Enzymic modification of a tyrosine residue to a stable free radical in ribonucleotide reductase.

Abstract

Protein B2, a subunit of ribonucleotide reductase from Escherichia coli, contains in its active form a tyrosyl free radical as part of the polypeptide chain and a dimeric Fe center that stabilizes the radical. The enzyme depends on this radical for its catalytic activity. Treatment with hydroxyurea scavenges the radical without disturbing the Fe center and, thereby, results in an inactive form of the subunit, B2/HU. A second inactive form, apoB2, lacking both the radical and the Fe center, is obtained by treatment of B2 with 8-hydroxyquinoline. Here an enzyme activity in extracts from E. coli is described that transforms the catalytically inactive B2/HU form into the active B2 subunit by regeneration of the tyrosyl radical. This reaction requires the presence of oxygen, dithiothreitol and Mg2+ and does not proceed through apoB2. Under anaerobic conditions, evidence was obtained for a second activity in the bacterial extract that destroys the free radical and transforms B2 into B2/HU. This novel type of protein modification may be functionally related to the synthesis of deoxyribonucleotides and DNA.