Electron paramagnetic resonance and nuclear magnetic resonance studies of class I ribonucleotide reductase

Ribonucleotide reductase catalyses the reduction of ribonucleotides to the corresponding deoxyribonucleotides needed for DNA synthesis. This review describes recent studies on the iron/tyrosyl free radical site in the R2 protein of iron-containing (class I) ribonucleotide reductases. The active enzyme is composed of two homodimeric proteins, R1 and R2. Active protein R2 contains a diiron-oxygen site and a neighboring free radical on a tyrosyl residue per polypeptide chain. The properties of the different redox states of the diiron center in protein R2 are discussed, as well as the formation of the iron/radical site and its possible involvement in long range electron transfer from the substrate binding site in protein R1. The EPR properties of oxidized neutral tyrosyl free radicals are described, and also of tryptophan free radicals found in studies of a mutant of the R2 protein, which lacks the tyrosyl radical site. NMR studies on protein R2 include observations of paramagnetically shifted resonances. Structural NMR studies have been performed on its highly mobile C-terminal domain as well as the corresponding oligopeptide which interacts with protein R1.