The class Ib ribonucleotide reductase of Escherichia coli can initiate reduction of nucleotides to deoxynucleotides with either a MnIII 2-tyrosyl radical (Y•) or a FeIII2-Y• cofactor in the NrdF subunit. Whereas FeIII2-Y• can self-assemble from FeII2-NrdF and O2, activation of MnII2-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 FeII2-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.
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