Argininosuccinate synthase (ASS; EC 6.3.4.5) 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. In humans, a defect in the ASS gene causes citrullinemia, a genetic disease characterized by severe vomiting spells and mental retardation. ASS is a homotetrameric enzyme of about 400 amino-acid residues. An arginine residue seems to be important for the enzyme's catalytic mechanism. The sequences of ASS from various prokaryotes, archaeabacteria and eukaryotes show significant similarity.
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