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Amidophosphoribosyltransferase (ATase), also known as glutamine phosphoribosylpyrophosphate amidotransferase (GPAT), is an enzyme that in humans is encoded by the PPAT (phosphoribosyl pyrophosphate amidotransferase) gene.

Function
ATase is an enzyme that converts α-phosphoribosylpyrophosphate (α-PRPP) into 5-β-phosphoribosylamine. The enzyme uses the ammonia group from the glutamine side-chain. This is the committing step in de novo purine synthesis. It is allosterically inhibited by AMP, GMP, and IMP. 6TGMP (6-thioguanine monophosphate) is a pseudo inhibitor for ATase

ATase is a member of the purine/pyrimidine phosphoribosyltransferase family. This protein is a regulatory allosteric enzyme that catalyzes the first step of de novo purine nucleotide biosynthesis.