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

Abstract

Copyright © 2020 The Authors, 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, a and b. Because of the transient subunit association, an atomic resolution structure of an active a2b2 RNR complex has been elusive. We used a doubly substituted b2, E52Q/(2,3,5)-trifluorotyrosine122-b2, to trap wild-type a2 in a long-lived a2b2 complex. We report the structure of this complex by means of cryo–electron microscopy to 3.6-angstrom resolution, allowing for structural visualization of a 32-angstrom-long radical transfer pathway that affords RNR activity.