Glycoproteins have carbohydrate attached to them — a process called glycosylation.The attachment is a covalent linkage to:
Sugars are very hydrophilic thanks to their many -OH groups. Their presence
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.
Two polymorphic versions of glycophorin A, which differ only at residues 1 and 5, occur in humans.These give rise to the
Glycophorin A is the most important attachment site by which the parasite Plasmodium falciparum invades human red blood cells.
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.