Revisiting the specificity of an α-(1→4)-mannosyltransferase involved in mycobacterial methylmannose polysaccharide biosynthesis

In order to evaluate the proposed biosynthetic pathway for the methylmannose (MMPs) polysaccharides produced by mycobacteria, two homologous series of synthetic α-(1→4)-linked 3-O-methyl-mannopyranosides, one terminated at the non-reducing end by a free mannopyranose residue (unmethylated oligosaccharides; OS) and the other terminated by a 3-O-methyl-mannopyranose residue (methylated OS), were prepared and evaluated as potential acceptors of an α-(1→4)-mannosyltransferase. Using a mycobacterial membrane preparation as the source of the transferase, it was found that unmethylated OS are better substrates for the enzyme compared to the methylated OS of the same length. These results are inconsistent with the proposed MMP biosynthetic pathway, which suggests only methylated OS are acceptors of this transferase. To confirm that the observed activity arose from the desired α-(1→4)-mannosyltransferase, as opposed to other mannosyltransferases present in the membrane preparation, the products resulting from tetrasaccharides 4 (unmethylated OS) and 9 (methylated OS), which only differ in the terminal residue, were further analyzed. MALDI-MS, exo-glycosidase digestion and (1) H NMR spectroscopy were used to evaluate the structures of these reaction products. These experiments revealed that the enzymatic products of both 4 and 9 contain only α-(1→4)-linked mannose residues, confirming the activity of the α-(1→4)-mannosyltransferase. This supports the finding that both methylated and unmethylated OS are acceptors of the enzyme. It was also demonstrated that a homologous series of oligosaccharides with different number of mannose residues were formed from both 4 and 9, as opposed to a single reaction product. These results, again, challenge the previously proposed MMP biosynthetic pathway involving alternating methylation and mannosylation reactions.