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Glycoproteins | Function & Examples

Haripriya Munipalli, Amanda Robb, Christianlly Cena
  • Author
    Haripriya Munipalli

    Haripriya Munipalli has taught botany and biochemistry to undergraduates for 7 years. She has M.Sc. in Plant Sciences degree from University of Hyderabad, India and Master of Philosophy degree from Annamalai University.

  • Instructor
    Amanda Robb

    Amanda has taught high school science for over 10 years. They have a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. They also are certified in secondary special education, biology, and physics in Massachusetts.

  • Expert Contributor
    Christianlly Cena

    Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

Learn about Glycoproteins. Explore the different types of glycoproteins, their names and their examples. Discover the function they do and their importance to the human body. Updated: 10/25/2021

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What are Glycoproteins?

Glycoproteins are the group of proteins in which the oligosaccharide chains are covalently attached to the amino acid side chains or polypeptide backbones. In various organisms, this process occurs through glycosylation. The type and number of sugar molecules attached to the glycosylation region on the protein differ based on the kind and function of a glycoprotein. This is possible because the proteins have several sites for glycosylation and form many glycosidic linkages.

Glycoprotein definition

Glycoproteins are polymer proteins that are covalently attached to carbohydrates. Glycosylation takes place after the protein is synthesized by the addition of a carbohydrate component. But in some cases, the protein is added with carbohydrate components, as the protein continues to grow. Hence, it is understood that carbohydrate is considered to be involved in the major biological function of the glycoprotein. The carbohydrates in glycoproteins are specifically called oligosaccharides, which are polymers that comprise 3 to 10 monosaccharides. Oligosaccharides generally do not exist freely in the cells. They are usually attached to the proteins with N-linkage or O-linkage. Several types of sugars are found to be forming oligosaccharides in human beings. These sugars include

  • Hexoses like mannose, galactose, and glucose.
  • Deoxyhexoses like fucose
  • Sialic acid-like N-acetyl neuraminic acid
  • Amino hexoses like N-acetyl glucosamine and N-acetyl galactosamine
  • Pentoses like xylose

Glycosylation

Glycosylation is a process in which the carbohydrate molecules are attached to the protein molecule resulting in a glycoprotein. In the cells of eukaryotes, this process begins from the endoplasmic reticulum and continues into the Golgi complex. Generally, glycoproteins are formed by the linking of carbohydrates with proteins and they are categorized based on the type of linkage. The oligosaccharides are attached to the proteins via certain specific amino acids like threonine, serine, and asparagine.

N-linked glycoprotein -- This class of glycoproteins is created through N-linked glycosylation. The carbohydrates are attached to the nitrogen of the side chain of the amino acid asparagine. The oligosaccharide that is linked with the asparagine amino acid of the protein first appears in the endoplasmic reticulum. The gathering begins with the binding of 2 N-acetyl glucosamine and 5 mannose residues to dolichol phosphate with the help of several cytoplasmic enzymes. These enzymes catalyze the movement of monosaccharides from the sugar nucleotides.

This newly created molecule moves from the membrane of the endoplasmic reticulum to its lumen where the sugar molecules are attached to it again. This entire process leads to the linking of a large oligosaccharide with the dolichol phosphate. The fully created carbohydrate moves to the region of the developing polypeptide chain so that it can get linked to the asparagine amino acid residue of the protein. This process involves the activated sugars to have specific recognition ability.

After the carbohydrate is attached to the asparagine residue of the protein, dolichol pyrophosphate is recreated as dolichol phosphate with the aid of enzyme phosphatase. This procedure is repeated with the addition of the oligosaccharide molecule. N-linked glycosylation is observed to be absent in prokaryotes and occurs in eukaryotes

N-linked glycosidic bond

N-linked glycosidic bond

O-linked glycoproteins -- These are created by the linking of oligosaccharides to the protein residues with the O-link glycosylation. The carbohydrate links to the oxygen atom of the side chain of the serine or threonine amino acid residue. The oligosaccharide molecule again prepares before getting attached to the protein having come from nucleotide precursors.

The oligosaccharide first enters the endoplasmic reticulum and then is linked to the hydroxyl group of the serine or threonine amino acid of the protein in the Golgi complex. This process is unique to O-linked glycosylation compared to N-linked glycosylation.

O-linked glycosidic bond

O-linked glycosidic bond

Overall Glycosylation -- Proteins are initially synthesized at the cytoplasmic side of the endoplasmic reticulum on ribosomes. After their synthesis, they move into the lumen of the endoplasmic reticulum. Here, the oligosaccharides are linked to the proteins via N-linkages. The N-linkage of oligosaccharides occurs in the lumen of the endoplasmic reticulum.

