Acetate Binding at the Photosystem II Oxygen Evolving Complex:  An S₂-State Multiline Signal ESEEM Study

Abstract

Previously, using acetate deuterated in the methyl hydrogen positions, we showed that acetate binds in close proximity to the Mn cluster/Y(.)(z) tyrosine dual spin complex in acetate-inhibited photosystem II (PSII) preparations exhibiting the "split" EPR signal arising from the S(2)-Y(.)(z) interaction [Force, D. A.; Randall, D. W.; Britt, R. D. Biochemistry 1997, 36, 12062-12070]. By using paramagnetic NO to quench the paramagnetism of Y(.)(z), we are able to observe the ESEEM spectrum of deuterated acetate interacting with only the Mn cluster. A good fit of the ESEEM data indicates two (2)H dipolar hyperfine couplings of 0.097 MHz and one of 0.190 MHz. Modeling of these dipolar interactions, using our "dangler" 3 + 1 model for the S(2)-state of the Mn cluster, reveals distances consistent with direct ligation of acetate to the Mn cluster. As acetate inhibition is competitive with the essential cofactor Cl(-), this suggests that Cl(-) ligates directly to the Mn cluster. The effect of acetate binding on the structure of the Mn cluster is investigated by comparing the Mn-histidine coupling in NO/acetate-treated PSII and untreated PSII using ESEEM. We find that the addition of acetate and NO does not affect the histidine ligation to the Mn cluster. We also investigate the ability of acetate to access Y(.)(z) in Mn-depleted PSII, a PSII preparation expected to be more solvent accessible than intact PSII. We detect no coupling between Y(.)(z) and acetate. We have previously shown that small alcohols such as methanol can ligate to the Mn cluster with ease, while larger alcohols such as 2-propanol, as well as DMSO, are excluded [Force, D. A.; Randall, D. W.; Lorigan, G. A.; Clemens, K. L.; Britt, R. D. J. Am. Chem. Soc. 1998, 120, 13321-13333]. We probe the effect of acetate binding on the ability of methanol and DMSO to bind to the Mn cluster. We find that methanol is able to bind to the Mn cluster in the presence of acetate. We detect no DMSO binding in the presence of acetate. Thus, acetate binding does not increase the affinity or accessibility for DMSO binding at the Mn cluster. We also explore the possibility that the acetate binding site is also a binding site for substrate water. By comparing the ratioed three-pulse ESEEM spectra of a control, untreated PSII sample in 50% D(2)O to an NO/acetate-treated PSII sample in 50% D(2)O, we find that the binding of acetate to the oxygen evolving complex of photosystem II displaces deuterons bound very closely to the Mn cluster.

Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography

“…However, they did not provide any information about the number and location of Cl Ϫ ions within the OEC. The inhibition of particular S-state transitions upon depletion of Cl Ϫ has been taken as evidence of the close proximity of Cl Ϫ to the Mn 4 Ca cluster, a proximity that also was suggested by studies of electron spin echo envelope modulation (19,20) and Fourier transform infrared spectroscopic studies (21). Recent EXAFS studies, however, suggested that Cl Ϫ ions do not bind to the Mn 4 Ca cluster in its S 1 state, and that no Cl Ϫ is present within a distance of 5 Å from the metal center (13).…”

Multifrequency pulsed EPR studies of biologically relevant manganese(II) complexes

“…Much research effort has been focused on obtaining ESEEM and ENDOR spectra of the various states of PSII [67][68][69][70][71][72][73][74][75]. While the Mn(II) oxidation state is likely not accessed during any of the so-called S states of the water oxidation reaction2 [76,77], Mn(II) ions are necessary for the in vivo photoassembly of the Mn cluster [78].…”

Manganese Efficiency in Barley: Identification and Characterization of the Metal Ion Transporter HvIRT1

“…These include (1) oxalate oxidase, which catalyzes the conversion of oxalate and oxygen into hydrogen peroxide and carbon dioxide (Requena and Bornemann, 1999); (2) the Mn-containing superoxide dismutase, located in the mitochondria, which protects the tissue from oxidative stress (Bowler et al, 1991;Alscher et al, 2002); and (3) the oxygen-evolving complex of PSII (Britt, 1996;Clemens et al, 2002;Rutherford and Boussac, 2004). In addition, Mn is an unspecific activator of a number of different enzymes, such as decarboxylases and dehydrogenases in the tricarboxylic acid cycle, Phe ammonia-lyase in the shikimic acid pathway, and in several glycosyltransferases in the Golgi apparatus (Marschner, 1995;Nunan and Scheller, 2003).…”