Peptidoglycan

Peptidoglycan is found in both Gram-negative and Gram-positive bacteria. It is present in larger amounts in Gram-positive bacteria as it appears as a multimolecular layer and can be found in association with additional compounds. In Gram-negative bacteria, it is found in either 1 or 2 layers between the inner and outer membrane . 

Peptidoglycan is a macromolecule made up of alternating residues of N-acetyl-β-D-glucosamine (NAG) and N-acetylmuramic acid (NAM) joined together by a glycosidic bond. There is then a pentapeptide attatched to the NAM amino acid which forms cross-links with other polysaccharide chains forming a 3D mesh-like layer. These cross-links are formed using the enzyme glycopeptide transpeptidase and provide peptidoglycan with stability. This stability allows it to provide mechanical support and prevent osmotic lysis from occurring in bacteria .

Penicillin inhibits the effect of peptidoglycan in bacteria by binding to serine in the enzyme glycopeptide transpeptidase. As a result, it can no longer catalyse the formation of cross-links. The enzyme is irreversibly inhibitted which therefore results in the termination of bacterial growth.

Penicillin inhibits the transpeptidase linking together the polysaccharide chains and cells become osmotically sensitive and burst.