Structure of a trapped radical transfer pathway within a ribonucleotide reductase holocomplex.

Ribonucleotide reductases (RNRs) are a diverse family of enzymes that are alone capable of generating 2'-deoxynucleotides de novo and are thus critical in DNA biosynthesis and repair. The nucleotide reduction reaction in all RNRs requires the generation of a transient active site thiyl radical, and in class I RNRs this process involves a long-range radical transfer between two subunits, α and β. Due to the transient subunit association, an atomic resolution structure of an active α2β2 RNR complex has been elusive. Here we use a doubly-substituted β2, E52Q/(2,3,5)-trifluorotyrosine122-β2 to trap wildtype-α2 in long-lived α2β2 complex. We report the structure of this complex by cryo-electron microscopy to 3.6-Å resolution, allowing for structural visualization of a 32-Å-long radical transfer pathway that affords RNR activity.