REVERSIBLE BINDING OF NITRIC-OXIDE TO TYROSYL RADICALS IN PHOTOSYSTEM-II - NITRIC-OXIDE QUENCHES FORMATION OF THE S3 EPR SIGNAL SPECIES IN ACETATE-INHIBITED PHOTOSYSTEM-II

Continuous illumination at temperatures above 350 K of photosystem II samples which have been depleted of calcium or chloride or treated wit h fluoride, acetate. or ammonia results in production of a broad radic al EPR signal centered at g = 2.0. This EPR signal, called the S3 EPR signal, has been attributed to an organic radical interacting with the St state of the oxygen-evolving complex to give the species S(2)X(+) (X(+) = organic radical), A tyrosine radical has been proposed as the species responsible for the S3 EPR signal, On the basis of experiments demonstrating that nitric oxide binds reversibly to the tyrosyl radic al in ribonucleotide reductase, nitric oxide has been used to probe th e S3 EPR signal in acetate-treated photosystem II, In experiments usin g manganese-depleted photosystem II, nitric oxide was found to bind re versibly to both redox-active tyrosines, Y-D(.) and Y-Z(.), to form EP R-silent adducts, Next, acetate-treated photosystem II was illuminated to form the S3 EPR signal in the presence of nitric oxide to test whe ther the S3 EPR signal behaves like Y-Z(.). Under conditions that prod uce the maximum yield of the S3 EPR signal in acetate-treated photosys tem II no S3 EPR signal was observed in the presence of nitric oxide. Upon removal of nitric oxide, the S3 EPR signal could be induced. Quen ching of the S3 EPR signal by nitric oxide yielded an S-2-state multil ine EPR signal, Its amplitude was 45% of that found for uninhibited ph otosystem II illuminated at 200 K: this yield is the same as the yield of the S3 EPR signal under equivalent conditions but without nitric o xide, These results suggest that the S3 EPR signal is due to the confi guration S2YZ. in which the S-2 state of the oxygen-evolving complex g ives a broadened multiline EPR signal as a result oi exchange and dipo lar interactions with Y-Z(.). The binding of nitric oxide to Y-Z(.) to form a diamagnetic Y-Z-NO species uncouples the S-2 state from Y-Z(.) , yielding a noninteracting S-2-state multiline EPR signal species.