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

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

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.