sentences:  The crystal structure of PurE indicates a unique quaternary structure that confirms the octameric nature of the enzyme.In the bacterium "Escherichia coli" the reaction is catalyzed in two steps carried out by two separate enzymes, PurK and PurE.PurK, "N"5-carboxyaminoimidazole ribonucleotide synthetase, catalyzes the conversion of 5-aminoimidazole ribonucleotide ("AIR"), ATP, and bicarbonate to "N"5-carboxyaminoimidazole ribonucleotide ("N5-CAIR"), ADP, and Pi.