Abstract
Glycan structures in non-vertebrates are highly variable; it can be assumed that this is a product of evolution and speciation, not that it is just a random event. However, in animals and protists, there is a relatively limited repertoire of around ten monosaccharide building blocks, most of which are neutral in terms of charge. While two monosaccharide types in eukaryotes (hexuronic and sialic acids) are anionic, there are a number of organic or inorganic modifications of glycans such as sulphate, pyruvate, phosphate, phosphorylcholine, phosphoethanolamine and aminoethylphosphonate that also confer a ‘charged’ nature (either anionic or zwitterionic) to glycoconjugate structures. These alter the physicochemical properties of the glycans to which they are attached, change their ionisation when analysing them by mass spectrometry and result in different interactions with protein receptors. Here, we focus on N-glycans carrying anionic and zwitterionic modifications in protists and invertebrates, but make some reference to O-glycans, glycolipids and glycosaminoglycans which also contain such moieties. The conclusion is that ‘charged’ glycoconjugates are a widespread, but easily overlooked, feature of ‘lower’ organisms.
Highlights
Glycans modify a range of proteins and lipids, thereby altering their properties and their interactions
While sialic and hexuronic acids are well-known in vertebrates as components of glycosaminoglycans, glycoproteins and glycolipids [2,3,4], sulphates are widespread on glycans of many species [5, 6]
Phosphates are known in the context of the mannose-6-phosphate marker for trafficking of lysosomal enzymes [7], but phosphodiesters are less common: exceptions being that phosphoethanolamine is a component of glycosylphosphatidylinositol anchors [8] and ribitol phosphodiesters are part of the modification of dystroglycan [9]
Summary
Glycans modify a range of proteins and lipids, thereby altering their properties and their interactions. The enzymatic basis for addition of these moieties is far from resolved, but conceivably CDPcholine/ CDP-ethanolamine or phosphatidylcholine/ phosphatidylethanolamine could be the donor substrates for PC/PEtransferases [132,133,134] Both phosphorylcholine and phosphoethanolamine are ligands for pentraxins (e.g., human C-reactive protein and serum amyloid P), which are components of the innate immune system [135]; at least phosphorylcholine modifications on glycolipids and glycoproteins are associated with immunomodulatory activity, as especially shown for the Acanthocheilonema viteaeES-62 protein, which is an excretory-secretory protein of a rodent nematode parasite, whereby the Toll-like receptor 4 (TLR4) may be involved in the relevant signalling pathway [136]. Methylaminoethylphosphonate and aminoethylphosphonate were detected on N-glycans as well as O-fucose-based structures from a marine snail [20], while there is evidence that oligomannosidic glycans from Euglena gracilis, a free-living protist, carry aminoethylphosphonate [145] It seems that either mannose or GlcNAc residues can be modified by these phosphonates; like phosphorylcholine and phosphoethanolamine, these can be cleaved by hydrofluoric acid. Abbreviated compositions of the form HxNyF0–1 U0–1 correspond to HexxHexNAcyFuc0–1HexA0–1; MEAP (or *), methylaminoethylphosphonate; PC, phosphorylcholine; PE, phosphoethanolamine; S, sulphate
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