The differences in microenvironments and functions of tyrosine radicals YZ and YD in photosystem II studied by EPR.

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were performed to investigate the difference in microenvironments and functions between tyrosine Z (Y(Z)) and tyrosine D (Y(D)). Mn-depletion or Ca(2+)-depletion causes extension of the lifetime of tyrosine radical Y(Z)(*), which can be trapped by rapid freezing after illumination at about 250 K. Above pH 6.5, Y(Z)(*) radical in Mn-depleted PS II shows similar EPR and ENDOR spectra similar to that of Y(D)(*) radical, which are ascribed to a typical neutral tyrosine radical. Below pH 6.5, Y(Z)(*) radical shows quite different EPR and ENDOR spectra. ENDOR spectra show the spin density distribution of the low-pH form of Y(Z)(*) that has been quite different from the high-pH form of Y(Z)(*). The spin density distribution of the low-pH Y(Z)(*) can be explained by a cation radical or the neutral radical induced by strong electrostatic interaction. The pH dependence of the activation energy of the recombination rate between Y(Z)(*) and Q(A)(-) shows a gap of 4.4 kJ/mol at pH 6.0-6.5. In the Ca(2+)-depleted PS II, Y(Z)(*) signal was the mixture of the cation-like and normal neutral radicals, and the pH dependence of Y(Z)(*) spectrum in Ca(2+)-depleted PS II is considerably different from the neutral radical found in Mn-depleted PS II. Based on the recent structure data of cyanobacterial PS II, the pH dependence of Y(Z)(*) could be ascribed to the modification of the local structure and hydrogen-bonding network induced by the dissociation of ASP170 near Y(Z).