An introduction to the glycoproteins
The glycoprotein is a type of glycoconjugate. It is a biological molecule that contains proteins as well as carbohydrates (especially oligosaccharides) linked to each other through the covalent linkage between the sugar unit of oligosaccharide and a side chain of certain types of amino acids of protein moiety.
Based on the linkage between the carbohydrate moiety and the amino acid residue of the protein, there are two types of glycoproteins O-linked (ether linkage) or N-linked (amine linkage). Thus, the glycoproteins comprise two classes one is an N-linked glycoprotein and the next is an O-linked glycoprotein.
Generally in N-linked glycoproteins, an amino acid linked to the oligosaccharide is asparagine and it is the side chain amino group the argentine that is involved in amine linkage with the sugar moiety. While in the case of O-linked glycoproteins, the sugar residue of carbohydrate moiety is linked to the protein either through the O atom of the side chain of threonine or serine.
Glycosylation of the proteins
Glycosylation of the proteins to form a glycoprotein is necessary for different biological purposes. Glycosylation is a type of post-translational modification of the protein that occurs in the lumen of the endoplasmic reticulum. Glycosylated proteins are then transported to different cellular locations through the Golgi complex.
First, an oligosaccharide is synthesized by the enzyme-catalyzed continuous addition of different sugar residues (nucleotide-activated sugar residues) to the dolichol phosphate embedded in the membrane of the endoplasmic reticulum.
Once the oligosaccharide of an appropriate number of sugars is synthesized, it is then transferred from dolichol residue to the proteins coming from ribosomal synthesis and, thus, the glycoprotein is formed. Now it’s time to transport the protein.
Biological importance of the glycosylation
Glycosylation of protein is necessary because of its role in different biological processes as described below.
Importance of the glycosylation in protein targeting
Newly synthesized proteins in the ribosome are attached to the endoplasmic reticulum and need to be transported to different cellular locations. Some proteins are transported to the plasma membrane while some proteins are transported into the nucleus. Some proteins are transported into the extracellular matrix.
Thus, transportation of the protein requires a tag to be displayed on the protein so that it can be targeted in specific locations where it is required. The glycosylation of the protein acts as a tag for that particular protein so that proteins can be recognized by a specific receptor and then can be targeted to specific locations.
This phenomenon of transporting protein from the synthetic machinery to the target location is called protein targeting.
Importance of glycosylation in protection
Newly synthesized proteins also need to be protected from being degraded by the action of different hydrolytic enzymes. Thus, glycosylation of the proteins serves as a protection against proteolytic degradation.
The glycan (oligosaccharides) portion of the glycoproteins has glycosidic bonds. These glycosidic bonds are ether bonds that are quite resistant to hydrolytic enzymes and, therefore, act as a shield for the hydrolytic enzymes preventing their action. Thus, newly synthesized proteins are glycosylated to protect themselves from being degraded by the hydrolytic enzymes.
However, old proteins are designated for proteolytic degradation. As glycoproteins become old, they lose glycan portion, and once there is no glycan portion they are marked for degradation and eventually degraded to conserve the biomolecules.
Importance of the glycosylation in recognition and receptor function
Some glycoproteins (after glycosylation) acts as a recognition site and, thus, function as receptor located at the cell surface, such as G-protein-linked cell surface receptor. In this case, the protein portion of glycoproteins is embedded into the lipid bilayer membrane and the glycan (oligosaccharide) portion is located outward.
Once a particular type of signal comes in contact with the receptor proteins, it is recognized by the oligosaccharide portion and then the signaling cascade will start. Oligosaccharides bound to the protein, therefore, play an important role in recognition.
Importance of glycosylation in immune function
Most of the antibodies such as immunoglobulin are glycoproteins. Immunoglobulin is of Y-shaped globulin protein with two chains (Heavy chain and Light chain) attached to each other through the disulfide bonds.
At the tip of the variable region of the light as well as heavy chain, there is an oligosaccharide moiety of a certain length that is involved in the recognition of antigens. This is called an antigen-binding site. This oligosaccharide is linked to the immunoglobulin at its tip in the antigen-binding site, therefore, plays an important role in immunological functions.
