Structure of the Tyrosyl Radical in Bacteriophage T4-induced Ribonucleotide Reductase

Ribonucleotide reductase induced by bacteriophage T4 in Escherichia coli contains an organic free radical necessary for enzymatic activity. Its EPR spectrum at 77K is similar to but not identical with that of the corresponding radical in the enzyme from uninfected E. coli studied previously. Isotope substitutions now show that the radical in the T4-induced enzyme also is localized to a tyrosine residue with its spin density delocalized over the aromatic ring of tyrosine. The difference between the radicals of the T4-induced and the E. coli ribonucleotide reductases, as reflected in the hyperfine patterns of their EPR spectra, is suggested to be due to slightly different radical geometries, resulting from a twist of about 10 degrees around the bond between the aromatic ring and the methylene group in the tyrosine radical. Hydroxyurea destroys the free radicals of both ribonucleotide reductases and also their catalytic activities. Both enzymes are considerably more sensitive to hydroxyurea during catalysis than in the noncatalytic state. However, when compared to the bacterial ribonucleotide reductase, the T4-induced enzyme shows an overall approximately 10 times higher sensitivity to hydroxyurea, judging from the drug concentrations needed to destroy the radicals and inhibit the activities. This result may reflect a difference in accessibility for the drug to the active sites of the enzymes.