This subgroup contains EcASL, the product of the purB gene in Escherichia coli, and related proteins. It is a member of the Lyase class I family of the Lyase_I superfamily. Members of the Lyase class I family function as homotetramers to catalyze similar beta-elimination reactions in which a Calpha-N or Calpha-O bond is cleaved with the subsequent release of fumarate as one of the products. The four active sites of the homotetrameric enzyme are each formed by residues from three different subunits. ASL catalyzes two non-sequential steps in the de novo purine biosynthesis pathway: the conversion of 5-aminoimidazole-(N-succinylocarboxamide) ribotide (SAICAR) into 5-aminoimidazole-4-carboxamide ribotide (AICAR) and; the conversion of adenylosuccinate (SAMP) into adenosine monophosphate (AMP).