Catalyzes two reactions in de novo purine nucleotide biosynthesis. Catalyzes the breakdown of 5-aminoimidazole- (N-succinylocarboxamide) ribotide (SAICAR or 2-[5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamido]succinate) to 5-aminoimidazole-4-carboxamide ribotide (AICAR or 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide) and fumarate, and of adenylosuccinate (ADS or N(6)-(1,2-dicarboxyethyl)-AMP) to adenosine monophosphate (AMP) and fumarate. N(6)-(1,2-dicarboxyethyl)-AMP = AMP + fumarate (2S)-2-[5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamido]succinate = 5-amino-1-(5-phospho-beta-D-ribosyl)imidazole-4-carboxamide + fumarate Purine metabolism; AMP biosynthesis via de novo pathway; AMP from IMP: step 2/2. Purine metabolism; IMP biosynthesis via de novo pathway; 5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxamide from 5-amino-1-(5-phospho-D-ribosyl)imidazole-4-carboxylate: step 2/2. Homooligomer. Residues from neighboring subunits contribute catalytic and substrate-binding residues to each active site (By similarity). Belongs to the lyase 1 family. Adenylosuccinate lyase subfamily.