Synthesis of a homologous series of galactofuranose-containing mycobacterial arabinogalactan fragments

Abstract

Mycobacteria, including the human pathogen Mycobacterium tuberculosis, the causative agent of tuberculosis, produce a complex cell wall structure made of carbohydrates and lipids. The major structural element of the mycobacterial cell wall is a glycoconjugate called the mycolic acid – arabinogalactan – peptidoglycan (mAGP) complex. Inhibition of mAGP biosynthesis is a proven strategy for developing anti-mycobacterial drugs, and thus, understanding the pathways and enzymes involved in the assembly of this molecule is of interest. In this paper, we describe the chemical synthesis of a panel of nine oligosaccharide fragments (4–12) of the galactan domain of the mAGP complex designed as biosynthetic probes. These structures, ranging in size from a hexasaccharide to a tetradecasaccharide, are potential substrates for two biosynthetic enzymes, GlfT2 and AftA, and represent the largest mycobacterial galactan fragments synthesized to date. The route developed was iterative and provided multimilligram quantities of the target molecules 4–12 in good overall yield.

Use of Synthetic Glycolipids to Probe the Number and Position of Arabinan Chains on Mycobacterial Arabinogalactan

“…The activity of Mtb AftA overexpressed in Msmg was studied in detail using linear β- d -Gal f -(1→5)-β- d -Gal f -(1→6)-β- d -Gal f -octyl and β- d -Gal f -(1→6)-β- d -Gal f -(1→5)-β- d -Gal f -octyl trisaccharide acceptors mimicking the repeating internal core of the galactan backbone of AG [Figure A]. Recently, the chemical synthesis of a series of galactan mimetics ranging from 4 to 12 Gal f residues has been reported [Figure B].…”

“…Well-defined, synthetic substrates mimicking the interior structure of AG, in contrast, have played an essential role in characterizing glycosyltransferases involved in AG biosynthesis − and, most recently, an exo-β- d -galactofuranose hydrolase involved in AG remodeling . In this study, membrane preparations from Mycobacterium smegmatis ( Msmg ) overexpressing aftA and synthetic galactan substrates of different lengths were used to determine the number and position of the priming Ara f units transferred by AftA.…”

Progress and challenges in the synthesis of sequence controlled polysaccharides

“…The migration could be reduced by using morpholine, instead of piperidine, during the Fmoc deprotection permitting the synthesis of a 7mer upon stepwise chain elongation [ 358 – 359 ]. Other approaches employing Lev as a temporary PG yielded a 14mer galactan [ 361 ]. Interestingly, while a good reactivity of acceptor 136 was observed, the poor reactivity of acceptor 137 hindered the stepwise synthesis of the linear backbone, already at the 3mer stage ( Figure 14 ).…”

Automated Glycan Assembly of Oligogalactofuranosides Reveals the Influence of Protecting Groups on Oligosaccharide Stability

“… 6 Solution phase syntheses of galctofuranosyl oligomers ranging from 4 to 12 Gal f residues have been reported. 7 A stepwise synthesis of a galactan tetramer revealed structural constraints in the trisaccharide nucleophile that resulted in drastically reduced reactivity. Therefore, a “nonreducing to reducing end” strategy relying on monosaccharide nucleophiles was employed, to prepare a tetrasaccharide galactan.…”

“…Well-defined synthetic galactofuranosides that resemble the interior portion of AG are necessary to establish structure–activity relationships for these carbohydrates . Solution phase syntheses of galctofuranosyl oligomers ranging from 4 to 12 Gal f residues have been reported . A stepwise synthesis of a galactan tetramer revealed structural constraints in the trisaccharide nucleophile that resulted in drastically reduced reactivity.…”