HIGH-FIELD EPR STUDIES OF MOUSE RIBONUCLEOTIDE REDUCTASE INDICATE HYDROGEN-BONDING OF THE TYROSYL RADICAL

Ribonucleotide reductase catalyzes by free radical chemistry the reduc tion of ribonucleotides to deoxyribonucleotides. The R2 protein of a c lass 1 ribonucleotide reductase contains a stable tyrosyl radical of n eutral phenoxy character, which is necessary for normal enzymatic acti vity, Here we present the EPR spectra from the tyrosyl free radical in the R2 protein from mouse at 9.62, 115, and 245 GHz. We show that the g-value anisotropy of the mouse R2 radical, when precisely determined from high field EPR spectra, is similar to that of the hydrogen bonde d dark stable Y-D(.) tyrosyl radical of photosystem II and different f rom that of the Escherichia coli R2 radical. Because the g-value aniso tropy is an important indicator of the hydrogen bonding status of the tyrosyl radical, this result suggests that the mouse R2 radical has it s tyrosylate oxygen hydrogen bonded with a D2O exchangeable proton, wh ereas this hydrogen bond is absent in the E. coli enzyme. It is sugges ted that the observed proton may be derived from the tyrosine that wil l become a tyrosyl radical.