Structural domains comprising this superfamily share the structure of C-terminal subdomain of the FGAM (formylglycinamidine ribonucleotide) synthetase domain, found in the enzyme formylglycinamide ribonucleotide amidotransferase (FGAR-AT), encoded by the purL gene and also known as PurL, which catalyzes the ATP-dependent synthesis of formylglycinamidine ribonucleotide (FGAM) from formylglycinamide ribonucleotide (FGAR) and glutamine, the fourth step of the purine biosynthetic pathway.

Two forms of PurL have been characterized, large and small. Large PurL is present in most Gram-negative bacteria and eukaryotes and consists of a single polypeptide chain comprised by an an N-terminal domain, a FGAM synthetase domain, and a glutaminase domain, with a putative ammonia channel located between the active sites of the latter two. Small PurL, present in Gram-positive bacteria and archaea, is structurally homologous to the FGAM synthetase domain of large PurL, and forms a complex with two additional gene products, PurQ and PurS.

The FGAM synthetase domain is divided into four subdomains: A1 and A2 which make up the core of the enzyme and B1 and B2 which represent the N-terminal and C-terminal subdomains, respectively PMID:17154526,PMID:18597481.