Glycoproteins

Glycoproteins have carbohydrate attached to them.

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 sugars and nitrogen-containing amino sugars.

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 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.

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3 August 2005