Electron Paramagnetic Resonance Investigations of a Kinetically Competent Intermediate Formed in Ribonucleotide Reduction: Evidence for a Thiyl Radical-Cob(II)alamin Interaction

The ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichmannii requires adenosylcobalamin (AdoCbl) as a cofactor to catalyze the conversion of nucleotides to deoxynucleotides. RTPR has previously been shown to catalyze the homolytic cleavage of the carbon−cobalt bond of AdoCbl, and the resulting paramagnetic species has been characterized by rapid freeze-quench EPR spectroscopy (Orme-Johnson, W. H.; Beinert, H.; Blakley, R. L. J. Biol. Chem. 1974, 249, 2338−2343. Licht, S.; Gerfen, G. J.; Stubbe, J. Science 1996, 271, 477−481). This study presents simulations of X- and Q-band EPR spectra of this intermediate. Modeling this species as a thiyl radical coupled to cob(II)alamin by electron-electron exchange and dipolar interactions yields reasonable fits to spectra obtained at both microwave frequencies, whereas simulations that employ a single-spin model do not. This modeling provides support for the intermediacy of a thiyl radical in this system. The techniques employed here may prove generally useful in simulation of similar spectra observed in other B 12 -dependent enzyme systems.