High-field EPR study of tyrosyl radicals in prostaglandin H₂ synthase-1

Various tyrosyl radicals generated by reaction of both native and indomethacin-inhibited ovine prostaglandin H synthase-1 with ethyl hydrogen peroxide were examined by using high-field/high-frequency EPR spectroscopy. The spectra for the initially formed tyrosyl radical commonly referred to as the "wide-doublet" species and the subsequent "wide-singlet" species as well as the indomethacin-inhibited "narrow-singlet" species were recorded at several frequencies and analyzed. For all three species, the g-values were distributed. In the case of the wide doublet, the high-field EPR spectra indicated that dominant hyperfine coupling was likely to be also distributed. The g_x-values for all three radicals were found to be consistent with a hydrogen-bonded tyrosyl radical. In the case of the wide-doublet species, this finding is consistent with the known position of the radical and the crystallographic structure and is in contradiction with recent ENDOR measurements. The high-field EPR observations are consistent with the model in which the tyrosyl phenyl ring rotates with respect to both the protein backbone and the putative hydrogen bond donor during evolution from the wide-doublet to the wide-singlet species. The high-field spectra also indicated that the g-values of two types of narrow-singlet species, self-inactivated and indomethacin-inhibited, were likely to be different, raising the possibility that the site of the radical is different or that the binding of the inhibitor perturbs the electrostatic environment of the radical. The 130 GHz pulsed EPR experiments performed on the wide-doublet species indicated that the possible interaction between the radical and the oxoferryl heme species was very weak.