Carbamoyl phosphate synthetase I

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carbamoyl-phosphate synthetase 1, mitochondrial
Identifiers
Symbol CPS1
Entrez 1373
HUGO 2323
OMIM 608307
RefSeq NM_001875
UniProt P31327
Other data
EC number 6.3.4.16
Locus Chr. 2 p

Carbamoyl Phosphate Synthetase I (E.coli: PDB 1jdb, EC 6.3.5.5) is a ligase enzyme located in the mitochondria involved in the production of urea. Carbamoyl Phosphate Synthetase I (CPSI) transfers an ammonia from glutamine to a molecule of bicarbonate that has been phosphorylated by a molecule of ATP. The resulting carbamate is then phosphorylated with another molecule ATP. The resulting molecule of carbamoyl phosphate leaves the enzyme.

Contents

[edit] Structure of Carbamoyl Phosphate Synthetase I

CPSI is a heterodimer with a small subunit and a larger subunit with are about 382 and 1073 amino acid residues in size[1]. The small subunit contains one active site for the binding and deamination of glutamine to make ammonia and glutamate. The large subunit contains two active sites, one for the production of carboxyphosphate, and the other for the production of carbamoyl phosphate[2][3]. Within the large subunit there are two domains (B and C) each with an active site of the ATP-grasp family[1]. Connecting the two subunits a tunnel of sorts which directs the ammonia from the small subunit to the large subunit[4].

Glutamine binding active site with Cys269 and His353 residues showing with glutamine present
Glutamine binding active site with Cys269 and His353 residues showing with glutamine present
View of an ATP binding site of the B domain
View of an ATP binding site of the B domain
View of the IMP allosteric binding site CPSI in E.coli
View of the IMP allosteric binding site CPSI in E.coli

[edit] Mechanism of Carbamoyl Phosphate Synthetase I

The overall reaction that occurs in CPSI is:

2ATP + HCO3-- + Glutamine --> 2ADP + Carbamoyl phosphate + glutamate + Pi[4]

This reaction can be thought of occurring in four distinct steps[5].

   1.  Bicarbonate is phosphorylated
   2.  Ammonia is taken off glutamine
   3.  The ammonia from glutamine attacks the carboxyphosphate resulting in carbamate
   4.  Carbamate is phosphorylated to give Carbamoyl phosphate

Of these four steps only step two, the deamination of glutamine to get ammonia is known to have actively participating amino acid residues, Cys269 and His353. The other three steps mostly utilize amino acid residue to form hydrogen bonds with substrates. A video of a simplified version of this mechanism is available here

[edit] Recent Mechanism Studies

It has been found that both ATP-binding sites in the large subunit of CPSI are structurally equivalent. A recent study has investigated the interlinking between these two domains (domain B and domain C) and has found evidence that they are coupled. This ATP-binding domain coupling works such that a molecule of ATP binding at one site (domain C) conformationally allows synthesis at the other domain (domain B). If this turns out to be true then the mechanism for carbamoyl phosphate synthesis is significantly altered. Instead of forming carbamoyl phosphate in step 5 (of the included mechanism below) by ejecting ADP it would be formed at step 4 by protonating the alcohol group and then kicking it off as water.[6].

Formation of carbamoyl phosphate mechanism
Formation of carbamoyl phosphate mechanism
Deamination of glutamine to form ammonia and glutamate
Deamination of glutamine to form ammonia and glutamate[7]

[edit] Regulation

CPSI is allosterically regulated by purine and pyrimidine nucleotides where the former is allosterically activates the enzyme and the later allosterically inhibits CPSI. Also, it has been found that ornithine is an allosteric activator of CPSI. Increased levels of ammonia also activate CPSI while decreased levels of ATP inhibit CPSI activity. All allosteric binding sites are located in the large subunit of the enzyme[5].

