The conversion of 5'-phosphoribosyl-5-aminoimidazole-4-carboxylate (CAIR) and aspartate to 5'-phosphoribosyl-5-aminoimidazole-4-N-succinocarboxamide (SAICAR), accompanied by the conversion of ATP to ADP and orthophosphate, is catalyzed by the phosphoribosylaminoimidazole succinocarboxamide synthetase domain of the bifunctional protein "phosphoribosylaminoimidazole carboxylase, phosphoribosylaminoimidazole succinocarboxamide synthetase" (PAICS) (Schild et al. 1990). The enzyme is an octamer (Li et al. 2007); it may associate in the cytosol with other enzymes of de novo IMP biosynthesis under some metabolic conditions (An et al. 2008).