A [2.1.0]-Fused Bicyclic Intermediate Is Produced during the Biosynthesis of Oxetane Nucleosides

AlsB and OxsB are homologous B₁₂-dependent radical S-adenosyl-l-methionine enzymes involved in the biosynthesis of the oxetane nucleosides albucidin and oxetanocin A. Both pathways also require a second enzyme (AlsA and OxsA, respectively) to complete the biosynthesis of the oxetane ring. Herein, OxsB and AlsB are both shown to catalyze an intramolecular C-C bond formation between two sp³-hybridized carbons in 5'-phosphates of 2'-deoxyadenosine to yield corresponding phosphates of the same highly strained [2.1.0]-bicyclic species. This compound undergoes nonenzymatic decomposition; however, in the presence of OxsA or AlsA, it is instead converted to oxetanocin A aldehyde phosphate or albucidin phosphate, respectively. This completes the description of oxetanocin A and albucidin biosynthetic pathways. Formation of the bicyclic intermediate involves hydrogen atom abstraction from both C2' and C4' in the substrate, which suggests an unprecedented catalytic role for cobalamin in stabilizing a carbon radical intermediate, thereby facilitating an intramolecular radical recombination reaction.