Chapter 10 - Modifications of Glycans and Glycoconjugates

Abstract

This chapter provides an overview of the occurrence, function, and biosynthesis of glycosylationally-modified carbohydrates that are modified by carboxylate, sulfate, and phosphate esters, by methylation, and by epimerization reactions. One of the simplest carbohydrate modifications is the epimerization of a single stereocenter in a sugar residue. Epimerizations may occur preglycosylationally at the level of the sugar nucleotide. Alternatively, epimerizations can occur after the synthesis of the glycan or glycoconjugate, as seen in the biosynthesis of certain polysaccharides that contain hexuronic acid residues. Epimerizations are involved in the biosynthesis of three major glycans: β-D-mannuronic acid (ManA) to α-L-guluronic acid (GulA) in alginate biosynthesis, and β-D-glucuronic acid to α-L-iduronic acid in heparin/heparan sulfate, and dermatan sulfate biosynthesis. There are other carbohydrate modifications that endow glycoconjugates with intriguing structural accents. These are either seen more rarely, or less is understood regarding their biological roles. For example, O-methylation is widespread and occurs through various carbohydrate O-methyltransferases that use S-methyladenosine as the methyl donor. O-Methylation typically occurs post- glycosylationally but has been observed preglycosylationally, at the level of the sugar nucleotide. O-methylation also results in large changes in hydrophobicity of the resultant methylated glycoconjugate. For example, some mycobacteria produce various 3-O-methylmannose-containing polysaccharides (MMP) and 6-O-methylglucose-containing LPSs that form stoichiometric complexes with C16- or longer acyl-CoA and that influence fatty acid biosynthesis in Mycobacterium smegmatis.