The sugar group(s) can assist in protein folding or improve proteins' stability.
Examples
One example of glycoproteins found in the body are mucins, which 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.
Glycoproteins are important for white blood cell recognition, especially in mammals.[citation needed] Examples of glycoproteins in the immune system are:
molecules such as antibodies (immunoglobulins), which interact directly with antigens
molecules of the major histocompatibility complex (or MHC), which are expressed on the surface of cells and interact with T cells as part of the adaptive immune response.
Other examples of glycoproteins include:
components of the zona pellucida, which surrounds the oocyte, and is important for sperm-egg interaction.
structural glycoproteins, which 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.
Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.
Specific glycoproteins on the surface membranes of platelets
Analysis
A variety of methods used in detection, purification, and structural analysis of glycoproteins are[4]
Some important methods used to study glycoproteins
Method
Use
Periodic acid-Schiff stain
Detects glycoproteins as pink bands after electrophoretic separation.
Incubation of cultured cells with glycoproteins as radioactive decay bands
Leads to detection of a radioactive sugar after electrophoretic separation.
Treatment with appropriate endo- or exoglycosidase or phospholipases
Resultant shifts in electrophoretic migration help distinguish among proteins with N-glycan, O-glycan, or GPI linkages and also between high mannose and complex N-glycans.