The enzyme Phosphoribosylaminoimidazole carboxylase, or AIR carboxylase () 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 + CO  5'-phosphoribosyl-4-carboxy-5-aminoimidazole + 2 H