Catalysis in Penicillin G Amidase – a Member of the Ntn (N Terminal Nucleophile) Hydrolase Family*

The Ntn (N terminal nucleophile) hydrolases are a new family of hydrolytic enzymes with a characteristic fold in their catalytic domain. These enzymes act on a range of substrates, cleaving amide or ester bonds by a nucleophilic reaction. The catalytic nucleophile is the oxygen in serine or threonine side chains, or sulphur in the cysteine side chain. The active site is often created by autocatalytic cleavage at the nucleophile-containing catalytic residue. As a result of cleavage a free alpha amino group is generated which is an essential component in the enzyme’s catalytic structure. The crystal structure of the precursor of the enzyme penicillin amidase has been determined, revealing the stereochemistry at the scissile bond prior to autocatalytic cleavage.