Peptidase T is an aminopeptidase, which hydrolyzes tripeptides containing N-terminal methionine, leucine, or phenylalanine. The protein comprises of two domains: a catalytic domain with an active site containing two metal ions (Zn2+), and a smaller domain, which is involved in a dimer formation. Metalloproteases are the most diverse of the four main types of protease, with more than 50 families identified to date. In these enzymes, a divalent cation, usually zinc, activates the water molecule. The metal ion is held in place by amino acid ligands, usually three in number. The known metal ligands are His, Glu, Asp or Lys and at least one other residue is required for catalysis, which may play an electrophillic role. Around half of the known metalloproteases contain an HEXXH motif, which forms part of the metal-binding site. Comparison of the active sites of peptidase T, carboxypeptidase G2, Aeromonas proteolytica aminopeptidase, carboxypeptidase A and leucine aminopeptidase reveals a common structural framework with interesting similarities and differences in the active sites and in the zinc coordination.