<p>Amidase signature (AS) enzymes are a large group of hydrolytic enzymes that contain a conserved stretch of approximately 130 amino acids known as the AS sequence. They are widespread, being found in both prokaryotes and eukaryotes. AS enzymes catalyse the hydrolysis of amide bonds (CO-NH2), although the family has diverged widely with regard to substrate specificity and function. Nonetheless, these enzymes maintain a core alpha/beta/alpha structure, where the topologies of the N- and C-terminal halves are similar. AS enzymes characteristically have a highly conserved C-terminal region rich in serine and glycine residues, but devoid of aspartic acid and histidine residues, therefore they differ from classical serine hydrolases. These enzymes posses a unique, highly conserved Ser-Ser-Lys catalytic triad used for amide hydrolysis, although the catalytic mechanism for acyl-enzyme intermediate formation can differ between enzymes [<cite idref="PUB00035563"/>].</p> <p>Examples of AS enzymes include:</p> <ul> <li>Peptide amidase (Pam) [<cite idref="PUB00035563"/>], which catalyses the hydrolysis of the C-terminal amide bond of peptides.</li><li>Fatty acid amide hydrolases [<cite idref="PUB00035564"/>], which hydrolyse fatty acid amid substrates (e.g. cannabinoid anandamide and sleep-inducing oleamide), thereby controlling the level and duration of signalling induced by this diverse class of lipid transmitters.</li><li>Malonamidase E2 [<cite idref="PUB00035565"/>], which catalyses the hydrolysis of malonamate into malonate and ammonia, and which is involved in the transport of fixed nitrogen from bacteroids to plant cells in symbiotic nitrogen metabolism.</li><li>Subunit A of Glu-tRNA(Gln) amidotransferase [<cite idref="PUB00035566"/>],a heterotrimeric enzyme that catalyses the formation of Gln-tRNA(Gln) by the transamidation of misacylated Glu-tRNA(Gln) via amidolysis of glutamine.</li> </ul>