<p>Argininosuccinate synthase (<db_xref db="EC" dbkey="6.3.4.5"/>) (AS) is a urea cycle enzyme that catalyzes the penultimate step in arginine biosynthesis: the ATP-dependent ligation of citrulline to aspartate to form argininosuccinate, AMP and pyrophosphate [<cite idref="PUB00001799"/>, <cite idref="PUB00002066"/>].</p><p>In humans, a defect in the AS gene causes citrullinemia, a genetic diseasecharacterised by severe vomiting spells and mental retardation.</p><p>AS is a homotetrameric enzyme of chains of about 400 amino-acid residues. An arginine seems to be important for the enzyme's catalytic mechanism. The sequences of AS from various prokaryotes, archaebacteria and eukaryotes show significant similarity.</p>This entry represents argininosuccinate synthase, type 1 subfamily.