The proteins are transferred to the Golgi complex from the endoplasmic reticulum lumen where the N-linked sugars are modified again. Some of the proteins undergo O-linked glycosylation. When the two kinds of glycoproteins are modified in the Golgi complex, they are released onto the cell membrane where the proteins carry out their functions.

What do glycoproteins do?

Importance of Glycosylation -- The reasons for the importance of glycoproteins include

  • They aid in stabilizing and protecting a few proteins
  • They are vital for blood grouping
  • The give rise to the production of various types of glycoproteins
  • They allow a few proteins in the body to perform the functions efficiently

Glycoprotein Function

Location of Glycoproteins

Where are glycoproteins found?

Cell membrane -- Glycoproteins are found in the cell membrane of prokaryotes, eukaryotes, and viruses. They are part of the cell envelope/cell membrane and perform various functions. These molecules protrude outwards into the extracellular matrix in the complex multicellular organisms allowing them to perform their function efficiently.

Function of P-glycoprotein

Function of P-glycoprotein

Plasma -- A few glycoproteins are found in the plasma. Some of the glycoprotein examples are mucopolysaccharides, albumin, mucoproteins, and embryo globulin. The glycoproteins are specifically identified by galactose and mannose in the oligosaccharide side chain. There are different functions of plasma glycoproteins. Mucopolysaccharides are important proteins not associated with the disease-causing agents but involved in the elevation of different disease types. A few of the glycoproteins are synthesized inside the cell and then released into the extracellular matrix

Function of Glycoproteins

Protect the epithelial cells

Mucins are glycosylated proteins that consist of several oligosaccharides as they have many serine and threonine residues. In several multicellular organisms, mucins are the main portions of the mucous membrane and are found lining different parts of the body including the nasal cavity, bronchioles, uterus, and gastric mucosa. Most of the time, mucins are synthesized and released into the extracellular matrix while a few are present in the cell membrane.

Studies showed that a negative charge prevails on the glycoprotein possessing modified sugar. This negative charge of the modified sugar molecule attracts the water molecule. This is how mucin can maintain the moisture on the epithelial tissue. The carbohydrates are sufficiently sticky to trick the pathogenic agents and are preventing them from entering the cell and causing infection.

Mucin acts as a physical barrier on the epithelial tissue and is shown to aid in defending the cells from stress-induced damage.

Hormonal action

Gonadotropins are glycoproteins. They include follicle-stimulating hormone, luteinizing hormone, thyroid-stimulating hormone, and human chorionic gonadotropin.

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  • Activities
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Glycoproteins True or False Activity

This activity will help you assess your knowledge of the definition, functions, and examples of glycoproteins.

Directions

Determine whether the following statements are true or false. To do this, print or copy this page on a blank paper and underline or circle the answer.

1. The endoplasmic reticulum is represented by a membrane system that produces E-linked sugars.

True | False

2. Glycogens are proteins that contain chains of sugar and are involved in most cellular processes.

True | False

3. Lectins are a type of glycoproteins that cover or form a lining on the surface of most cells.

True | False

4. Salts mixed with water in the body generate a smooth mucus that protects certain organs.

True | False

5. Mucus, formed using glycoproteins, are used in the lungs to trap bacteria.

True | False

6. Cadherins are transmembrane proteins important in maintaining tissue structure, such as in the skin.

True | False

7. The sperm finds the egg cell and attaches to it through the help of glycoproteins in the zona pellucida.

True | False

8. MHC molecules bind with white blood cells to allow them to communicate with the T-cells.

True | False

9. Glycoproteins play a vital role in immunology as they serve as transport systems of red blood cells.

True | False

10. Hemophilia is a rare disorder in which your blood doesn't clot normally because it lacks sufficient blood-clotting proteins.

True | False


Answer Key

  1. False, because the correct statement is, The endoplasmic reticulum is represented by a membrane system that produces N-linked sugars.
  2. False, because the correct statement is, Glycoproteins are proteins that contain chains of sugar and are involved in most cellular processes.
  3. True
  4. False, because the correct statement is, Sugars mixed with water in the body generate a smooth mucus that protects certain organs.
  5. True
  6. True
  7. True
  8. True
  9. False, because the correct statement is, Glycoproteins play a vital role in immunology as they serve as transport systems of white blood cells.
  10. True

What does glycoprotein do in the cell membrane?

Glycoproteins are found in the cell membranes. They extend outwards into the extracellular matrix and perform various functions. For example, mucins are the primary components of the mucous membrane. Some mucins are retained in the membrane while a few are released into the extracellular matrix. Mucins lubricate epithelial tissue and trap pathogenic agents.

What is the function of a glycoprotein?

Some of the functions of the glycoproteins include protecting the epithelial cells, release glycoprotein hormones from the anterior pituitary gland like follicle stimulating hormone, luteinizing hormone, thyroid stimulating hormone, and human chorionic gonadotropin. They aid in cell adhesion, as cellular markers and in cell invasion.

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