Phosphoribosylaminoimidazole carboxylase

The enzyme Phosphoribosylaminoimidazole carboxylase, or AIR carboxylase (EC 4.1.1.21) is involved in nucleotide biosynthesis and in particular in purine biosynthesis. It catalyzes the conversion of 5'-phosphoribosyl-5-aminoimidazole ("AIR") into 5'-phosphoribosyl-4-carboxy-5-aminoimidazole ("CAIR") as described in the reaction:

5-aminoimidazole ribonucleotide + CO₂ ${\displaystyle \rightleftharpoons }$ 5'-phosphoribosyl-4-carboxy-5-aminoimidazole + 2 H⁺

Phosphoribosylaminoimidazole carboxylase
Phosphoribosylaminoimidazole carboxylase octamer, Human
Identifiers
EC no.	4.1.1.21
CAS no.	9032-04-6
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / QuickGO
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phosphoribosylaminoimidazole carboxylase, phosphoribosylaminoimidazole succinocarboxamide synthetase
Identifiers
Symbol	PAICS
Alt. symbols	PAIS
NCBI gene	10606
HGNC	8587
OMIM	172439
RefSeq	NM_006452
UniProt	P22234
Other data
EC number	4.1.1.21
Locus	Chr. 4 pter-q21
Search for Structures Swiss-model Domains InterPro

In plants and fungi

Phosphoribosylaminoimidazole carboxylase is a fusion protein in plants and fungi, but consists of two non-interacting proteins in bacteria, PurK and PurE.

The crystal structure of PurE indicates a unique quaternary structure that confirms the octameric nature of the enzyme.^[1]

In Escherichia coli

In the bacterium Escherichia coli the reaction is catalyzed in two steps carried out by two separate enzymes, PurK and PurE.

PurK, N5-carboxyaminoimidazole ribonucleotide synthetase, catalyzes the conversion of 5-aminoimidazole ribonucleotide ("AIR"), ATP, and bicarbonate to N5-carboxyaminoimidazole ribonucleotide ("N5-CAIR"), ADP, and phosphate.

PurE, N5-carboxyaminoimidazole ribonucleotide mutase, converts N5-CAIR to CAIR, the sixth step of de novo purine biosynthesis. In the presence of high concentrations of bicarbonate, PurE is reported able to convert AIR to CAIR directly and without ATP. Some members of this family contain two copies of this domain.^[2]