Glycoconjugates are simply molecules which have a carbohydrate bonded to a fat or protein. When bonded to a fat, glycoconjugates are called ‘glycolipids’. When carbohydrates are bonded to proteins, they are called ‘glycoproteins’.
A glycoprotein is a compound containing carbohydrate (or glycans) covalently linked to protein. The carbohydrate may be in the form of a monosaccharide, disaccharide(s),oligosaccharide(s), polysaccharide(s), or their derivatives (e.g. sulfo- or phospho-substituted).
The monosaccharide or oligosaccharide units are referred to as the glycone) and are are covalently linked to a noncarbohydrate moiety, the aglycone.
One, a few, or many carbohydrate units may be present. Proteoglycans are a subclass of glycoproteins in which the carbohydrate units are polysaccharides that contain amino sugars. Such polysaccharides are also known as glycosaminoglycans.
Common monosaccharides found in higher animal oligosaccharides
- Sialic Acids: Family of nine-carbon acidic sugars (generic abbreviation is Sia), of which the most common is N-acetyl neuraminic acid (Neu5Ac, also sometimes called NeuNAc, NeuAc, or NANA) (for more details, see Chapter 15).
- Hexoses: Six-carbon neutral sugars, including glucose (Glc), galactose (Gal), mannose (Man).
- Hexosamines: Hexose with an amino group at the 2-position, which can be either free or, more commonly, N-acetylated: N-acetylglucosamine (GlcNAc) and N-acetylgalactosamine (GalNAc).
- Deoxyhexoses: Six-carbon neutral sugar without the hydroxyl group at the 6-position, fucose (Fuc).
- Pentoses: Five-carbon sugar, xylose? (Xyl).
- Uronic Acids: Hexose with a negatively charged carboxylate at the 6-position, glucuronic acid (GlcA) and iduronic acid (IdA).
Glycoconjugates as glycoproteins and glycolipids play an essential role in many physiological processes. Consequently they are potential candidates for the development of new drugs. Important glycoproteins for the pharmaceutical research and development are for example tumor markers, enzymes, receptors, cytokines or antibodies.
In naturally occurring glycoconjugates, the portion of the molecule comprising the glycans can vary greatly, from being very minor in amount to being the dominant component. Indeed, it is striking that sugar chains make up a substantial portion of the mass of most glycoconjugates. For this reason, the surfaces of most types of cells (which are heavily decorated with different kinds of glycoconjugates) are effectively covered with a dense coating of sugars, giving rise to the so-called glycocalyx.