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Glycoprotein Definition

Polymer proteins that are attached covalently to carbohydrates are glycoproteins. During protein synthesis, the carbohydrate component of the protein is added. Sometimes, however, protein is combined with carbohydrate components as the protein continues to grow. It is therefore clear that glycoproteins are considered to have a major biological function involving carbohydrates. Glycoproteins contain carbohydrates called oligosaccharides, which are polymers composed of 3 to 10 monosaccharides. Liposaccharides are rarely found free in cells. The N- and O-linked amino acids are usually attached to the proteins. Oligosaccharides are formed by several types of sugar in humans. This includes:

  • Glucose, galactose, and mannose are hexoses.

  • Fucoses are deoxyhexose.

  • N-acetyl neuraminic acid, similar to sialic acid.

  • Glucosamine and galactosamine are amino hexoses.

  • Xylose-like pentoses.

Glycoprotein Structure

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Glycoprotein structures have been described by their name - a sugar portion (glycol) attached to a protein. They are covalently bonded together.

Carbohydrates (sugar) are commonly referred to as glycans in the glycoprotein structures. A glycoprotein can have between one and eighty percent oligosaccharides, known as glycan. Sugar molecules that are glycosylated before binding to proteins are called glycosyl groups.

Having the hydroxyl group (-OH) removed from a monosaccharide will result in a glycosyl group with an empty spot, which will make it unstable. A total of thirteen monosaccharides are currently attached to eight different amino acids. To make an unstable monosaccharide stable again, these attachments are needed. Glycosyl groups are replaced by hydroxyl groups in amino acids associated with them.

In glycosylation, glycosyl groups are added to peptide chains, proteins, or lipids. Human body proteins are glycosylated to more than half. Cellular glycosylation takes place in the Golgi and endoplasmic reticulum of eukaryotic cells.

Glycoprotein Function

Glycoproteins work to structure and generate cells, reproduce, regulate the immune system, produce hormones, and protect organisms.

Membrane lipid bilayers are coated with glycoproteins. They can function in the aqueous environment because of their hydrophilic nature, which enables them to act in the recognition of cell-cell and binding of other molecules. A tissue's stability and strength are enhanced by crosslinking cell surface glycoproteins (e.g., collagen). A plant's ability to stand upright against gravity is due to glycoproteins found in its cells.

Intercellular communication is not the only important function of glycosylated proteins. The proteins also facilitate communication between organ systems. Gray matter in the brain contains glycoproteins, which work in conjunction with axons and synaptosomes.

Glycoprotein Examples

There are too many examples of glycoprotein functions to list them all. Different species of glycoproteins have distinct characteristics; rat glycoproteins are very different from those in mice. The following article discusses just the glycoprotein function of the most important human glycoproteins.

  • Serum Glycoprotein: The serum glycoproteome is composed of hundreds of glycoproteins that exist in our blood plasma. A simple blood test can be used to determine one or more of these glycoproteins. 

  • Zona Pellucida Glycoprotein: We are unable to reproduce without glycoproteins. A sperm cell's ability to bind to and move into released, unfertilized eggs is controlled by the zona pellucida that surrounds human egg cells.

  • Cartilage Glycoprotein: Excess levels of human cartilage glycoprotein YKL-40 cause cartilage remodelling to fail during osteoarthritis.

  • Mucin-Type Glycoprotein: Mucin is an O-linked oligosaccharide found in airways, digestive systems, sweat glands, and cancer cells, among others. The mucin substance surrounds tissues and cells with a protective barrier.

  • Glycoprotein Hormones: These three hormone glycoproteins are follicle-stimulating hormone, luteinizing hormone, and thyroid-stimulating hormone, just to name a few.

  • Immune Glycoprotein: Galectins, C-type lectins, and singles are some of the receptors our immune system uses to recognize surface membrane glycoproteins.

  • Glycoprotein IIb IIIa Inhibitors: Inhibitors of glycoprotein IIb IIIa are also related to the blood clotting process. As a result of their blocking glycoprotein IIb and IIIa receptors, platelets cannot join together to form a clot. Some types of coronary artery procedures use medicines based on this principle to prevent blood clots.

Glycoprotein Hormones

  • The name of the hormones are:

  • Follicle-stimulating hormone

  • Luteinizing hormone

  • Thyroid-stimulating hormone

  • Human chorionic gona

Glycoprotein and Glycolipid

The main difference between glycoprotein and glycolipid is that glycolipids are lipids that are attached to carbohydrate molecules, whereas glycoproteins are proteins that are attached to carbohydrate molecules. Additionally, glycolipids serve as cell markers or antigens that the immune system recognizes as self or non-self, while glycoproteins serve as receptors for chemical signals and play a role in adhesion.

Glycoprotein and glycolipids are two types of molecules mainly found in the cell membrane. Glycolipids and glycoproteins play several important roles in the cell.

Do You Know?

The followings are functions of Glycolipids:

  1. Cells receive energy from it.

  2. Cell membranes are made up of phospholipids.

  3. It helps determine a person's blood type.

  4. They serve as receptors on the surface of red blood cells.

  5. The immune system, the last function of Glycolipids,  is also assisted by destroying and eliminating pathogens from the body.

Some Types of Glycolipids Examples Are:

  • Glyceroglycolipids

  • Galactolipids

  • Sulfolipids

  • Cerebrosides

  • Glucocerebrosides

  • Sulfatides

  • Globosides

Last updated date: 27th Sep 2023
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FAQs on Glycoprotein

1. Do you think that antibodies are glycoproteins?

An antibody is a glycoprotein molecule produced by plasma cells (white blood cells). In the immune system, antibodies recognize and bind to specific antigens, such as bacteria or viruses, and aid in their destruction. The immune system's immune response to antibodies is extremely specific and complex. As a result, antibodies are complex glycoproteins. Any immunochemical technique works on the premise that a certain antibody will combine with an exclusive antigen to form an antibody-antigen complex.

2. What is the difference between glycation and glycosylation?

Between glycation and glycosylation, the main difference is that glycation cannot be enzymatic while glycosylation can.

Sugar molecules are added to proteins via glycation and glycosylation. Glycation occurs spontaneously in the bloodstream and is a non-enzymatic process of adding free sugars to proteins in a covalent way. The glycosylation process takes place within the endoplasmic reticulum and the Golgi apparatus after a protein has been translated and is ready for use. Although glycation and glycosylation have some similarities, this article focuses on the differences between the two.

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