Amidophosphoribosyltransferase
![](/Images/godic/202412/20/ATase_crystal_structure2441.png")
![First half of the catalytic mechanism of ATase occurring in the glutaminase domain active site. The catalytic cysteine performs a nucleophilic attack on the substrate to form an acyl-enzyme intermediate, which is resolved by hydrolysis. Ammonia is produced in the third step, which is used in the second half of the mechanism.Second half of the catalytic mechanism of ATase occurring in the phosphoribosyltransferase domain active site. The ammonia liberated in the first half of the reaction replaces pyrophosphate in PRPP, yielding phosphoribosylamine. A tyrosine residue stabilizes the transition state and allows the reaction to occur.](/Images/godic/202412/20/Mechanism_of_ATase,_first_half2441.png")
![Arrow pushing mechanism for the reaction catalyzed by ATase.](/Images/godic/202412/20/Mechanism_of_ATase,_second_half2441.png")
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Amidophosphoribosyltransferase (ATase), also known as glutamine phosphoribosylpyrophosphate amidotransferase (GPAT), is an enzyme responsible for catalyzing the conversion of 5-phosphoribosyl-1-pyrophosphate (PRPP) into 5-phosphoribosyl-1-amine (PRA), using the ammonia group from a glutamine side-chain. This is the committing step in de novo purine synthesis. In humans it is encoded by the PPAT (phosphoribosyl pyrophosphate amidotransferase) gene. ATase is a member of the purine/pyrimidine phosphoribosyltransferase family.