Proximity of acetate, manganese, and exchangeable deuterons to tyrosine YZ. in acetate-inhibited photosystem II membranes: implications for the direct involvement of YZ. in water-splitting.
The environment of the photosystem II YZ. radical, trapped in the "split-signal" form, is examined in acetate-treated PSII membranes using pulsed EPR methods. The split-signal line shape is simulated with dipolar and exchange couplings to the Mn cluster of 1260 and -28 MHz, respectively. The 1260-MHz dipolar coupling corresponds to a Mn-YZ. distance of 3.5 A in the point dipole limit. A 0.117-MHz dipolar coupling is observed between nonexchangeable deuterons of methyl-deuterated acetate and YZ.. This interaction is modeled with a 3.1-A distance between an acetate methyl group deuteron and the phenoxy oxygen of YZ*. Since acetate inhibition is competitive with Cl-, this result strongly suggests a close proximity between YZ. and the Cl- cofactor binding site. Analysis of pulsed ENDOR and ESEEM experiments investigating the proximity of deuterons exchanged into the vicinity of YZ. after incubation in 2H2O-enriched buffer demonstrates that YZ. trapped in the split-signal form participates in two hydrogen-bonding interactions, in contrast to YD*, which forms a single hydrogen bond. This result is inconsistent with a simple electron transfer role for YZ* and provides direct experimental evidence for a role for YZ* in proton or hydrogen atom transfer.
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Department of Chemistry, University of California, Davis 95616, USA.
Randall
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Britt
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D000085:Acetates / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D000458:Cyanobacteria / Q000378:metabolism; D003903:Deuterium / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D008345:Manganese / Q000378:metabolism*; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D014443:Tyrosine / Q000378:metabolism*; D014867:Water / Q000378:metabolism*
D000085:Acetates; D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D014867:Water; D014443:Tyrosine; D008345:Manganese; D003903:Deuterium