Structural basis for activation of class Ib ribonucleotide reductase

The class Ib ribonucleotide reductase of Escherichia coli can initiate reduction of nucleotides to deoxynucleotides with either a Mn^III ₂-tyrosyl radical (Y•) or a Fe^III₂-Y• cofactor in the NrdF subunit. Whereas Fe^III₂-Y• can self-assemble from Fe^II₂-NrdF and O₂, activation of Mn^II₂-NrdF requires a reduced flavoprotein, NrdI, proposed to form the oxidant for cofactor assembly by reduction of O ₂. The crystal structures reported here of E. coli Mn ^II₂-NrdF and Fe^II₂-NrdF reveal different coordination environments, suggesting distinct initial binding sites for the oxidants during cofactor activation. In the structures of Mn ^II₂-NrdF in complex with reduced and oxidized NrdI, a continuous channel connects the NrdI flavin cofactor to the NrdF Mn ^II₂ active site. Crystallographic detection of a putative peroxide in this channel supports the proposed mechanism of Mn ^III₂-Y• cofactor assembly.