Glycoconjugates
Many membrane proteins and certain classes of membrane lipids have more or less complex arrays of covalently attached oligosaccharides; these are glycoproteins and glycolipids, which are components of extracellular matrix and serve as information system between the cells.
Some glycoproteins have only one or a few carbohydrate group; others have numerous oligosaccharides side chains, which may be linear or branched, which are covalently attached to residues near the amino terminus of the polypeptide chains. Oligosaccharide units are O-linked to Ser or Thr side chains, and can also be N-linked when it happens to an Asn residue. These two are basic types of linkages in glycoproteins.
The carbohydrate chains covalently attached to glycoproteins may change the way the protein is formed, changing its structure at time it is attached.
Some lipids also contain covalently bound oligosaccharide chains. In gangliosides, the polar head group is a complex oligosaccharide containing sialic acid and other monosaccharide units. Lipopolysaccharides are major components of the outer membrane of gram-negative bacteria such as Escherichia coli and Salmonella typhimurium.