Structural basis for carbapenamC-alkylation by TokK, a B₁₂-dependent radical SAM enzyme

Abstract Cobalamin- or B 12 -dependent radical S -adenosylmethionine (SAM) enzymes acting during carbapenem antibiotic biosynthesis carry out radical-mediated methyl transfers that underlie the therapeutic usefulness of these essential medicines. Here we present x-ray crystal structures of TokK, which are representative of this functional class, containing its two metallocofactors and determined in the presence and absence of carbapenam substrate. The structures give the first visualization of a cobalamin-dependent radical SAM methylase that employs the radical mechanism shared by a vast majority of these enzymes. The structures provide insight into the stereochemistry of initial C6 methylation and suggests that substrate positioning governs the rate of each methylation event. One Sentence Summary Structural insight into a cobalamin-dependent radical SAM methylase that performs three sequential radical-mediated methylations to install the C6 side chain of a carbapenem antibiotic.