What is the Difference Between Peptidoglycan and Glycoprotein

The main difference between peptidoglycan and glycoprotein is that peptidoglycan is a polymer of glycan and peptides found in bacterial cell walls whereas glycoprotein is a protein with covalently bonded carbohydrates. 

Peptidoglycan and glycoprotein are two types of glycoconjugates characterized by the presence of carbohydrates covalently attached to other types of chemical constituent. Furthermore, peptidoglycans do not contain a protein portion and the sugars in the peptidoglycans do not occur elsewhere while glycoproteins contain oligosaccharide chains attached to amino acid side chains of a defined protein.

Key Areas Covered

1. What is a Peptidoglycan
     – Definition, Structure, Importance
2. What is Glycoprotein
     – Definition, Structure, Importance
3. What are the Similarities Between Peptidoglycan and Glycoprotein
     – Outline of Common Features
4. What is the Difference Between Peptidoglycan and Glycoprotein
     – Comparison of Key Differences

Key Terms

Bacterial Cell Wall, Cell Membrane, Glycoconjugates, Glycoprotein, NAG, NAM, Peptidoglycan

What is a Peptidoglycan

Peptidoglycan or murein is the polymer present in the bacterial cell wall. The two sugar components in peptidoglycan are β-(1, 4) linked N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). These two sugar components are alternatively attached to the N-acetylmuramic acid, which is a peptide chain made up of 3-5 amino acids. The peptide chains can be cross-linked by other peptide chains. However, these sugars and amino acids form a mesh-like layer outside of the plasma membrane in most bacteria. And, this layer is the bacterial cell wall. The main function of the bacterial cell wall is to provide structural strength to the bacterial cell while controlling the osmotic pressure. Also, peptidoglycans play a role in binary fission of bacteria as well.

Figure 1: Peptidoglycan

Furthermore, the thickness of the peptidoglycan layer of the bacterial cell wall is the feature used to characterize bacteria as Gram-positive and Gram-negative. Generally, Gram-positive bacteria have a peptidoglycan layer with a thickness of 20 to 80 nm while the thickness of the peptidoglycan layer of the Gram-negative bacteria is 7 to 8 nm. Also, peptidoglycans account for 90% of the dry weight of Gram-positive bacteria while they account for 10% of the dry weight of Gram-negative bacteria.

What is a Glycoprotein

A glycoprotein is a type of glycoconjugate made up of an oligosaccharide attached to a protein. Significantly, glycoproteins occur in the cell membrane of eukaryotic cells and in the blood. Here, the oligosaccharide chains of glycoprotein molecules protrude out from the cell membrane while the protein molecule occurs as an integral membrane protein. One of the main function of glycoproteins on the cell membrane is to stabilize the membrane by making hydrogen bonds with the surrounding water molecules. On the other hand, glycoproteins on the cell membrane and in the blood serve as antigens, facilitating cell recognition as either self or non-self.  Glycoproteins on the cell membrane also play a role in cell attachment. In addition, glycoproteins serve as receptor molecules for hormones and neurotransmitters, triggering intercellular signaling pathways.

Figure 2: Glycoproteins in the Cell Membrane

Moreover, glycoproteins are a diverse group of molecules. Some examples of glycoproteins are collagen, a structural molecule, mucins that are lubricants, transport molecules such as transferrin, immunologic molecules such as immunoglobulins and histocompatibility antigens, hormones such as HCG and TSH, etc.

Similarities Between Peptidoglycan and Glycoprotein

• Peptidoglycan and glycoprotein are two types of glycoconjugates which contain carbohydrates attached to another chemical constituent.
• Both occur on the outer surface of different types of cells.
• Also, covalent bonds occur between the components of each glycoconjugate.

Difference Between Peptidoglycan and Glycoprotein

Definition

Peptidoglycan refers to a substance that forms the cell walls of many bacteria, consisting of glycosaminoglycan chains interlinked with short peptides while glycoprotein refers to any of a class of proteins which have carbohydrate groups attached to the polypeptide chain. Thus, this is the main difference between peptidoglycan and glycoprotein.

Type of Molecule

Furthermore, peptidoglycan is a polymer while glycoprotein is not a polymer.

Components

Peptidoglycan is composed of NAG, NAM attached to N-acetylmuramic acid while a glycoprotein is composed of oligosaccharide chains attached to a protein. Hence, this is an important difference between peptidoglycan and glycoprotein.

Sugars

Moreover, the sugars in peptidoglycans are not available elsewhere while the sugars in glycoproteins naturally occur in the other biological systems as well.

Proteins

Also, proteins do not occur in peptidoglycans while proteins occur in glycoproteins.

Occurrence

Another difference between peptidoglycan and glycoprotein is that the peptidoglycans occur in the bacterial cell wall while glycoproteins occur on the eukaryotic cell membrane and in the blood.

Role

Besides, their role is another difference between peptidoglycan and glycoprotein. Peptidoglycans give structural strength to the bacterial cell wall while counteracting osmotic pressure whereas glycoproteins help in cell recognition, cell attachment, signal recognition, etc.

Conclusion

Peptidoglycan is a polymer containing NAG, NAM, and N-acetylmuramic acid. It exclusively occurs in the bacterial cell wall, providing structural strength and controlling osmotic pressure. Moreover, its sugars do not occur elsewhere and it does not contain a defined protein. In contrast, glycoprotein is a protein attached to oligosaccharides. It occurs both on the cell membrane of eukaryotes and in the blood. It is responsible for cellular recognition, attachment, and signal transduction of chemical signals. Therefore, the main difference between peptidoglycan and glycoprotein is their structure, occurrence, and function.

References:

1. Moss, G P. “Nomenclature of Glycoproteins, Glycopeptides and Peptidoglycans.” Queen Mary University of London, Available Here.

Image Courtesy:

1. “Mureine” By MouagipThis W3C-unspecified vector image was created with Adobe Illustrator. – Own work (Public Domain) via Commons Wikimedia  
2. “OSC Microbio 03 04 EukPlasMem” By CNX OpenStax (CC BY 4.0) via Commons Wikimedia