Phosphoribosylamine synthesis is the first committed and highly regulated step in the purine de novo synthesis pathway. In this reaction, the enzyme glutamine phosphoribosyl amidotransferase catalyzes the addition of ammonia to PRPP, producing 5-phosphoribosyl-1-amine and PPi. This enzyme is allosterically regulated and is inhibited by the downstream products AMP, ATP, GMP, and GTP.