Glutamine-PRPP amidotransferase !

The main switch of purine synthesis: the committed step that turns off when there are enough nucleotides.

Updated 2026-11-01

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Key facts

PRPP + glutamine + H₂O → 5-phosphoribosylamine + glutamate + PPi.
Committed, rate-limiting step of de novo purine synthesis.
AMP, GMP and IMP inhibit it (synergistic inhibition); PRPP activates it and shifts it out of its inactive form.
One domain hydrolyses glutamine and sends NH₃ through a tunnel to the PRPP site.

Excess PRPP (PRPP synthetase superactivity, HGPRT deficiency) over-activates it and raises uric acid.
Metabolites of 6-mercaptopurine and azathioprine inhibit it as if they were the cell's own nucleotides.

Think of the answer, then tap the question to see it.

Which step of purine synthesis does the amidotransferase catalyse?

The first step unique to the pathway (committed): it moves glutamine nitrogen onto PRPP, forming phosphoribosylamine.

Which end products inhibit the amidotransferase?

AMP, GMP and IMP; the effect is stronger when both adenine and guanine nucleotides are present.

Why does HGPRT deficiency speed this enzyme up?

Without salvage there is spare PRPP, its activator, and less IMP and GMP, its brakes.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: PPAT: Amidophosphoribosyltransferase (human) (UniProt, 2026); Purine ribonucleoside monophosphate biosynthesis (Reactome, 2026); 1ECC: ESCHERICHIA COLI GLUTAMINE PHOSPHORIBOSYLPYROPHOSPHATE (PRPP) AMIDOTRANSFERASE COMPLEXED WITH MN-CPRPP AND 5-OXO-NORLEUCINE (RCSB Protein Data Bank, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.