Construction of the octose 8-phosphate intermediate in lincomycin A biosynthesis: Characterization of the reactions catalyzed by LmbR and LmbN

Lincomycin A is a potent antimicrobial agent noted for its unusual C1 methylmercapto-substituted 8-carbon sugar. Despite its long clinical history for the treatment of Gram-positive infections, the biosynthesis of the C ₈-sugar, methylthiolincosamide (MTL), is poorly understood. Here, we report our studies of the two initial enzymatic steps in the MTL biosynthetic pathway leading to the identification of d-erythro-d-gluco-octose 8-phosphate as a key intermediate. Our experiments demonstrate that this intermediate is formed via a transaldol reaction catalyzed by LmbR using d-fructose 6-phosphate or d-sedoheptulose 7-phosphate as the C₃ donor and d-ribose 5-phosphate as the C₅ acceptor. Subsequent 1,2-isomerization catalyzed by LmbN converts the resulting 2-keto C₈-sugar (octulose 8-phosphate) to octose 8-phosphate. These results provide, for the first time, in vitro evidence for the biosynthetic origin of the C₈ backbone of MTL.