F
IPR006275

Carbamoyl-phosphate synthase, large subunit

InterPro entry
Short nameCarbamoylP_synth_lsu

Description

This entry represents glutamine-dependent CPSase (
6.3.5.5
) from prokaryotes and eukaryotes (CPSase II).

Carbamoyl phosphate synthase (CPSase) is a heterodimeric enzyme composed of a small and a large subunit (with the exception of CPSase III, see below). CPSase catalyses the synthesis of carbamoyl phosphate from biocarbonate, ATP and glutamine (
6.3.5.5
) or ammonia (
6.3.4.16
), and represents the first committed step in pyrimidine and arginine biosynthesis in prokaryotes and eukaryotes, and in the urea cycle in most terrestrial vertebrates
[1, 2]
. CPSase has three active sites, one in the small subunit and two in the large subunit. The small subunit contains the glutamine binding site and catalyses the hydrolysis of glutamine to glutamate and ammonia. The large subunit has two homologous carboxy phosphate domains, both of which have ATP-binding sites; however, the N-terminal carboxy phosphate domain catalyses the phosphorylation of biocarbonate, while the C-terminal domain catalyses the phosphorylation of the carbamate intermediate
[3]
. The carboxy phosphate domain found duplicated in the large subunit of CPSase is also present as a single copy in the biotin-dependent enzymes acetyl-CoA carboxylase (
6.4.1.2
) (ACC), propionyl-CoA carboxylase (
6.4.1.3
) (PCCase), pyruvate carboxylase (
6.4.1.1
) (PC) and urea carboxylase (
6.3.4.6
).

Most prokaryotes carry one form of CPSase that participates in both arginine and pyrimidine biosynthesis, however certain bacteria can have separate forms. The large subunit in bacterial CPSase has four structural domains: the carboxy phosphate domain 1, the oligomerisation domain, the carbamoyl phosphate domain 2 and the allosteric domain
[4]
. CPSase heterodimers from Escherichia coli contain two molecular tunnels: an ammonia tunnel and a carbamate tunnel. These inter-domain tunnels connect the three distinct active sites, and function as conduits for the transport of unstable reaction intermediates (ammonia and carbamate) between successive active sites
[5]
. The catalytic mechanism of CPSase involves the diffusion of carbamate through the interior of the enzyme from the site of synthesis within the N-terminal domain of the large subunit to the site of phosphorylation within the C-terminal domain.

Eukaryotes have two distinct forms of CPSase: a mitochondrial enzyme (CPSase I) that participates in both arginine biosynthesis and the urea cycle; and a cytosolic enzyme (CPSase II) involved in pyrimidine biosynthesis. CPSase II occurs as part of a multi-enzyme complex along with aspartate transcarbamoylase and dihydroorotase; this complex is referred to as the CAD protein
[6]
. The hepatic expression of CPSase is transcriptionally regulated by glucocorticoids and/or cAMP
[7]
. There is a third form of the enzyme, CPSase III, found in fish, which uses glutamine as a nitrogen source instead of ammonia
[8]
. CPSase III is closely related to CPSase I, and is composed of a single polypeptide that may have arisen from gene fusion of the glutaminase and synthetase domains
[9]
.

References

1.The amidotransferase family of enzymes: molecular machines for the production and delivery of ammonia. Raushel FM, Thoden JB, Holden HM. Biochemistry 38, 7891-9, (1999). View articlePMID: 10387030

2.Carbamoyl phosphate synthetase: an amazing biochemical odyssey from substrate to product. Holden HM, Thoden JB, Raushel FM. Cell. Mol. Life Sci. 56, 507-22, (1999). View articlePMID: 11212301

3.Role of conserved residues within the carboxy phosphate domain of carbamoyl phosphate synthetase. Stapleton MA, Javid-Majd F, Harmon MF, Hanks BA, Grahmann JL, Mullins LS, Raushel FM. Biochemistry 35, 14352-61, (1996). View articlePMID: 8916922

4.The structure of carbamoyl phosphate synthetase determined to 2.1 A resolution. Thoden JB, Raushel FM, Benning MM, Rayment I, Holden HM. Acta Crystallogr. D Biol. Crystallogr. 55, 8-24, (1999). View articlePMID: 10089390

5.Structural defects within the carbamate tunnel of carbamoyl phosphate synthetase. Kim J, Howell S, Huang X, Raushel FM. Biochemistry 41, 12575-81, (2002). View articlePMID: 12379099

6.Cloning, expression, and functional interactions of the amidotransferase domain of mammalian CAD carbamyl phosphate synthetase. Guy HI, Evans DR. J. Biol. Chem. 269, 7702-8, (1994). View articlePMID: 7907330

7.cyclicAMP and glucocorticoid responsiveness of the rat carbamoylphosphate synthetase gene requires the interplay of upstream regulatory units. Schoneveld OJ, Hoogenkamp M, Stallen JM, Gaemers IC, Lamers WH. Biochimie 89, 574-80, (2007). View articlePMID: 17397987

8.Air-breathing catfish, Clarias batrachus upregulates glutamine synthetase and carbamyl phosphate synthetase III during exposure to high external ammonia. Saha N, Datta S, Kharbuli ZY, Biswas K, Bhattacharjee A. Comp. Biochem. Physiol. B, Biochem. Mol. Biol. 147, 520-30, (2007). View articlePMID: 17451989

9.Carbamyl phosphate synthetase III, an evolutionary intermediate in the transition between glutamine-dependent and ammonia-dependent carbamyl phosphate synthetases. Hong J, Salo WL, Lusty CJ, Anderson PM. J. Mol. Biol. 243, 131-40, (1994). View articlePMID: 7932737

Cross References

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