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 with fluoride, acetate. or ammonia results in production of a broad radical 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 radical in ribonucleotide reductase, nitric oxide has been used to probe the S3 EPR signal in acetate-treated photosystem II, In experiments using manganese-depleted photosystem II, nitric oxide was found to bind reversibly to both redox-active tyrosines, Y-D(.) and Y-Z(.), to form EPR-silent adducts, Next, acetate-treated photosystem II was illuminated to form the S3 EPR signal in the presence of nitric oxide to test whether the S3 EPR signal behaves like Y-Z(.). Under conditions that produce the maximum yield of the S3 EPR signal in acetate-treated photosystem 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. Quenching of the S3 EPR signal by nitric oxide yielded an S-2-state multiline EPR signal, Its amplitude was 45% of that found for uninhibited photosystem II illuminated at 200 K: this yield is the same as the yield of the S3 EPR signal under equivalent conditions but without nitric oxide, These results suggest that the S3 EPR signal is due to the configuration S2YZ. in which the S-2 state of the oxygen-evolving complex gives a broadened multiline EPR signal as a result oi exchange and dipolar 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.