Purine and pyrimidine synthesis and salvage pathways and the disorders (gout, Lesch-Nyhan syndrome) tied to their enzyme deficiencies tested on USMLE Step 1.
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- What is the rate-limiting enzyme of de novo purine synthesis?
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Glutamine-PRPP amidotransferase- What reaction does PRPP synthetase catalyze?
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Ribose-5-phosphate + ATP form PRPP- In de novo purine synthesis, is the ring built directly on PRPP or built first and then attached?
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Built directly on PRPP- In de novo pyrimidine synthesis, is the ring built first or built directly on PRPP?
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The ring is built first, then attached to PRPP- What is the rate-limiting enzyme of de novo pyrimidine synthesis?
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Carbamoyl phosphate synthetase II (CPS II)- Where in the cell is carbamoyl phosphate synthetase II (CPS II) located?
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The cytosol- Carbamoyl phosphate synthetase I (CPS I) functions in which pathway and organelle?
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The urea cycle, in the mitochondria- Lesch-Nyhan syndrome is caused by a deficiency of ____.
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HGPRT (hypoxanthine-guanine phosphoribosyltransferase)- What is the inheritance pattern of Lesch-Nyhan syndrome?
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X-linked recessive- What is the classic clinical triad of Lesch-Nyhan syndrome?
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Hyperuricemia, self-mutilation, and intellectual disability with dystonia- What reaction does HGPRT normally catalyze?
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Salvages hypoxanthine and guanine into IMP and GMP using PRPP- Why does HGPRT deficiency lead to excess uric acid production?
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Unused PRPP accumulates and drives excess de novo purine synthesis, increasing purine degradation to uric acid- Does allopurinol relieve the neurologic symptoms of Lesch-Nyhan syndrome?
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No, it only lowers uric acid; it does not improve the neurologic symptoms- Adenine phosphoribosyltransferase (APRT) salvages which base into which nucleotide?
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Adenine into AMP- Deficiency of which enzyme is the most common metabolic cause of severe combined immunodeficiency (SCID)?
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Adenosine deaminase (ADA)
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