Lec 28 - Nucleotide: Purine Metabolism

Purine Metabolism - In Depth Notes

Lecture Overview

This lecture focuses on purine metabolism, including nucleotide biosynthesis, degradation, and their regulation. The key topics include the roles of PRPP, AMP, and GMP syntheses, purine salvage pathways, and conditions like gout and Lesch-Nyhan syndrome.

Page 1: Introductions

Instructor Information

  • Instructor Name: J. Scott Pattison, Ph.D.

  • Office Location: Lee Med Bldg, Room 210

  • Contact: james.pattison@usd.edu

  • Date: April 4, 2025

Page 2: Objectives of Purine Metabolism

  1. Describe the role of PRPP in nucleotide biosynthesis.

  2. Define the rate-limiting step of purine synthesis and its regulation.

  3. Identify the atom sources of the purine ring.

  4. Describe how IMP serves as a precursor for AMP and GMP.

  5. Explain the purine salvage process.

  6. Predict outcomes of enzyme deficiencies, notably Lesch-Nyhan syndrome.

  7. Understand purine catabolism and how it generates uric acid.

  8. Predict consequences related to excess uric acid (gout) and treatment rationale.

Page 3: Overview of Nucleotide Metabolism

  • Purine Metabolism includes:

    • De novo Synthesis

    • Salvage (recycling)

    • Degradation

    • Related drugs

  • Pyrimidine Metabolism includes similar pathways as purines.

Page 4: Purine Synthesis

  • Purines originated from nucleosides and ribose.

  • They are phosphorylated into ATP and GTP, derived from IMP (Inosine monophosphate).

  • Key locations for purine synthesis are the liver, brain, neutrophils, and immune cells.

Page 5: Steps of Purine Biosynthesis

  1. Synthesis of PRPP.

  2. Reaction: PRPP + Gln → PRA (5-phosphoribosylamine).

  3. Assembly of IMP from PRA.

  4. IMP + Asp + GTP → AMP.

  5. IMP + Gln + ATP → GMP.

Page 6: Key Enzymes in Purine Biosynthesis

  • PRPP Synthetase - synthesizes PRPP from Ribose-5-P; inhibited by high concentrations of purine nucleotides.

  • Glutamine-PRPP Amidotransferase - rate-limiting step, inhibited by AMP, GMP, and IMP.

Page 7: IMP Synthesis Origin

  • The purine ring is constructed using atoms from:

    • Glycine

    • Aspartate

    • Glutamine

    • CO₂

    • 10-Formyltetrahydrofolate (THF donates 2 carbons).

Page 8: IMP Structural Synthesis

  • The construction involves several compounds and ATP influences. The sequence culminates in Inosine 5'-monophosphate (IMP).

Page 9: Role of IMP in Nucleotide Synthesis

  • IMP (hypoxanthine base) serves as the primary precursor for both AMP and GMP pathways.

Page 10: AMP Formation

  • AMP is formed through the condensation of aspartate onto IMP. The enzyme adenylosuccinate synthetase uses GTP and is inhibited by AMP.

  • Fumarate is released during this process.

Page 11: GMP Formation

  • IMP dehydrogenase oxidizes IMP to xanthosine monophosphate (XMP) using NAD+. It is the third committed step and is inhibited by GMP and XMP.

Page 12: Regulation of Purine Biosynthesis

  • PRPP Synthetase: inhibited by IMP, AMP, GMP.

  • Glutamine-PRPP Amidotransferase: feedback inhibited by IMP, AMP, GMP.

  • Adenylosuccinate Synthetase: negatively regulated by AMP; IMP Dehydrogenase is negatively regulated by XMP, GMP.

Page 13: Purine Salvage Pathway

  • This pathway recycles purine bases into nucleotides from dietary sources and cellular breakdown of nucleic acids.

  • HGPRT (Hypoxanthine-guanine phosphoribosyltransferase) converts purines into the corresponding nucleotides.

Page 14: Comparison of Pathways

  • De novo synthesis is energetically costly and highly regulated, while salvage pathways are more efficient.

  • Ribonucleotide reductase is essential for dNTP synthesis required for DNA.

Page 15: Digestion of Dietary Nucleotides

  • Dietary nucleic acids are digested by enzymes and absorbed, with most bases being degraded to form uric acid.

Page 16: Urea vs. Uric Acid

  • Distinguishes between the two products of nitrogen metabolism, where uric acid is precipitated out, leading to conditions like gout.

Page 17: Lesch-Nyhan Syndrome

  • A genetic disorder caused by defective HGPRT, leading to increased purine synthesis and uric acid excretion, characterized by behavioral issues and no effective symptom treatments.

Page 18: Nucleotide Salvage Pathway Functions

  • Used to reconstitute nucleotide monophosphates with less energy than de novo synthesis.

Page 19: Purine Degradation Overview

  • Main pathway leads to uric acid synthesis primarily in the liver, utilizing salvage enzymes.

Page 20: Key Enzymes in Purine Degradation

  • Xanthine oxidase and xanthine dehydrogenase are critical for oxidizing hypoxanthine and xanthine to uric acid.

Page 21: Gout and Its Causes

  • Gout arises from excess uric acid due to either overproduction or under-excretion, leading to joint inflammation from uric acid crystal deposits.

Page 22: Gout Treatment Strategies

  • Allopurinol: a xanthine oxidase inhibitor, facilitates the excretion of more soluble intermediates like xanthine.

Page 23: Mechanism of Allopurinol

  • An inhibitor mechanism detailed its competitive and suicide inhibition characteristics on xanthine oxidase.

Page 24: Clinical Considerations

  • Adenosine Deaminase Deficiency leads to immune dysfunction and SCID, necessitating treatments like enzyme replacements and gene therapy.

Page 25: Gene Therapy for ADA-SCID

  • Describes a cutting-edge treatment approach using retroviral vectors to insert functional ADA into a patient's hematopoietic stem cells.

Page 26: Summary of Purine-Related Diseases

  • Highlights conditions like Gout and ADA-SCID, along with affected metabolic pathways and clinical impacts.

Page 27: Conclusion on Purine Metabolism

  • Recap of pathways and synthesis types emphasizing their regulation and clinical significance.

Page 28: Questions and Clarifications

  • Open session to clarify nucleotide structures and metabolism implications.