High-frequency (140-GHz) time domain EPR and ENDOR spectroscopy: The tyrosyl radical-diiron cofactor in ribonucleotide reductase from yeast

High-frequency pulsed EPR and ENDOR have been employed to characterize the tyrosyl radical (Y .)-diiron cofactor in the Y2-containing R2 subunit of ribonucleotide reductase (RNR) from yeast. The present work represents the first use of 140-GHz time domain EPR and ENDOR to examine this system and demonstrates the capabilities of the method to elucidate the electronic structure and the chemical environment of protein radicals. Low-temperature spin-echo-detected EPR spectra of yeast Y . reveal an EPR line shape typical of a tyrosyl radical; however, when compared with the EPR spectra of Y . from E. coli RNR, a substantial upfield shift of the gl-value is observed. The origin of the shift in g(1) was investigated by 140-GHz H-1 and H-2 pulsed ENDOR experiments of the Y2-containing subunit in protonated and D2O-exchanged buffer. H-2 ENDOR spectra and simulations provide unambiguous evidence for one strongly coupled H-2 arising from a bond between the radical and an exchangeable proton of an adjacent residue or a water molecule. Orientation-selective 140-GHz ENDOR spectra indicate the direction of the hydrogen bond with respect to the molecular symmetry axes and the bond length (1.81 Angstrom). Finally, we have performed saturation recovery experiments and observed enhanced spin lattice relaxation rates of the Y . above 10 K. Ar temperatures higher than 20 K, the relaxation rates are isotropic across the EPR line, a phenomenon that we attribute to isotropic exchange interaction between Y . and the first excited paramagnetic state of the diiron cluster adjacent to it. From the activation energy of the rates, we determine the exchange interaction between the two irons of the cluster, J(exc) = -85 cm(-1). The relaxation mechanism and the presence of the hydrogen bond are discussed in terms of the differences in the structure of the Y . -diiron cofactor in yeast Y2 and other class I R2s.