PRPP synthetase
Summary
PRPP synthetase catalyzes the formation of phosphoribosyl pyrophosphate (PRPP) from ribose-5-phosphate and ATP, a key committed step in both purine and pyrimidine nucleotide synthesis. Overactivity of this enzyme causes excess purine production leading to hyperuricemia and gout, and can be seen in X-linked PRPP synthetase superactivity syndrome. It is a critical regulatory point in nucleotide metabolism, analogous to glycolysis/gluconeogenesis rate-limiting enzymes.
Detail
PRPP synthetase (phosphoribosyl pyrophosphate synthetase) catalyzes: Ribose-5-phosphate + ATP → PRPP + AMP. PRPP is a key precursor molecule required for de novo and salvage pathways of both purine and pyrimidine synthesis, as well as histidine and tryptophan synthesis.
Regulation: PRPP synthetase is allosterically inhibited by purine nucleotides (ADP, GDP) via feedback inhibition — this is the primary regulatory checkpoint in purine synthesis (along with glutamine-PRPP amidotransferase, the rate-limiting enzyme).
Clinical significance: 1. PRPP synthetase superactivity (X-linked): Gain-of-function mutations cause increased PRPP production, leading to purine overproduction, hyperuricemia, gout, and potentially sensorineural hearing loss and neurodevelopmental abnormalities. Distinguished from Lesch-Nyhan syndrome (HGPRT deficiency) by normal HGPRT activity. 2. Contrast with Lesch-Nyhan syndrome: HGPRT deficiency causes accumulation of PRPP (since it's not consumed in the salvage pathway), which drives excess purine synthesis and hyperuricemia — different enzyme defect, similar downstream consequence. 3. Relevant in gout pathophysiology: understanding PRPP metabolism helps explain hyperuricemia mechanisms (both overproduction and underexcretion causes).
High-yield connections: PRPP is required by: - Purine synthesis (de novo and salvage - HGPRT, APRT) - Pyrimidine synthesis (orotate phosphoribosyltransferase, part of UMP synthase) - Amino acid synthesis (histidine, tryptophan)
This makes PRPP synthetase a convergence point tested via biochemistry pathways and connected clinical disorders (gout, Lesch-Nyhan, orotic aciduria).
Sources
- First Aid for the USMLE Step 1
- Lippincott Biochemistry
- Harper's Illustrated Biochemistry
- UpToDate - Disorders of purine metabolism
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