Penicillin functions so well because it acts as a suicide inhibitor to glycopeptide transpeptidase. The function of penicillin is a thiazolidine ring with a beta-lactam ring and a variable R group. This will inhibit the cross –linking of glycopeptide transpeptidase, which forms the crosslinks between amino acid residues in bacteria. In bacteria, there are peptidoglycan walls, with NAM and NAG that are linked by tetrapeptides and peptaglycines. Glycopeptide does this by reacting with the penultimate D-Ala of the substrate, forming the acyl intermediate, which then goes on to react with an glycine in another peptide to form the cross-link. Because penicillin has the D-Ala-D-Ala moiety of the usual substrate, it will begin to form the acyl intermediate that glycopeptide transpeptidase normally forms. However, instead of binding the alanine, a serine in the active site attacks the carbonyl carbon of the beta-lactam ring, forming a covalent bond that prevents the enzyme from reacting any further.