Binding of NH3 to the S2 state of the O2-evolving complex of photosystem II (PSII) causes a structural change in the Mn site that is detectable with low-temperature electron paramagnetic resonance (EPR) spectroscopy. Untreated spinach PSII membranes at pH 7.5 produce a S2 state multiline EPR spectrum when illuminated at either 210 K or at 0°C in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) having an average hyperfine line spacing of 87.5 G. The temperature dependence of the S2 state multiline EPR signal observed from untreated samples deviates from the Curie law above 5 K, with a maximum signal intensity at 6.9 K as has been previously observed. In contrast, 100 mM NH4Cl-treated PSII membranes at pH 7.5 exhibit a new S2 state EPR spectrum when illuminated at 0°C in the presence of DCMU. The novel S2 state EPR spectrum from NH4Cl-treated PSII membranes has an average hyperfine line spacing of 67.5 G and a temperature dependence obeying the Curie law except for small deviations at low temperature. We assign the new S2 state EPR signal from NH4Cl-treated PSII membranes to a form of the S2 state having one or more NH3 molecules directly coordinated to the Mn site. NH3 does not bind to Mn in the dark-stable S1 state present before illumination, since generation of the S2 state in NH4Cl-treated PSII membranes by illumination at 210 K does not yield the new S2 state EPR spectrum. Since inhibition of O2 evolution activity in the presence of NH4Cl probably occurs through binding of NH3 to the O2-evolving complex in competition with substrate H2O molecules, these results indicate that the EPR-detectable Mn site functions as the substrate-binding site of the O2-evolving complex.
|Number of pages||5|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1986|
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