The glycoside is formed by the union of a sugar molecule with another non - carbohydrate, we call aglycone.
The main types of association between carbohydrates and other types of molecules are:
- Protein-bound carbohydrates: peptidoglycans, proteoglycans, and glycoproteins.
- Carbohydrates bound to lipids: glycolipids and lipopolysaccharides.
The proteoglycans are molecules made up of a large fraction of polysaccharide (80% of the molecule), called glycosaminoglycan, and a small protein fraction (approximately 20%).
Glycosaminoglycan chains are made up of linear polymers of N-acetylglucosamine (or N-acetylgalactosamine) and glucuronic acid. The most common are:
- The hyaluronic acid (in connective tissue, vitreous humor or synovial fluids).
- The chondroitin sulfate (woven bone and cartilage).
- The heparin (anticoagulant in lung, liver and skin).
The glycoproteins are molecules formed by a small carbohydrate fraction (between 5% and 40%) and a protein fraction, which are joined by strong bonds (covalent).
They differ from proteoglycans in that the carbohydrate part contains neither hyaluronic acid nor chondroitin sulfates.
The main ones are: secretory mucins, such as salivary ones; the glycoproteins of blood, such as prothrombin and immunoglobulins; the gonadotropic hormones; some ribonuclease enzymes and the so-called glycoproteins of cell membranes.
The glycolipid consist of monosaccharides or oligosaccharides bound to lipids (ceramide). They are generally found in the cell membrane. The best known are:
- The cerebrosides are glycolipids in which ceramide is attached to a monosaccharide or an oligosaccharide.
- The gangliosides are glycolipids in which ceramide binds to an oligosaccharide complex that always appears sialic acid.