Reverse Electron Transfer Completes the Catalytic Cycle in a 2,3,5-Trifluorotyrosine-Substituted Ribonucleotide Reductase

Escherichia coli class Ia ribonucleotide reductase is composed of two subunits (α and β), which form an α2β2 complex that catalyzes the conversion of nucleoside 5′-diphosphates to deoxynucleotides (dNDPs). β2 contains the essential tyrosyl radical (Y[subscript 122][superscript •]) that generates a thiyl radical (C[subscript 439][superscript •]) in α2 where dNDPs are made. This oxidation occurs over 35 Å through a pathway of amino acid radical intermediates (Y[subscript 122] → [W[subscript 48]] → Y[subscript 356] in β2 to Y[subscript 731] → Y[subscript 730] → C[subscrip 439] in α2). However, chemistry is preceded by a slow protein conformational change(s) that prevents observation of these intermediates. 2,3,5-Trifluorotyrosine site-specifically inserted at position 122 of β2 (F[subscript 3]Y[superscript •]-β2) perturbs its conformation and the driving force for radical propagation, while maintaining catalytic activity (1.7 s[superscript –1]). Rapid freeze–quench electron paramagnetic resonance spectroscopy and rapid chemical-quench analysis of the F[subscript 3]Y[superscript •]-β2, α2, CDP, and ATP (effector) reaction show generation of 0.5 equiv of Y[subscript 356] and 0.5 equiv of dCDP, both at 30 s[superscript –1]. In the absence of an external reducing system, Y[subscript 356] reduction occurs concomitant with F3Y reoxidation (0.4 s–1) and subsequent to oxidation of all α2s. In the presence of a reducing system, a burst of dCDP (0.4 equiv at 22 s–1) is observed prior to steady-state turnover (1.7 s–1). The [Y[subscript 356][superescript •]] does not change, consistent with rate-limiting F[subscript 3]Y reoxidation. The data support a mechanism where Y[subscript 122][superscript •] is reduced and reoxidized on each turnover and demonstrate for the first time the ability of a pathway radical in an active α2β2 complex to complete the catalytic cycle.National Institutes of Health (U.S.) (Grant GM29595