Penicillin is a class of antibiotics that are derived from the Penicillium fungi. The mechanism of action of penicillin involves targeting and inhibiting bacterial cell wall synthesis, specifically by interfering with the formation of peptidoglycan, a major component of the bacterial cell wall.Peptidoglycan is a polymer made up of alternating units of N-acetylglucosamine NAG and N-acetylmuramic acid NAM , which are linked together by - 1,4 -glycosidic bonds. The NAM units have short peptide chains attached to them, which are cross-linked to form a strong and rigid structure that provides mechanical strength and protection to the bacterial cell.The key enzyme involved in the cross-linking of peptidoglycan is called penicillin-binding protein PBP , which is a transpeptidase. PBPs catalyze the formation of peptide bonds between the amino acid side chains of adjacent NAM units, creating the cross-links that stabilize the peptidoglycan structure.Penicillin and other -lactam antibiotics work by mimicking the structure of the terminal D-alanyl-D-alanine D-Ala-D-Ala dipeptide found in the peptide chains of peptidoglycan. When penicillin binds to the active site of the PBP, it forms a covalent bond with a serine residue in the enzyme's active site. This bond is stable and irreversible, effectively inactivating the PBP and preventing it from catalyzing the cross-linking reaction.As a result, the bacterial cell wall becomes weakened and compromised, leading to osmotic instability and ultimately cell lysis and death. Since human cells do not have peptidoglycan cell walls, penicillin and other -lactam antibiotics are selectively toxic to bacteria and have minimal effects on human cells, making them effective treatments for bacterial infections.