LEARNING OBJECTIVES
In this article, author has explained the introduction, occurrence, classification, functions and examples of glycoproteins.
Definition
Table of Contents
The proteins which are bounded to carbohydrates are known as glycoproteins.
The carbohydrate content of glycoproteins can be from 1% to 90% by weight. The oligosaccharides are covalently bounded to amino acids of proteins. Carbohydrates have an OH group. Due to this property, glycoproteins are more hydrophilic.
Occurrence
Cell membrane
Glycoproteins are found on the surface of the cell membrane of various eukaryotic and prokaryotic organisms. Being a part of the cell envelope they perform so many vital functions.
Plasma
Glycoproteins are also found in the plasma. Mucopolysaccharides, albumin, and embryo globulin are found in the plasma. These glycoproteins consist of galactose and maltose in their oligosaccharide chain. These plasma glycoproteins have many important functions.
Glycosylation
The process by which carbohydrates are covalently attached to the proteins is called glycosylation. Glycosylation starts in the endoplasmic reticulum in the case of a eukaryotic cell. Further modifications occur in the Golgi complex. Glycosylation is a post-translational modification that occurs after the formation of protein.
Classification of glycoproteins
Glycoproteins are classified based on the attachment pattern of carbohydrates to amino acids to proteins. There are two major categories of glycoproteins on the basis that glycosylation occurs. These are O-linked and N-linked glycoproteins.
1. O-linked glycoproteins
When carbohydrates are attached to the oxygen atom of the hydroxyl group of the R group of amino acids, these are called O-linked glycoproteins. OH-containing amino acids are serine and threonine. The process is known as O-glycosylation. O-linked carbohydrates are also bound to hydroxylysine or hydroxyproline. O-linked glycoproteins are founded in the Golgi complex.
2. N-linked glycoproteins
When carbohydrates are linked to the nitrogen atom of the amine group of amino acids, these carbohydrates are called N-linked amino acids. This process is called N-glycosylation. The sugar for N-linked glycoproteins comes from the endoplasmic reticulum and modification occurs in the Golgi complex.
3. P-linked glycoproteins
When carbohydrates attach to the phosphorus of the phosphoserine, they are called P-linked glycoproteins.
4. C-linked glycoproteins
When carbohydrates attach to the carbon atom of amino acid, they are called C-linked glycoproteins. Mannose attached to the carbon atom of tryptophan is an example of C glycosylation.
Functions of glycoproteins
Glycoprotein has so many functions including structure, reproduction, cell communication and protection of cells, etc.
- Prothrombin, thrombin, and fibrinogen are the glycoproteins that play role in blood clotting. They occur in blood plasma and are essential components of blood clotting.
- Glycoproteins are also found on the surface of lipid bilayer membranes. Glycoproteins are hydrophilic (water-loving) in nature. Due to this nature, glycoproteins can work in an aqueous environment. Glycoproteins are present on the surface and help in cell-cell signaling. They recognize the receptor and bind to them.
- Glycoproteins also play role in the communication of organs. They are found in the brain where they work with the axons. Glycoproteins are found in the grey matter of the brain.
- Many hormones are glycoproteins. For example, human chorionic gonadotropin and erythropoietin hormones are glycoproteins.
- Glycoproteins play role in reproduction. They allow the attachment of sperm to the surface of the egg.
- Glycoproteins play role in immunity. Antibodies that are immunity-producing substances have carbohydrates. These carbohydrates are glycoproteins. These glycoproteins interact with specific antigens. Glycoproteins are also present on the surface of B and T cells.
- Mucus has special substances which protect the urinary tract, respiratory tract, and digestive tract. These substances are called mucins. Mucins are glycoproteins in nature.
- Glycoproteins also act as cell biomarkers. For example, Glycophorins A and B. These biomarkers are used to group the human MN blood group system.
Examples of glycoproteins
- Collagen is a major structural glycoprotein.
- Hydrolases, proteases, and glycosidases enzymes are glycoproteins
- Ceruloplasmin which helps in transport is a glycoprotein.
- Immunoglobulins are glycoproteins. They defend against the infection.
- Synovial glycoproteins perform the function of lubrication.
- Thyrotropins and erythropoietin hormones are glycoproteins.
- Fibronectin and laminin are glycoproteins. They help in cell-cell recognition and adhesion.
- Fibrinogen plays role in blood clotting and is also a glycoprotein.