The purine biosynthetic pathway in prokaryotes enlists eleven enzymes, six of which use ATP. Enzymes 5 and 6 of this pathway, formylglycinamide ribonucleotide (FGAR) amidotransferase (PurL) and aminoimidazole ribonucleotide (AIR) synthetase (PurM or AIRS) utilise ATP to activate the oxygen of an amide within their substrate toward nucleophilic attack by a nitrogen. PurM (also known as phosphoribosyl-aminoimidazole synthetase or phosphoribosylformylglycinamidine cyclo-ligase) uses the product of PurL, formylglycinamidine ribonucleotide (FGAM) and ATP to make AIR, ADP and P(i). The N-terminal domain of PurM is related to the ATP-binding domains of hydrogen expression/formation protein HypE, the AIR synthases, selenophosphate synthetase (SelD), and FGAM synthase and is thought to bind ATP.