Activation of the iron-containing B2 protein of ribonucleotide reductase by hydrogen peroxide

The active form of protein B2, the small subunit of ribonucleotide reductase, contains two dinuclear Fe(III) centers and a tyrosyl radical. The inactive metB2 form also contains the same diferric complexes but lacks the tyrosyl radical. We now demonstrate that incubation of metB2 with hydrogen peroxide generates the tyrosyl radical. The reaction is optimal at 5.5 nM hydrogen peroxide, with a maximum of 25-30% tyrosyl radical being formed after approximately 1.5 hr of incubation. The activation reaction is counteracted by a hydrogen peroxide-dependent reduction of the tyrosyl radical. It is likely that the generation of the radical proceeds via a ferryl intermediate, as in the proposed mechanisms for cytochrome P-450 and the peroxidases.