Bifunctional phosphoribosylaminoimidazole carboxylase and phosphoribosylaminoimidazole succinocarboxamide synthetase catalyzing two reactions of the de novo purine biosynthetic pathway. {ECO:0000269|PubMed:17224163, ECO:0000269|PubMed:2183217, ECO:0000269|PubMed:31600779}. This is the function of PAICS (phosphoribosylaminoimidazole carboxylase and phosphoribosylaminoimidazolesuccinocarboxamide synthase, ENSG00000128050).