Possible ligation of the Mn cluster in photosystem II by the carboxyl-terminus of the D1 polypeptide: An FTIR study.

Abstract

To obtain insight into the mechanism of water oxidation in PSII, we are probing changes in the ligation environment of the Mn cluster that occur during the S state transitions with a combination of site-directed mutagenesis, isotopic labeling, ESEEM spectroscopy, and FTIR difference spectroscopy. On the basis of this approach, we recently showed that D1-His332 almost certainly ligates the assembled Mn cluster [Debus, R. J., Campbell, K. A., Gregor, W., Li, Z.-L., Burnap, R. L., and Britt, R. D. (2001) Biochemistry 40, 3690-3699] and that D1-Asp170 either ligates Mn or Ca or participates in a hydrogen bond to the Mn4-Ca cluster [Chu, H.-A., Debus, R. J., and Babcock, G. T. (2001) Biochemistry 40, 2312-2316]. To determine if Asp170 ligates the assembled Mn cluster, we have conducted an ESEEM study of D1-Asp170His PSII particles purified from the cyanobacterium, Synechocystis sp. PCC 6803. These PSII particles evolve O2 and show normal S1 and S2 state multiline EPR signals. Preliminary ESEEM spectra of the S2 state multiline EPR signal show that the amplitude of the histidyl nitrogen modulation near 5 MHz increases substantially in the mutant relative to that in wild-type. These data imply that D1-Asp170 ligates the assembled Mn cluster, with a carboxylate oxygen of Asp170 being replaced by a histidyl nitrogen in the mutant. No carboxylate modes in the mid-frequency region of the FTIR difference spectrum are altered by the mutation. We conclude that D1-Asp170 ligates the assembled Mn cluster in PSII, but does not ligate the Mn ion that is oxidized during the S1 ® S2 transition.