Synthetic polyprenol-pyrophosphate linked oligosaccharides are efficient substrates for mycobacterial galactan biosynthetic enzymes

Mycobacteria, including the human pathogen Mycobacterium tuberculosis, produce a complex cell wall that is critical for their survival. The largest structural component of the cell wall, the mycolyl-arabinogalactan-peptidoglycan complex, has at its core a galactan domain composed of d-galactofuranose residues. Mycobacterial galactan biosynthesis has been proposed to involve two glycosyltransferases, GlfT1 and GlfT2, which elongate polyprenol-pyrophosphate linked glycosyl acceptor substrates using UDP-galactofuranose as the donor substrate. We here report the first chemical synthesis of GlfT1 and GlfT2 acceptor substrates containing pyrophosphate and polyprenol moieties (compounds 3, 4, 22 and 23). The approach involves chemical synthesis of an oligosaccharide, subsequent phosphorylation at the reducing end and coupling to a polyprenol phosphate. These compounds were shown to be substrates for either GlfT1 (22 and 23) or GlfT2 (3 and 4) and all were substantially more active than the corresponding alkyl glycoside substrates reported previously. Mass spectrometric analysis of the products formed from the reaction of 3, 4, 22 and 23 with the respective cognate enzyme and UDP-galactofuranose provide additional evidence for the galactan biosynthetic model in which GlfT1 adds the first two galactofuranose residues with the remainder being installed via GlfT2. Overall, these results highlight the importance of the pyrophosphate motif in recognition of acceptor substrates by both enzymes and demonstrate a straightforward route for the preparation of such compounds. The work also provides additional support for the process by which this important glycan is biosynthesized using, for the first time, close structural analogs to the natural substrates.