Nature 544, 322–326 (2017)

The biosynthesis of oxetanocin A, a nucleoside analog that has antitumor, antiviral, and antibacterial activity, requires the function of two enzymes (OxsA and OxsB) to rearrange a purine nucleoside co-opted from primary metabolism. Bridwell-Rabb et al. have now biochemically and structurally characterized OxsB, a cobalamin (Cbl)-dependent radical S-adenosylmethionine (SAM) enzyme, and found that not only is it responsible for catalyzing the formation of the natural product's oxetane ring, but it also catalyzes a reaction that is unusual for this family of enzymes. Compared to both canonical radical SAM enzymes and traditional Cbl-dependent enzymes, the structure of OxsB (the first for this superfamily) reveals an unprecedented mode of Cbl binding and an expanded radical SAM fold that, in part, functions to tether the [4Fe–4S] cluster, the SAM co-substrate, and the Cbl cofactor close together within the enzyme. As the name of this enzyme family suggests, the OxsB-catalyzed reaction proceeds through a radical intermediate. However, OxsB is the first characterized Cbl-dependent radical SAM enzyme not catalyzing a C-methylation reaction; instead, it catalyzes a thermodynamically unfavorable carbon-ring contraction. While the exact catalytic role of the Cbl cofactor remains to be elucidated, the novel structural and mechanistic features of OxsB further expand the growing collection of diverse functions for enzymes in this superfamily.