Glycoproteins

Functionally significant covalent complexes of polypeptide and heterosaccharide chains that are widely distributed in nature. Among human glycoproteins, only 8 sugars play a significant role in the composition of the molecules: fucose (Fuc), glucose (Glc), galactose (Gal), xylose (Xyl), mannose (Man), N-acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc) and N-acetylneuraminic acid (Neu5Ac).

Glycoproteins include numerous enzymes, most proteohormones, plasma proteins, antibodies (e.g. antibodies and the proteins of the MHC system that react with T cells or T cell receptors), complement factors, membrane proteins, lectins, blood group and mucilage substances.

In the case of N-glycosidically bound glycoproteins (they are proportionally by far the most common group), the sugar is bound to the nitrogen of the free acid amide group of asparagine. A large part of plasma proteins belong to this group, but also of membrane-bound proteins.

Due to their high viscosity, glycoproteins have a lubricating and protective function (e.g. against proteolytically active enzymes), inhibitor functions against bacteria and viruses. They are jointly responsible for cellular adhesion and contact inhibition during cell growth.

Glycoproteins are (co-)responsible for the cellular recognition of foreign tissue and the antigenicity of tumour cells. In most cases, the carbohydrate part of the molecule plays the decisive role in these immunological recognition signals.