Sulfation:
So far sulfate residues have been found attached to GlcNAc (6-OH and 4-OH) and Man (6-OH).
Posttranslational sulfation at the 6- or 4-hydroxyl group of GlcNAc has been thought of as a marker for the clearance of glycoproteins (mainly glycoprotein hormones) from blood plasma (Lis 1993; Drickamer 1991). Other functions seem to be the regulation of secretion of certain glycoproteins and protection against proteolysis. (Hoogewerf 1991) For general information about sulfated oligosaccharides see also Varki 1988.
Sulfates might be involved in ionic interactions as sialic acid residues probably are.
Phosphorylation
Besides the commonly occuring phosphorylated amino acid residues in proteins, phosphomono- and -diesters have been found attached to sugar residues of the oligosaccharide sidechains. (Varki 1988) One of the well known examples for the function of phosphate esters is the targeting of lysosomal enzymes where mannose residues are phosphorylated at the C-6 hydroxyl group.
Acetylation of sialic acid besides the N-acetyl group.
Acetylation of N-acetylneuraminic acid (Neu5Ac) can occur at the C-4 hydroxyl and the C-7 to C-9 hydroxyl groups. It has been shown that the neuraminidase from vibrio cholerae is inhibited by an acetyl group at linked to the C-4 oxygen. Acetylation is in this case thought to possibly provide a protection against pathogens using neuraminidases for attaching themselves to cells prior to entering. Another role of the acetyl groups might be to provide a hydrophobic group on an otherwise hydrophilic sugar for certain interactions with other molecules.
Written by: Christian Frosch
frosch@mzdmza.zdv.uni-mainz.de
http://www.uni-mainz.de/~frosc000
Institute of Toxicology
University of Mainz
Last update: 11.07.1995