2.12: Glycoproteins

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.

A chain of labeled amino-acid circles runs from the N-terminal end outside the cell, through the lipid bilayer, to the C-terminal end inside. Residue numbers mark positions along the chain. The key identifies blue O-linked carbohydrate symbols attached to orange Ser or Thr residues, and green N-linked carbohydrate attached to a dark blue Asn residue. These carbohydrate attachments occur on the outside portion; the membrane-spanning and inside portions have none shown. The bilayer is drawn as two rows of heads with tails facing inward.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