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Class Ia ribonucleotide reductase (RNR) of Escherichia coli contains an unusually stable tyrosyl radical cofactor in the β 2 subunit (Y 122  • ) necessary for nucleotide reductase activity. Upon binding the cognate α 2 subunit, loaded with nucleoside diphosphate substrate and an allosteric/activity effector, a rate determining conformational change(s) enables rapid radical transfer (RT) within the active α 2 β 2 complex from the Y 122  • site in β 2 to the substrate activating cysteine residue (C 439 ) in α 2 via a pathway of redox active amino acids (Y 122 [β] ↔ W 48 [β]? ↔ Y 356 [β] ↔ Y 731 [α] ↔ Y 730 [α] ↔ C 439 [α]) spanning &gt;35 Å. Ionizable residues at the α 2 β 2 interface are essential in mediating RT, and therefore control activity. One of these mutations, E 350 X (where X = A, D, Q) in β 2 , obviates all RT, though the mechanism of control by which E 350 mediates RT remains unclear. Herein, we utilize an E 350 Q-photoβ 2 construct to photochemically rescue RNR activity from an otherwise inactive construct, wherein the initial RT event (Y 122  • → Y 356 ) is replaced by direct photochemical radical generation of Y 356  • . These data present compelling evidence that E 350 conveys allosteric information between the α 2 and β 2 subunits facilitating conformational gating of RT that specifically targets Y 122  • reduction, while the fidelity of the remainder of the RT pathway is retained.

Photochemical Rescue of a Conformationally Inactivated Ribonucleotide Reductase