Penicillin was discovered in 1928 by the Scottish bacteriologist, Fleming. Antibiotics of the penicillin family affect the biosynthesis of the cell walls of certain bacteria.

During this biosynthesis two polypeptide chains of the peptidoglycanes of the cell wall are bonded together by a peptide bond. This bonding is catalysed by a transpeptidase enzyme. Penicillin blocks this reaction by irreversibly inhibiting the enzyme.

In a normal situation (left) the enzyme forms a peptide bond with an amino group on chain 1 (red on the drawing) and a terminal alanine is split off. Chain 2 (green on the drawing) then forms a peptide with chain 1, with the enzyme splitting away (2x transpeptidase reaction).

In the presence of penicillin (right), which strongly resembles the alanine end of chain 1 (ß-lactam-structure), the enzyme bonds with the penicillin. The enzyme is thus blocked and can no longer act as a catalyst during the cell wall formation.