<p>The enzyme L-aspartate ammonia-lyase (aspartase) catalyses the reversible deamination of the amino acid L-aspartic acid, using a carbanion mechanism to produce fumaric acid and ammonium ion. Aspartases from different organisms show high sequence homology, and this homology extends to functionally related enzymes such as the class II fumarases, the argininosuccinate and adenylosuccinate lyases. The high-resolution structure of aspartase reveals a monomer that is composed of three domains oriented in an elongated S-shape [<cite idref="PUB00011785"/>]. The central domain, comprised of five-helices, provides the subunit contacts in the functionally active tetramer. The active sites are located in clefts between the subunits and structural and mutagenic studies have identified several of the active site functional groups. A separate regulatory site has been identified. The substrate, aspartic acid, can also play the role of an activator, binding at this site along with a required divalent metal ion.</p>