Glycoproteins have carbohydrate attached to them — a process called glycosylation. The attachment is a covalent linkage to:
- the hydroxyl (-OH) group of the R group of serine or threonine - called "O-linked" in both cases or to
- the amino group (-NH2) in the R group of asparagine - called "N-linked"
The carbohydrate consists of short, usually branched, chains of
- plain sugars (e.g., glucose, galactose)
- amino sugars (sugars with an amino group, e.g., N-acetylglucosamine), and
- acidic sugars (sugars with a carboxyl group, e.g., sialic acid)
Sugars are very hydrophilic thanks to their many -OH groups. Their presence
- makes glycoproteins far more hydrophilic than they would be otherwise and
- are often essential for the proper folding of the protein into its tertiary structure
Most of the proteins exposed to the watery surroundings at the surface of cells are glycoproteins.
This image shows the primary structure of glycophorin A, a glycoprotein that spans the plasma membrane ("Lipid bilayer") of human red blood cells. Each RBC has some 500,000 copies of the molecule embedded in its plasma membrane.
- Fifteen carbohydrate chains are "O-linked" to serine (Ser) and threonine (Thr) residues
- One carbohydrate chain is "N-linked" to the asparagine (Asn) at position 26
Two polymorphic versions of glycophorin A, which differ only at residues 1 and 5, occur in humans. These give rise to the MN blood groups
- The M allele encodes Ser at position 1 (Ser-1) and Gly at position 5 (Gly-5)
- The N allele encodes Leu-1 and Glu-5
Genotype to Phenotype
Individuals who inherit two N alleles (are homozygous) have blood group N.
- Individuals who are homozygous for the M allele have blood group M.
- Heterozygous individuals produce both proteins and have blood group MN.
Glycophorin A is the most important attachment site by which the parasite Plasmodium falciparum invades human red blood cells.
Glycosylation versus Glycation
The sugars on glycoproteins have been placed there by glycosylation — a precise enzymatic activity that makes a product which would otherwise not function correctly. However, sugars can also spontaneously form covalent links to proteins (and lipids) — a process called glycation. No enzymes are involved so the process is quite random. The products are apt to have reduced, or even no, function. The glycation of proteins and lipids is an inevitable outcome of aging. It is hastened in diabetics with their high blood sugar (glucose) levels. In fact, measuring the amount of glycation of hemoglobin is an important test for determining how well diabetes is being controlled.