Fig. 1: RNR catalysis requires formation of a transient thiyl radical species.

A) Radical-based reduction of ribonucleoside diphosphates by class Ia RNR. Reducing equivalents are provided by a pair of active site cysteines that are oxidized concomitant with product formation. The resulting disulfide is re-reduced via disulfide exchange with NADPH as the ultimate source of electrons (27). B) Proposed proton-coupled electron transfer pathway that results in the transient oxidation of C439-α by Y122•-β. The direct involvement of W48 in the pathway is uncertain. C) Symmetric “docking model” of 260 kDa α2β2 complex that was based on shape complementarity of the individual α2 and β2 crystal structures (5). α2 is shown in cyan/blue and β2 is shown in red/orange. D) 3.6-Å resolution cryo-EM map of an asymmetric, active RNR trapped using a doubly-substituted E52Q/(2,3,5)-trifluorotyrosine122(F₃Y122)-β2 with wildtype-α2 in the presence of substrate GDP and specificity effector TTP. Model colors are as in Fig. 1C with cryo-EM density shown in transparent grey.