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.
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.
Functions of peptidoglycan layer
- Peptidoglycan provides rigid support to bacterial cells and maintains the characteristic shape of the cell.
- 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.