Reduced forms of the iron-containing small subunit of ribonucleotide reductase from Escherichia coli

The B2 subunit of ribonucleotide reductase from Escherichia coli contains a stable tyrosyl free radical and an antiferromagnetically coupled dimeric iron center with high-spin ferric ions. The tyrosyl radical is an oxidized form of tyrosine-122. This study shows that the B2 protein has a fully reduced state, denoted reduced B2, characterized by a normal nonradical tyrosine-122 residue and a dimeric ferrous iron center. Reduced B2 can be formed either from active B2 by a three-electron reduction in the presence of suitable mediators or from apoB2 by addition of two equimolar amounts of ferrous ions in the absence of oxygen. The oxidized tyrosyl radical and the ferric iron center can be generated from reduced B2 by the admission of air. The tyrosyl radical can be selectively reduced by one-electron reduction in the presence of a suitable mediator, yielding metB2, a form that seems identical with the form resulting from treatment of active B2 with hydroxyurea. 1H NMR was used to characterize the paramagnetically shifted resonances associated with the reduced iron center. Prominent resonances were observed around 45 ppm (nonexchangeable with solvent) and 57 ppm (exchangeable with solvent) at 37 degrees C. From the temperature dependence of the chemical shifts of these resonances it was concluded that the ferrous ions in reduced B2 are only weakly, if at all, antiferromagnetically coupled. By comparison with data on the similar iron center of deoxyhemerythrin it is suggested that the 57 ppm resonance should be assigned to protons in histidine ligands of the iron center.