligates the assembled (Mn)4 cluster, but the hyperfine couplings to the ligating histidyl nitrogen of D1-His170 are too large or anisotropic to be detected by ESEEM analyses conducted at 9.2 GHz. (2) D1-Asp170 ligates the assembled (Mn)4 cluster, but D1-His170 does not. (3) D1-Asp170 does not ligate the assembled (Mn)4 cluster.

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