In enzymology, an aminoacylase () is an enzyme that catalyzes the chemical reaction N-acyl-L-amino acid + H2O carboxylate + L-amino acid Thus, the two substrates of this enzyme are N-acyl-L-amino acid and H2O, whereas its two products are carboxylate and L-amino acid. This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in linear amides. The systematic name of this enzyme class is N-acyl-L-amino acid amidohydrolase. Other names in common use include dehydropeptidase II, histozyme, hippuricase, benzamidase, acylase I, hippurase, amido acid deacylase, L-aminoacylase, acylase, aminoacylase I, L-amino-acid acylase, alpha-N-acylaminoacid hydrolase, long acyl amidoacylase, and short acyl amidoacylase. This enzyme participates in urea cycle and metabolism of amino groups. Enzyme structure As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and . These structures also correspond to two known primary amino acid sequences for aminoacylases. The associated papers identify two types of domains comprising aminoacylases: Zinc binding domains - which bind Zn2+ ions - and domains that facilitate dimerization of Zinc binding domains. It is this dimerization that allows catalysis to occur, since aminoacylase's active site lies between its two Zinc binding domains. Bound Zinc facilitates the binding of the N-acyl-L-amino acid substrate, causing a conformational shift that brings the protein's subunits together around the substrate and allowing catalysis to occur. Aminoacylase 1 exists in a heterotetrameric structure, meaning 2 Zinc binding domains and 2 dimerization domains come together to make aminoacylase 1's quaternary structure. Enzyme Mechanism Aminoacylase is a metallo-enzyme that needs Zinc (Zn2+) as a cofactor to function. The Zinc ions inside of aminoacylase are each coordinated to histidine, glutamate, aspartate, and water. The Zinc ion polarizes the water, facilitating its deprotonation by a nearby basic residue. The negatively charged hydroxide ion is nucleophilic and attacks the electrophilic carbonyl carbon of the substrate's acyl group. The exact mechanism after this point is unknown, with one possibility being that the carbonyl then reforms, breaks the amide bond, and forms the two products. At some point in the mechanism, another water molecule enters and coordinates with Zinc, returning the enzyme to its original state. The nucleophilic attack by water is the rate-limiting step of aminoacylase's catalytic mechanism. This nucleophilic attack is reversible while the subsequent steps are fast and irreversible. This reaction sequence is an example of Michaelis–Menten kinetics, allowing one to determine