Peptidoglycan (Murein/Mucopeptide): Structure and Medical Significance

The term peptidoglycan was derived from the peptides and the sugars (glycan) that make a molecule; it is also called ‘Murein’. It is found only in bacterial cell walls, thus, its synthesis can be targeted by antibiotics. Peptidoglycan, the polymer of sugars and amino acids,  is a complex, interwoven network that surrounds the entire bacterial cell.

Difference between Gram positive and Gram Negative bacterial cell wall
Difference between Gram positive and Gram Negative bacterial cell wall

Peptidoglycan consists of carbohydrate backbone (glycan chain) composed of alternating units of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) molecules.  Glycan chains are connected by short peptides.Attached to each of the muramic acid molecules are a tetrapeptide consisting of both D- and L- amino acids, the precise composition of which differs between bacteria. Teichoic acid and lipoteichoic acid which are polymers of a sugar alcohol (ribitol or glycerol) are embedded in it.

Special amino acids found in peptidoglycan layer:

  • Diaminopimelic acid: Unique to bacterial cells.
  • D- alanine: Involved in the cross links between tetrapeptides and in the action of penicillin.
Peptidoglycan structure of E. coli Peptidoglycan structure of Staphylococcus aureus Source:

Functions of peptidoglycan layer

  1. It provides rigid support to bacterial cells and maintains the characteristic shape of the cell.
  2. Allows bacterial cell to withstand media of low osmotic pressure, such as water.

Medical Importance of peptidoglycan layer

  • Peptidoglycan is a good target for antibacterial drugs.  Eg. Penicillins, cephalosporins etc inhibit transpeptidase reaction which makes cross-links between the two adjacent tetrapeptides.
  • Lysozyme enzyme present in human tears, mucus, and saliva cleave peptidoglycan backbone breaking its glycosyl bonds.

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