The 1.6 A crystal structure of E. coli argininosuccinate synthetase suggests a conformational change during catalysis.
BACKGROUND
Argininosuccinate synthetase (AS) is the rate-limiting enzyme of both the urea and arginine-citrulline cycles. In mammals, deficiency of AS leads to citrullinemia, a debilitating and often fatal autosomal recessive urea cycle disorder, whereas its overexpression for sustained nitric oxide production via the arginine-citrulline cycle leads to the potentially fatal hypotension associated with septic and cytokine-induced circulatory shock.


RESULTS
The crystal structure of E. coli AS (EAS) has been determined by the use of selenomethionine incorporation and MAD phasing. The structure has been refined at 1.6 A resolution in the absence of its substrates and at 2.0 A in the presence of aspartate and citrulline (EAS*CIT+ASP). Each monomer of this tetrameric protein has two structural domains: a nucleotide binding domain similar to that of the "N-type" ATP pyrophosphatase class of enzymes, and a novel catalytic/multimerization domain. The EAS*CIT+ASP structure clearly describes the binding of citrulline at the cleft between the two domains and of aspartate to a loop of the nucleotide binding domain, whereas homology modeling with the N-type ATP pyrophosphatases has provided the location of ATP binding.


CONCLUSIONS
The first three-dimensional structures of AS are reported. The fold of the nucleotide binding domain confirms AS as the fourth structurally defined member of the N-type ATP pyrophosphatases. The structures identify catalytically important residues and suggest the requirement for a conformational change during the catalytic cycle. Sequence similarity between the bacterial and human enzymes has been used for providing insight into the structural and functional effects of observed clinical mutations.
Lemke
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Structural Biology and Biochemistry, Research Institute, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada.
Howell
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D000255:Adenosine Triphosphate / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001124:Argininosuccinate Synthase / Q000737:chemistry* / Q000235:genetics*; D001224:Aspartic Acid / Q000737:chemistry; D001665:Binding Sites; D002384:Catalysis; D002956:Citrulline / Q000737:chemistry; D018360:Crystallography, X-Ray; D019281:Dimerization; D004926:Escherichia coli / Q000201:enzymology*; D006801:Humans; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D009711:Nucleotides / Q000378:metabolism; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid
D009711:Nucleotides; D002956:Citrulline; D001224:Aspartic Acid; D000255:Adenosine Triphosphate; D001124:Argininosuccinate Synthase