High-frequency EPR and ENDOR: Time-domain spectroscopy of ribonucleotide reductase

This paper discusses time-domain electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) experiments aimed at elucidating the enzymatic mechanism of ribonucleotide reductase (RNR), the enzyme responsible for the conversion of ribonucleotides to deoxyribonucleotides. The article begins with a discussion of the current state of the art of instrumentation for high-frequency EPR and ENDOR and some suggestions as to future developments. We then provide an introduction to the chemistry of RNR and a discussion of the high-field EPR and ENDOR spectra of the tyrosyl radical (Y-.) in the R2 subunit of class I RNR. Finally, we describe two examples illustrating the use of high-frequency EPR and ENDOR to elucidate the enzymatic mechanism of RNR. EPR and ENDOR have played an important role for these studies since the mechanism involves several different radical intermediates. These intermediates are all present in low concentrations relative to the Y-. concentration and they possess similar g-values. Spectral overlap, therefore, has been a problem with X-band EPR. At high frequencies the spectra are resolved to the point that individual powder lineshapes are discernible. In addition, we describe our approach, on the basis of differential relaxation, to suppress the spectrum of the dominant Y-.. High-frequency EPR and ENDOR therefore has permitted us to determine the structure of several radical intermediates which in rum have contributed to the understanding of the enzymatic mechanism of RNR.