PROSITE documentation PDOC00091

Aspartate carbamoyltransferase (EC 2.1.3.2) (ATCase) catalyzes the conversion of aspartate and carbamoyl phosphate to carbamoylaspartate, the second step in the de novo biosynthesis of pyrimidine nucleotides [1]. In prokaryotes ATCase consists of two subunits: a catalytic chain (gene pyrB) and a regulatory chain (gene pyrI), while in eukaryotes it is a domain in a multi-functional enzyme (called URA2 in yeast, rudimentary in Drosophila, and CAD in mammals [2]) that also catalyzes other steps of the biosynthesis of pyrimidines.

Ornithine carbamoyltransferase (EC 2.1.3.3) (OTCase) catalyzes the conversion of ornithine and carbamoyl phosphate to citrulline. In mammals this enzyme participates in the urea cycle [3] and is located in the mitochondrial matrix. In prokaryotes and eukaryotic microorganisms it is involved in the biosynthesis of arginine. In some bacterial species it is also involved in the degradation of arginine [4] (the arginine deaminase pathway).

It has been shown [5] that these two enzymes are evolutionary related. The predicted secondary structure of both enzymes are similar and there are some regions of sequence similarities. One of these regions includes three residues which have been shown, by crystallographic studies [6], to be implicated in binding the phosphoryl group of carbamoyl phosphate. We have selected this region as a signature for these enzymes.

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The residue in position 3 of the pattern allows to distinguish between an ATCase (Glu) and an OTCase (Lys).

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1	Authors	Lerner C.G., Switzer R.L.
	Title	Cloning and structure of the Bacillus subtilis aspartate transcarbamylase gene (pyrB).
	Source	J. Biol. Chem. 261:11156-11165(1986).
	PubMed ID	3015959

2	Authors	Davidson J.N., Chen K.C., Jamison R.S., Musmanno L.A., Kern C.B.
	Title	The evolutionary history of the first three enzymes in pyrimidine biosynthesis.
	Source	BioEssays 15:157-164(1993).
	PubMed ID	8098212

3	Authors	Takiguchi M., Matsubasa T., Amaya Y., Mori M.
	Title	Evolutionary aspects of urea cycle enzyme genes.
	Source	BioEssays 10:163-166(1989).
	PubMed ID	2662961

4	Authors	Baur H., Stalon V., Falmagne P., Luethi E., Haas D.
	Title	Primary and quaternary structure of the catabolic ornithine carbamoyltransferase from Pseudomonas aeruginosa. Extensive sequence homology with the anabolic ornithine carbamoyltransferases of Escherichia coli.
	Source	Eur. J. Biochem. 166:111-117(1987).
	PubMed ID	3109911

5	Authors	Houghton J.E., Bencini D.A., O'Donovan G.A., Wild J.R.
	Title	Protein differentiation: a comparison of aspartate transcarbamoylase and ornithine transcarbamoylase from Escherichia coli K-12.
	Source	Proc. Natl. Acad. Sci. U.S.A. 81:4864-4868(1984).
	PubMed ID	6379651

Aspartate and ornithine carbamoyltransferases signature

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6	Authors	Ke H.-M., Honzatko R.B., Lipscomb W.N.
	Title	Structure of unligated aspartate carbamoyltransferase of Escherichia coli at 2.6-A resolution.
	Source	Proc. Natl. Acad. Sci. U.S.A. 81:4037-4040(1984).
	PubMed ID	6377306