Crystallographic structure of human argininosuccinate synthetase.
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
|Argininosuccinate synthetase 1|
|Locus||Chr. 9 q34.1|
crystal structure of thermus thermophilus hb8 argininosuccinate synthetase in complex with atp and citrulline
Genetic structure 
The gene that encodes for this enzyme, ASS, is located on chromosome 9. In humans, ASS is expressed mostly in the cells of liver and kidney. The expressed ASS gene is at least 65 kb in length, including at least 12 introns.
Enzyme mechanism 
In the first step of the catalyzed reaction, citrulline attacks the α-phosphate of ATP to form citrulline adenylate, a reactive intermediate. The attachment of AMP to the ureido (urea-like) group on citrulline activates the carbonyl center for subsequent nucleophilic attack. This activation facilitates the second step, in which the α-amino group of aspartate attacks the ureido group. Attack by aspartate is the rate-limiting step of the reaction. This step produces free AMP and L-argininosuccinate.
Thermodynamically, adenylation of the citrulline ureido group is more favorable than the analogous phosphorylation. Additionally, attack by citrulline at the α-phosphate of ATP produces an equivalent of pyrophosphate, which can be hydrolyzed in a thermodynamically favorable reaction to provide additional energy to drive the adenylation.
Enzyme structure 
Argininosuccinate synthetase is a homotetramer, with each subunit consisting of 412 residues. The interfaces between subunits contain a number of bridges and hydrogen bonds, and the C-terminus of each subunit is involved in oligomerization by interacting with the C-termini and nucleotide-binding domains of the other subunits.
X-ray crystal structures have been generated for argininosuccinate synthetase from Thermus thermophilus, E. coli, Thermotoga maritime, and Homo sapiens. In ASS from T. thermophilus, E. coli, and H. sapiens, citrulline and aspartate are tightly bound in the active site by interactions with serine and arginine residues; interactions of the substrates with other residues in the active site vary by species. In T. thermophilus, the ureido group of citrulline appears to be repositioned during nucleophilic attack to attain sufficient proximity to the α-phosphate of ATP. In E. coli, it is suggested that binding of ATP causes a conformational shift that brings together the nucleotide-binding domain and the synthetase domain. An argininosuccinate synthetase structure with a bound ATP in the active site has not been attained, although modeling suggests that the distance between ATP and the ureido group of citrulline is smaller in human argininosuccinate synthetase than in the E. coli variety, so it is likely that a much smaller conformational change is necessary for catalysis. The ATP binding domain of argininosuccinate synthetase is similar to that of other N-type ATP pyrophosphatases.
Biological function 
The transformation of citrulline into argininosuccinate is the rate-limiting step in arginine synthesis. The activity of argininosuccinate synthetase in arginine synthesis occurs largely in at the outer mitochondrial membrane of periportal liver cells as part of the urea cycle, with some activity occurring in cortical kidney cells. Genetic defects that cause incorrect localization of argininosuccinate synthetase to the outer mitochondrial membrane cause type II citrullinemia.
In fetuses and infants, arginine is also produced via argininosuccinate synthetase activity in intestinal cells, presumably to supplement the low level of arginine found in mother’s milk. Expression of argininosuccinate synthetase in the intestines ceases after two to three years of life.
It is thought that regulation of argininosuccinate synthetase activity in arginine synthesis occurs primarily at the transcriptional level in response to glucocorticoids, cAMP, glucagon, and insulin. It has also been demonstrated in vitro that arginine down-regulates argininosuccinate synthetase expression, while citrulline up-regulates it.
The enzyme endothelial nitric oxide synthase produces nitric oxide from arginine in endothelial cells. Argininosuccinate synthetase and argininosuccinate lyase recycle citrulline, a byproduct of nitric oxide production, into arginine. Since nitric oxide is an important signaling molecule, this role of ASS is important to vascular physiology. In this role, argininosuccinate synthetase activity is regulated largely by inflammatory cellular signal molecules such as cytokines.
In endothelial cells, it has been shown that ASS expression is increased by laminar shear stress due to pulsative blood flow. Emerging evidence suggests that ASS may also be subject to regulation by phosphorylation at the Ser-328 residue by protein kinase C-α and by nitrosylation at the Cys-132 residue by nitric oxide synthase.
Role in disease 
Citrullinemia is an inherited autosomal recessive disease. At least 50 mutations that cause type I citrullinemia have been identified in the ASS gene. Most of these mutations substitute one amino acid for another in ASS. These mutations likely affect the structure of the enzyme and its ability to bind to citrulline, aspartate, and other molecules. A few mutations lead to the production of an abnormally short enzyme that cannot effectively play its role in the urea cycle.
Defects in ASS disrupt the third step of the urea cycle, preventing the liver from processing excess nitrogen into urea. As a result, nitrogen (in the form of ammonia) and other byproducts of the urea cycle (such as citrulline) build up in the bloodstream. Ammonia is toxic, particularly to the nervous system. An accumulation of ammonia during the first few days of life leads to poor feeding, vomiting, seizures, and the other signs and symptoms of type I citrullinemia.
Treatment for this defect includes a low-protein diet and dietary supplementation with arginine and phenylacetate. Arginine allows the urea cycle to complete itself, creating the substrates needed to originally fix ammonia. This will lower blood pH. Additionally, phenylacetate reacts with backed-up glutamine, resulting on phenylacetoglutamine, which can be excreted renally.
A lack of argininosuccinate synthetase expression has been observed in several types of cancer cells, including pancreatic cancer, liver cancer, and melanoma. For example, defects in ASS have been seen in 87% of pancreatic cancers. Cancer cells are therefore unable to synthesis enough arginine for cellular processes and so must rely on dietary arginine. Depletion of plasma arginine using arginine deiminase has been shown to lead to regression of tumours in mice.
