sentences:  Phosphoribosylaminoimidazole carboxylase
  
  Phosphoribosylaminoimidazole carboxylase (or AIR carboxylase) is an enzyme 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