The protein environment surrounding tyrosyl radicals D′ and Z′ in photosystem II:: A difference Fourier-transform infrared spectroscopic study

Photosystem II contains two redox-active tyrosine residues, termed D and Z, which have different midpoint potentials and oxidation/reduction kinetics. To understand the functional properties of redox-active tyrosines, we report a difference Fourier-transform infrared (FT-IR) spectroscopic study of these species. Vibrational spectra associated with the oxidation of each tyrosine residue are acquired; electron paramagnetic resonance (EPR) and fluorescence experiments demonstrate that there is no detectable contribution of Q(A)(-) to these spectra. Vibrational lines are assigned to the radicals by isotopic labeling of tyrosine. Global N-15 labeling, H-2 exchange, and changes in pH identify differences in the reversible interactions of the two redox-active tyrosines with N-containing, titratable amino acid side chains in their environments. To identify the amino acid residue that contributes to the spectrum of D, mutations at His(189) in the D2 polypeptide were examined. Mutations at this site result in substantial changes in the spectrum of tyrosine D, Previously, mutations at the analogous histidine, His(190) in the D1 polypeptide, were shown to have no significant effect on the FT-IR spectrum of tyrosine Z (Bernard, M. T., et at. 1995. J. Biol. Chem. 270:1589-1594). A disparity in the number of accessible, proton-accepting groups could influence electron transfer rates and energetics and account for functional differences between the two redox-active tyrosines.