EPR evidence that the M+ radical, which is observed in three site-directed mutants of photosystem II, is a tyrosine radical.
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.
Boerner
R J
RJ
Department of Biochemistry, University of Minnesota, St. Paul 55108.
Barry
B A
BA
D000458:Cyanobacteria / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045332:Photosystem II Protein Complex; D014443:Tyrosine / Q000737:chemistry*
D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D014443:Tyrosine