glycoprotein

 A glycoprotein is a type of protein molecule that has had a carbohydrate attached to it. 

The carbohydrate is an oligosaccharide chain (glycan) that is covalently bonded to the polypeptide side chains of the protein. Because of the -OH groups of sugars, glycoproteins are more hydrophilic than simple proteins. This means glycoproteins are more attracted to water than ordinary proteins. 

Glycoprotein Examples and Functions

Glycoproteins function in the structure, reproduction, immune system, hormones, and protection of cells and organisms.

Glycoproteins are found on the surface of the lipid bilayer of cell membranes. Their hydrophilic nature allows them to function in the aqueous environment, where they act in cell-cell recognition and binding of other molecules. Cell surface glycoproteins are also important for cross-linking cells and proteins (e.g., collagen) to add strength and stability to a tissue. 

Glycosylated proteins are not just critical for intercellular communication. They also help organ systems communicate with each other. Glycoproteins are found in brain gray matter, where they work together with axons and synaptosomes.

Hormones may be glycoproteins. Examples include human chorionic gonadotropin (HCG) and erythropoietin (EPO).

Blood clotting depends on the glycoproteins prothrombin, thrombin, and fibrinogen.

Cell markers may be glycoproteins. The MN blood groups are due to two polymorphic forms of the glycoprotein glycophorin A. The two forms differ only by two amino acid residues, yet that is enough to cause problems for persons receiving an organ donated by someone with a different blood group. The Major Histocompatibility Complex (MHC) and H antigen of the ABO blood group are distinguished by glycosylated proteins.

Glycophorin A is also important because it's the attachment site for Plasmodium falciparum, a human blood parasite.

Glycoproteins are important for reproduction because they allow for the binding of the sperm cell to the surface of the egg.

Mucins are glycoproteins found in mucus. The molecules protect sensitive epithelial surfaces, including the respiratory, urinary, digestive, and reproductive tracts.

The immune response relies on glycoproteins. The carbohydrate of antibodies (which are glycoproteins) determines the specific antigen it can bind. B cells and T cells have surface glycoproteins which bind antigens, as well.

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.

Glycosylation Versus Glycation

Glycoproteins get their sugar from an enzymatic process that forms a molecule that would not function otherwise. Another process, called glycation, covalently bonds sugars to proteins and lipids. Glycation is not an enzymatic process. Often, glycation reduces or negates the function of the affected molecule. Glycation naturally occurs during aging and is accelerated in diabetic patients with high glucose levels in their blood.