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J. Biol. Chem., Vol. 269, Issue 1, 134-137, 01, 1994

EPR evidence that the M+ radical, which is observed in three site- directed mutants of photosystem II, is a tyrosine radical

RJ Boerner and BA Barry
Department of Biochemistry, University of Minnesota, St. Paul 55108.

Isotopic labeling of Synechocystis sp. PCC 6803 and EPR spectroscopy have been used to demonstrate that photosystem II contains two redox active tyrosines, D and Z (Barry, B. A. and Babcock, G. T. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 7099-7103; Boerner, R. J. and Barry, B. A. (1993) J. Biol. Chem. 268, 17151-17154). Another organic radical, M+, has recently been observed in site-directed mutants in which a non- redox active amino acid is substituted at either the putative D or putative Z sites (Boerner, R. J., Bixby, K. A., Nguyen, A. P., Noren, G. H., Debus, R. J., and Barry, B. A. (1993) J. Biol. Chem. 268, 1817- 1823; Noren, G. H., and Barry, B. A. (1992) Biochemistry 31, 3335- 3342). Here, we use isotopic labeling to determine the chemical identity of M+. Upon incorporation of perdeuterated tyrosine into photosystem II, the M+ EPR signal narrows to approximately 12-13 G. Labeling with 3,5-deuterated tyrosine results in an isotropic doublet with splittings of 11 G. Our results show that M+ is a tyrosine radical with unique spectroscopic properties.
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