[edit] Carbamoyl Phosphate Synthetase I and Metabolism

CPSI plays a vital role in protein and nitrogen metabolism. Once ammonia has been brought into the mitochondria via glutamine, or glutamate it is CPSI’s job to add the ammonia to bicarbonate along with a phosphate group to form carbamoyl phosphate. Carbamoyl phosphate is then put into the urea cycle to eventually create urea. Urea can then be transferred back to the blood stream and to the kidneys for filtration and on to the bladder for excretion[8].

[edit] Health Problems related to Carbamoyl Phosphate Synthetase I

The main problem related to CPSI is genetically based. Sometimes the body doesn’t produce enough CPSI due to a mutation in the genetic code. When this happens the body can’t metabolize proteins and nitrogen as well and this can result in high levels of ammonia in the body. This is dangerous because ammonia is highly toxic to the body and especially the nervous system and can result in retardation and seizures. For more information check out the article on Carbamoyl phosphate synthetase I deficiency.

[edit] References

  1. ^ a b James B. Thoden‡, Xinyi Hua, et al. “Carbamoyl-phosphate Synthetase: Creation of an Escape Route for Ammonia.” From the ‡Department of Biochemistry, University of Wisconsin, Madison, Wisconsin, 53706-1544 and the §Department of Chemistry, Texas A&M University, College Station, Texas 77843-3012. http://www.jbc.org/cgi/reprint/277/42/39722.pdf
  2. ^ SUE GLENN POWERS, “Inhibition of Carbamyl Phosphate Synthetase by PI, P5-Di (adenosine 5’) - pentaphosphate EVIDENCE FOR TWO ATP BINDING SITES.” From the Department ofBiochemistry, Cornell University Medical College, New York, New York 10021. http://www.jbc.org/cgi/reprint/252/10/3558
  3. ^ James B. Thoden, Hazel M. Holden, et al (1997). “Structure of Carbamoyl Phosphate Synthetase: A Journey of 96 Å from Substrate to Product.” Institute for Enzyme Research, Graduate School, and Department of Biochemistry, College of Agriculture, UniVersity of Wisconsin, Madison, Wisconsin 53705, and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843. http://pubs.acs.org/cgi-bin/article.cgi/bichaw/1997/36/i21/pdf/bi970503q.pdf
  4. ^ a b Jungwook Kim and Frank M. Raushel (2004). “Perforation of the Tunnel Wall in Carbamoyl Phosphate Synthetase Derails the Passage of Ammonia between Sequential Active Sites.” Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842-3012. http://pubs.acs.org/cgi-bin/article.cgi/bichaw/2004/43/i18/pdf/bi049945+.pdf
  5. ^ a b ALTON MEISTER. “MECHANISM AND REGULATION OF THE GLUTAMINE-DEPENDENT CARBAMYL PHOSPHATE SYNTHETASE OF ESCHERZCHZA COLI.” Department of Biochemistry, Cornell University Medical College, New York, New York 10021
  6. ^ MICHAEL KOTH*, BINNUR EROGLU, et al. “Novel mechanism for carbamoyl-phosphate synthetase: A nucleotide switch for functionally equivalent domains.” Department of Biology, Northeastern University, Boston, MA 02115 http://www.pnas.org/cgi/reprint/94/23/12348.pdf?ck=nck
  7. ^ James B. Thoden, et al (1998). “Carbamoyl Phosphate Synthetase: Caught in the Act of Glutamine Hydrolysis.” Biochemistry 1998, 37, 8825-8831. http://pubs.acs.org/cgi-bin/article.cgi/bichaw/1998/37/i25/pdf/bi9807761.pdf
  8. ^ David Nelson and Michael Cox. “Principles of Biochemistry, fourth edition.” Pgs 666-669

[edit] External links

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Metabolism: amino acid metabolism - urea cycle enzymes
Main cycle
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see also disorders
v • d • e
Enzymes: ligases (EC 6)
6.1 - Carbon-Oxygen
6.2 - Carbon-Sulfur
6.3 - Carbon-Nitrogen
6.4 - Carbon-Carbon
6.5 - Phosphoric Ester
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Mitochondrial enzymes and transporters
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Complex I (7 units) - Complex III (1 unit) - Complex IV (3 units) - ATP synthase (2 units)
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