purine
Summary
Purines are nitrogen-containing heterocyclic compounds (adenine and guanine) that serve as building blocks of DNA, RNA, ATP, GTP, and cofactors like NAD+ and CoA. They are synthesized de novo or via salvage pathways and are catabolized to uric acid in humans, making purine metabolism clinically relevant to gout and immunodeficiencies.
Detail
Purines (adenine, guanine, plus hypoxanthine and xanthine as intermediates) are synthesized de novo starting from PRPP (phosphoribosyl pyrophosphate) and ribose-5-phosphate, incorporating atoms from glycine, aspartate, glutamine, CO2, and N10-formyl-THF, ultimately forming IMP, which is converted to AMP or GMP. The rate-limiting enzyme is glutamine-PRPP amidotransferase. Salvage pathways recycle free purine bases into nucleotides using HGPRT (hypoxanthine-guanine phosphoribosyltransferase) and adenine phosphoribosyltransferase (APRT), conserving energy. HGPRT deficiency causes Lesch-Nyhan syndrome (X-linked recessive), characterized by hyperuricemia, gout, intellectual disability, self-mutilation, and choreoathetosis, due to excess purine degradation to uric acid. PRPP synthetase overactivity also causes hyperuricemia/gout via excessive purine synthesis. Purine catabolism proceeds through hypoxanthine and xanthine to uric acid via xanthine oxidase; allopurinol and febuxostat inhibit this enzyme to treat gout and tumor lysis syndrome. Adenosine deaminase (ADA) deficiency impairs purine salvage/degradation, causing toxic accumulation of dATP that is lethal to lymphocytes, resulting in severe combined immunodeficiency (SCID). Purines are essential for nucleic acid synthesis, energy transfer (ATP), signal transduction (cGMP, cAMP as pyrimidine... no ATP/GTP), and coenzyme structure (NAD+, FAD, CoA). Clinically, purine metabolism disorders are tested via enzyme deficiencies and their downstream effects—recognizing hyperuricemia in gout, orotic aciduria differentiation (pyrimidine pathway), and immunodeficiency syndromes (ADA deficiency, PNP deficiency) is high-yield for boards.
Sources
- First Aid for the USMLE Step 1
- Lippincott Illustrated Reviews: Biochemistry
- Harper's Illustrated Biochemistry
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