Most membrane carbohydrates are associated with proteins. They are linked to proteins by specific bonds. How to define glycoproteins? What is their function ? and their role?

A glycoprotein is a molecule that is composed by the bond between a protein (set of amino acids) and a carbohydrate (sugar) which is an oside (one of the two main classes of carbohydrates). After a chemical reaction (hydrolysis), the osides provide one or more oses, they are simple sugars for which no chemical reaction is possible. Glycoproteins are more soluble in water thanks to carbohydrates, and help them against chemicals and infections.

The term glycoprotein (or glucoprotein) usually refers to a molecule having a specific size.

A glycoprotein is a heteroprotein, part of which is formed from one or more oligosaccharides covalently attached to side chains characteristic of polypeptides.

Also called “gp”, it is therefore a compound originating from the grouping of a protein and a carbohydrate prosthetic group.

Glycoproteins exist in all types of organisms, are very numerous and their functions are very diverse. They are found in all tissues and body fluids.

A glycoprotein is synthesized after the glycosylation of a protein. Glycosylation is an enzymatic reaction of covalently (solid) binding a carbohydrate to a peptide chain or protein.

It is present in all cells and is also found in bacteria.

There are two types of glycosylation:

• N glycosylation: sugars anchored on the nitrogen of the amide group of asparagine. The chains linked in N are longer and more persevering. N strings can be really very long;
• O glycosylation: sugars anchored on the oxygen of the hydroxyl group of serine, threonine, and hydroxylysine. O-linked chains are shorter and more variable and most often contain only one, two, or three carbohydrate residues.

The ABO blood group system is a system that allows, thanks to the presence of antigenic structures on the surface of these cells, to recognize red blood cells foreign to the body.

ABO antigens are made up of glycans (complex of carbohydrates) attached to proteins (glycoproteins) or membrane lipids (glycolipids).

The bond is of the O-glycosidic type, in the glycoproteins.

Viruses and other pathogens have changed in order to identify sugars present on the surface of cells and use it as entry points:

• The HIV virus enters cells of the immune system by attaching to receptors which are glycoproteins;
• The membrane glycoprotein “Dystroglycan” is a component of “dystrophin”. Two proteins, a-DG and b-DG, come from the dystroglycan gene: a-DG is the bacteria responsible for leprosy, as well as arenaviruses causing hemorrhagic fevers;
• Rhinoviruses, which cause the common cold, use the glycoprotein “ICAM-1”;
• The rabies virus attaches to the glycoprotein “N-CAM”;
• Malaria (malarial agent Plasmodium vivax) uses the “interleukin 8” receptor to enter red blood cells;
• Plasmodium falciparum, one of the forms of malaria, uses the sugars carried by glycophorin to the surface of red blood cells. Glycophorin also serves as an entry point for the influenza virus.

The pituitary hormones LH and FSH and TSH and the hormone HCG: glycoproteins composed by the bringing together of two polypeptide chains.

• The hormones LH and FSH help the proper functioning of the male reproductive system, the hormone TSH stimulates the thyroid function in women;
• The hormone HCG stimulates the corpus luteum during early pregnancy and, in the male fetus, stimulates the production of testosterone by the testes;
• Plasma glycoproteins: the concentration of orosomucoid increases in the event of acute inflammation, in particular when this is of infectious origin, and decreases in the event of hepatic insufficiency or nephrotic syndrome;
• Ovalbumin is a glycoprotein found in egg white;
• Lectins are glycoproteins that are found in high concentrations in most dried legume seeds (lentils, beans, peas), but also more widely in nature. Lectins are involved in recognition between cells (immune responses, infections). Lectins are used in hematology for the determination of blood groups.