Glycoproteins

 
Glycoproteins are proteins that have covalently attached sugar residues. One of the common linkages between the sugar(s) and the protein is an N-glycosidic likage between the -OH group of the sugar at C1 and the side chain of a an asparagine residue in the protein. The linkage is called an N-glycosidic linkage and the asparagine residue is part of a specific sequence within the protein where sugars will be attached.

The sugar residue shown here is a modified form of glucose called N-acetyl glucosamine or GlcNAc. Other kinds of sugars can be attached to proteins. Most of them are modified versions of the standard carbohydrates. Another example is N-acetylgalactosamine or GalNac (see Monday's Molecule #14).

As a general rule, a bunch of these sugars are strung together to form an oligosaccharide chain (see below) and it's this long chain that's attached to the protein to form a glycoprotein.


Glycoproteins are usually secreted proteins that normally function outside the cell. One of the roles of the attached sugars is to stabilize the folded protein in an exterior environment and another role is to protect the protein against degradation by shielding it from enzymes that degrade proteins.

Most secreted glycoproteins have a specific kind of polysaccharide decoration that's added by specific enzymes in the lumen of the endoplasmic reticulum. (Recall that secreted proteins are imported into the ER were they are then targeted for secretion though small vesicles that carry them to the cell membrane.) An example of a typical oligosaccharide chain is shown below. The common part, called the "core", is shown in red. Note that there are many different kinds of sugars and the oligosaccharide can have branches. The strange looking code (e.g., β-(1→4)) describes the specific type of linkage between sugar residues.


Many secreted glycoproteins are inserted into the outer membrane of the cell. This results in a cell surface that bristles with a protective covering of complex carbohydrates.