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