AN ENDOR STUDY OF STRUCTURAL-CHANGES IN THE ENVIRONMENT OF THE DARK STABLE TYROSINE RADICAL, Y-D, OF PHOTOSYSTEM-2 INDUCED BY INHIBITION OFTHE OXYGEN-EVOLVING COMPLEX

The oxygen evolving complex (OEC) of Photosystem II (PS II) may be inh ibited without inducing the loss of the manganese cofactor. Methods in clude depletion of the calcium cofactor by high salt concentration and the addition of ammonium ions at alkaline pH or acetate ions at acidi c pH. ENDOR spectra of the dark stable tyrosine radical Y-D(.) in PS I I have been obtained for samples treated in these ways. These show tha t the orientation of the tyrosine aromatic ring relative to the attach ed beta methylene protons is different in each case. The effects are s pecific to the inhibitory treatment employed and have been quantitated . Y-D is constrained by its protein environment and as such these orie ntation effects show that the protein environment around this tyrosine is affected by such inhibitory treatments. Since the distance between the OEC and Y-D has previously been estimated at 3-4 nm (Evelo, R.G., Styring, S., Rutherford, A.W. and Hoff, A.J. (1989) Biochim. Biophys. Acta 973, 428-442), the ENDOR data suggest that long-range protein-me diated communication occurs between them. The orientation of Y-D is al so shown to be sensitive to the removal of the light harvesting comple x, LHC II.