| Argininosuccinate synthetase 1 | |
| Symbol: | ASS1 |
| Hgncid: | 758 |
| Chromosome: | 9 |
| Arm: | q |
| Band: | 34.1 |
| Ecnumber: | 6.3.4.5 |
| Omim: | 603470 |
| Entrezgene: | 445 |
| Refseq: | NM_000050 |
| Uniprot: | P00966 |
| Symbol: | Arginosuc_synth |
| Argininosuccinate synthetase | |
| Pfam: | PF00764 |
| Pfam Clan: | CL0039 |
| Interpro: | IPR001518 |
| Prosite: | PDOC00488 |
| Scop: | 1kp2 |
Argininosuccinate synthase or synthetase (ASS;) is an enzyme that catalyzes the synthesis of argininosuccinate from citrulline and aspartate. In humans, argininosuccinate synthase is encoded by the ASS gene located on chromosome 9.
ASS is responsible for the third step of the urea cycle and one of the reactions of the citrulline-NO cycle.
The expressed ASS gene is at least 65 kb in length, including at least 12 introns.[1] In humans, ASS is expressed mostly in the cells of the liver and kidney.
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.[2]
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.[3]
Argininosuccinate synthetase is a homotetramer, with each subunit consisting of 412 residues.[4] The interfaces between subunits contain a number of salt 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.[5]
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.[6] 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.
Argininosuccinate synthetase is involved in the synthesis of creatine, polyamines, arginine, urea, and nitric oxide.[7]
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.