Biosynthesis of the tunicamycin antibiotics proceeds via unique exo-glycal intermediates

Abstract

The tunicamycins are archetypal nucleoside antibiotics targeting bacterial peptidoglycan biosynthesis and eukaryotic protein N-glycosylation. Understanding the biosynthesis of their unusual carbon framework may lead to variants with improved selectivity. Here, we demonstrate in vitro recapitulation of key sugar-manipulating enzymes from this pathway. TunA is found to exhibit unusual regioselectivity in the reduction of a key α,β-unsaturated ketone. The product of this reaction is shown to be the preferred substrate for TunF—an epimerase that converts the glucose derivative to a galactose. In Streptomyces strains in which another gene (tunB) is deleted, the biosynthesis is shown to stall at this exo-glycal product. These investigations confirm the combined TunA/F activity and delineate the ordering of events in the metabolic pathway. This is the first time these surprising exo-glycal intermediates have been seen in biology. They suggest that construction of the aminodialdose core of tunicamycin exploits their enol ether motif in a mode of C–C bond formation not previously observed in nature, to create an 11-carbon chain. Construction of the remarkable 11-carbon frame of the antibiotic tunicamycin is shown to use cyclic enol ethers (exo-glycals) — the first time such intermediates have been seen in biology. Exo-glycal synthase TunA uses an elegantly subtle mechanism to control regioselectivity and with exo-glycal epimerase TunF sets a logical chemical stage for downstream radical C–C coupling.

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Wyszynski, F., Lee, S., Yabe, T. et al. Biosynthesis of the tunicamycin antibiotics proceeds via unique exo-glycal intermediates. Nature Chem 4, 539–546 (2012). https://doi.org/10.1038/nchem.1351

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