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Meaning of Glycoprotein
The cell membrane contains proteins that are free to float within or close to the membrane. They may move and engage with the surroundings of the cell. In science, the prefix “glyco” stands for “sugar.”
Any protein molecule with a carbohydrate attached is known as a glycoprotein. The protein’s polypeptide side chains are covalently joined to the carbohydrate, an oligosaccharide chain (glycan). Either the process takes place during protein translation, or it occurs post-translationally via glycosylation.
Glycoconjugates are formed when carbohydrates are linked to proteins and lipids. They exist in three forms: glycoproteins, glycolipids and proteoglycans. Glycoproteins are formed when the protein component predominates in the combination of carbohydrates and proteins. It is referred to as a proteoglycan if the association comprises more carbohydrates than proteins. Glycolipids are formed when a carbohydrate combines with lipids.
The primary site of glycoprotein and glycolipid synthesis is the Golgi apparatus.
Structure
A sugar component (glyco) linked to a protein describes the structure of glycoproteins. Covalent bonds are used to bind the two components together.
Glycoproteins have higher hydrophilicity than simple proteins due to the -OH groups of sugars. This implies that compared to other proteins, glycoproteins are more drawn towards water. The hydrophilic properties of the molecule also result in the distinctive folding of the tertiary structure of the protein.
N-Linked Glycoproteins and O-Linked
Based on where the carbohydrate attaches to an amino acid in the protein, glycoproteins are divided into different groups.
A carbohydrate is attached to the nitrogen (N) of the amino group (-NH2) of the R group of the amino acid asparagine in N-linked glycoproteins. The amide side chain of asparagine often serves as the R group. The process of bonding is known as N-glycosylation. The endoplasmic reticulum (ER) membrane provides sugar to N-linked glycoproteins, which are then transferred to the Golgi complex for processing.
O-linked glycoproteins are those in which the carbohydrate forms a chemical bond with the hydroxyl group (-OH) of either the R group of the amino acid threonine or the R group of the amino acid serine. A hydroxylysine or hydroxyproline molecule can also form a connection with O-linked carbohydrates. The action is known as O-glycosylation. In the Golgi complex, sugar is bound to O-linked glycoproteins.
Functions
Nearly all cellular processes involve glycoproteins. They play various roles in our body, including those related to our immune systems, physical protection, cell-to-cell communication, and reproductive systems.
- Glycoproteins are present on the lipid bilayer of cell membranes. They can operate in the aqueous environment due to their hydrophilic character, which plays a role in chemical bonding and cell-cell recognition.
- Cell surface glycoproteins are crucial for cross-linking proteins (such as collagen) and cells to strengthen and stabilise a tissue.
- Plants can resist gravity because of glycoproteins found in their cells.
- White blood cells guard the blood arteries as they search for prospective invaders. They use lectin-type glycoproteins to adhere to the blood vessel lining.
- Glycoproteins are present in the grey matter of the brain, where they collaborate with synaptosomes and axons.
- The glycoproteins thrombin, prothrombin, and fibrinogen are necessary for blood coagulation.
- Red blood cells also depend on glycoproteins for their function. The type of glycoprotein on human red blood cells is referred to as the blood type. Red blood cells with type A blood have A antigens or A glycoproteins. As a result, the body learns that the blood is a component of oneself and is instructed not to fight it.
- Due to their ability to facilitate sperm cell attachment to the egg’s surface, glycoproteins are essential for reproduction.
- Glycoproteins called mucins are present in the mucus. The molecules protect delicate epithelial surfaces in the digestive, reproductive, urinary, and respiratory tracts.
- Glycoproteins support the immunological response. The specific antigen to which an antibody (or glycoproteins) can bind depends on the carbohydrate it contains. Surface glycoproteins on B and T cells also bind antigens.
- Glycoproteins also maintain the health of our skin. The epithelial cells that form skin have glycoproteins on their surface—these aid in bonding the skin cells in our bodies, creating a strong barrier to protect them.
- Another glycoprotein that helps in the stability of human skin is cadherin.
Examples
There are numerous uses for the distinctive interaction between the oligosaccharide chains. Different varieties of glycoproteins with various structures and activities can be produced due to the diversity of interactions.
White blood cell identification depends on glycoproteins. The immune system uses a variety of glycoproteins, including,
- Molecules that directly interact with antigens, such as antibodies (immunoglobulins).
- Major histocompatibility complex or MHC molecules, which interact with T cells as a component of the adaptive immune response, are molecules expressed on the surface of cells.
- Blood compatibility antigen H of the ABO blood type is another example of glycoprotein. A few additional examples of glycoproteins are:
- Gonadotropins (luteinizing hormone).
- Components of the zona pellucida, which protects the oocyte and is essential for sperm-egg interaction.
- Connective tissue also contains structural glycoproteins, facilitating the interaction between the connective tissue’s fibres and ground substance.
- Soluble glycoproteins often exhibit a high viscosity in blood plasma and egg white.
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