See also 
- PDB 2nz2; Karlberg T, Collins R, van den Berg S, Flores A, Hammarström M, Högbom M, Holmberg Schiavone L, Uppenberg J (March 2008). "Structure of human argininosuccinate synthetase". Acta Crystallogr. D Biol. Crystallogr. 64 (Pt 3): 279–86. doi:10.1107/S0907444907067455. PMID 18323623.
- Goto M, Omi R, Miyahara I, Sugahara M, Hirotsu K (June 2003). "Structures of argininosuccinate synthetase in enzyme-ATP substrates and enzyme-AMP product forms: stereochemistry of the catalytic reaction". J. Biol. Chem. 278 (25): 22964–71. doi:10.1074/jbc.M213198200. PMID 12684518.
- Freytag SO, Beaudet AL, Bock HG, O'Brien WE (October 1984). "Molecular structure of the human argininosuccinate synthetase gene: occurrence of alternative mRNA splicing". Mol. Cell. Biol. 4 (10): 1978–84. PMC 369014. PMID 6095035.
- Ghose C, Raushel FM (October 1985). "Determination of the mechanism of the argininosuccinate synthetase reaction by static and dynamic quench experiments". Biochemistry 24 (21): 5894–8. doi:10.1021/bi00342a031. PMID 3878725.
- Kumar S, Lennane J, Ratner S (October 1985). "Argininosuccinate synthetase: essential role of cysteine and arginine residues in relation to structure and mechanism of ATP activation". Proc. Natl. Acad. Sci. U.S.A. 82 (20): 6745–9. doi:10.1073/pnas.82.20.6745. PMC 390763. PMID 3863125.
- Husson A, Brasse-Lagnel C, Fairand A, Renouf S, Lavoinne A (May 2003). "Argininosuccinate synthetase from the urea cycle to the citrulline-NO cycle". Eur. J. Biochem. 270 (9): 1887–99. doi:10.1046/j.1432-1033.2003.03559.x. PMID 12709047.
- Karlberg T, Collins R, van den Berg S, Flores A, Hammarström M, Högbom M, Holmberg Schiavone L, Uppenberg J (March 2008). "Structure of human argininosuccinate synthetase". Acta Crystallogr. D Biol. Crystallogr. 64 (Pt 3): 279–86. doi:10.1107/S0907444907067455. PMID 18323623.
- Lemke CT, Howell PL (December 2001). "The 1.6 A crystal structure of E. coli argininosuccinate synthetase suggests a conformational change during catalysis". Structure 9 (12): 1153–64. doi:10.1016/S0969-2126(01)00683-9. PMID 11738042.
- Haines RJ, Pendleton LC, Eichler DC (2011). "Argininosuccinate synthase: at the center of arginine metabolism". Int J Biochem Mol Biol 2 (1): 8–23. PMC 3074183. PMID 21494411.
- Morris SM (2002). "Regulation of enzymes of the urea cycle and arginine metabolism". Annu. Rev. Nutr. 22: 87–105. doi:10.1146/annurev.nutr.22.110801.140547. PMID 12055339.
- Mun GI, Boo YC (April 2012). "A regulatory role of Kruppel-like factor 4 in endothelial argininosuccinate synthetase 1 expression in response to laminar shear stress". Biochem. Biophys. Res. Commun. 420 (2): 450–5. doi:10.1016/j.bbrc.2012.03.016. PMID 22430140.
- Haines RJ, Corbin KD, Pendleton LC, Eichler DC (July 2012). "Protein kinase Cα phosphorylates a novel argininosuccinate synthase site at serine 328 during calcium-dependent stimulation of endothelial nitric-oxide synthase in vascular endothelial cells". J. Biol. Chem. 287 (31): 26168–76. doi:10.1074/jbc.M112.378794. PMC 3406701. PMID 22696221.
- Häberle J, Pauli S, Linnebank M, Kleijer WJ, Bakker HD, Wanders RJ, Harms E, Koch HG (April 2002). "Structure of the human argininosuccinate synthetase gene and an improved system for molecular diagnostics in patients with classical and mild citrullinemia". Hum. Genet. 110 (4): 327–33. doi:10.1007/s00439-002-0686-6. PMID 11941481.
- Devlin TM (2002). Textbook of biochemistry: with clinical correlations. New York: Wiley-Liss. p. 788. ISBN 0-471-41136-1.
- Wu L, Li L, Meng S, Qi R, Mao Z, Lin M (February 2013). "Expression of argininosuccinate synthetase in patients with hepatocellular carcinoma". J. Gastroenterol. Hepatol. 28 (2): 365–8. doi:10.1111/jgh.12043. PMID 23339388.
- Yoon JK, Frankel AE, Feun LG, Ekmekcioglu S, Kim KB (2013). "Arginine deprivation therapy for malignant melanoma". Clin Pharmacol 5: 11–9. doi:10.2147/CPAA.S37350. PMC 3534294. PMID 23293541.
- Bowles TL, Kim R, Galante J, Parsons CM, Virudachalam S, Kung HJ, Bold RJ (October 2008). "Pancreatic cancer cell lines deficient in argininosuccinate synthetase are sensitive to arginine deprivation by arginine deiminase". Int. J. Cancer 123 (8): 1950–5. doi:10.1002/ijc.23723. PMID 18661517.
- GeneReviews/NCBI/NIH/UW entry on Argininosuccinate Synthetase Deficiency; ASS Deficiency; Argininosuccinic Acid Synthetase Deficiency; CTLN1; Citrullinemia, Classic