glycoprotein

Definition: Glycoproteins are proteins that contain oligosaccharide chains (glycans) covalently attached to their polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as protein glycosylation.

In proteins that have segments extending extracellularly, the extracellular segments are often glycosylated.

Glycoproteins are often important integral membrane proteins, where they play a role in cell-cell interactions. Glycoproteins also occur in the cytosol, but their functions and the pathways producing these modifications in this compartment are less well-understood.

N-glycosylation and O-glycosylation

There are two types of glycosylation:

 In N-glycosylation, the addition of sugar chains can happen at the amide nitrogen on the side chain of the asparagine.

 In O-glycosylation, the addition of sugar chains can happen on the hydroxyl oxygen on the side chain of hydroxylysine, hydroxyproline, serine, or threonine.

Functions

 structural glycoproteins

• occur in connective tissue. These help bind together the fibers, cells, and ground substance of connective tissue. They may also help components of the tissue bind to inorganic substances, such as calcium in bone.
• examples: collagens

 Lubricant and protective agent: mucins
* The mucins are secreted in the mucus of the respiratory and digestive tracts. The sugars attached to mucins give them considerable water-holding capacity and also make them resistant to proteolysis by digestive enzymes.

 transport molecules

• transferrin
• ceruloplasmin

 immunologic molecule

• immunoglobulins, which interact directly with antigens
• major histocompatibility complex proteins (or MHC proteins), which are expressed on the surface of cells and interact with T cells as part of the adaptive immune response.

 enzymes

• alkaline phosphatase

 adhesion proteins (cell attachment-recognition site)

• glycoprotein IIb/IIIa, an integrin found on platelets that is required for normal platelet aggregation and adherence to the endothelium.
• components of the zona pellucida, which surrounds the oocyte and is important for sperm-egg interaction
• cell-cell interaction (eg, sperm-oocyte interaction)
• virus-cell interaction
• bacterium-cell interaction
• hormone-cell interaction

 proteic hormones

• follicle-stimulating hormone (FSH)
• luteinizing hormone (LH)
• thyroid-stimulating hormone (TSH)
• human chorionic gonadotropin (HCG)
• alpha-fetoprotein (AFP)
• erythropoietin (EPO)

 HIV viral coat proteins.

• glycoprotein-41 (gp41)
• glycoprotein-120 (gp120)

 soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.

 interaction with specific carbohydrates

• lectins
• selectins (cell adhesion lectins)
• antibodies

 receptors

• various proteins involved in hormone and drug action

 folding of certain proteins

• calnexin
• calreticulin

 regulation of development

• Notch and its analogs

 hemostasis (and thrombosis)
 specific glycoproteins on the surface membranes of platelets