Pentose phosphate pathway

PPP Overview - The Sugar Shuffle

• The non-oxidative phase of the PPP; a reversible "sugar shuffle."
• Interconverts pentose phosphates (e.g., Ribose-5-P) into glycolytic intermediates (Fructose-6-P, Glyceraldehyde-3-P).
• Key enzymes: Transketolase and Transaldolase.
  • Allows nucleotide synthesis (from Ribose-5-P) independent of the oxidative phase.
  • No ATP is consumed or produced.

⭐ Transketolase requires Thiamine (Vitamin B1) as a cofactor. Measuring its activity in RBCs is a classic diagnostic test for thiamine deficiency (e.g., in Wernicke-Korsakoff syndrome).

Pathway Phases - Oxidative vs. Non-Oxidative

• Oxidative Phase: Irreversible & NADPH-Producing

  • Function: Generates 2 NADPH per Glucose-6-P for reductive processes (e.g., glutathione reduction, fatty acid synthesis).
  • Rate-Limiting Enzyme: Glucose-6-phosphate dehydrogenase (G6PD).
  • Regulation: G6PD is induced by insulin. It is allosterically activated by its substrate NADP⁺ and inhibited by its product NADPH.
  • End Products: 2 NADPH, Ribulose-5-P, and $CO_2$.

• Non-Oxidative Phase: Reversible & Biosynthetic

  • Function: Synthesizes Ribose-5-P for nucleotide/nucleic acid synthesis and converts pentoses into glycolytic intermediates.
  • Key Enzymes: Transketolase (requires Thiamine/B1) and Transaldolase.
  • Versatility: Allows cells to generate nucleotide precursors without producing NADPH, or vice-versa, by recycling intermediates back to glycolysis.
  • 📌 Mnemonic: Transketolase needs Thiamine.

⭐ In tissues with high cell turnover (bone marrow, skin) or cancer cells, the non-oxidative pathway is highly active to supply Ribose-5-P for DNA/RNA synthesis.

G6PD - The Rate-Limiting Boss

• Irreversible, rate-limiting step of the Pentose Phosphate Pathway (PPP).
• Reaction: Glucose-6-Phosphate + NADP+ → 6-Phosphogluconolactone + NADPH.
• Regulation is key:
  • Activated by: ↑ NADP+ (indicates need for biosynthetic precursors/reductive power).
  • Inhibited by: ↑ NADPH (product feedback inhibition).
  • Induction: Insulin upregulates G6PD gene transcription, linking PPP to the fed state (e.g., for fatty acid synthesis).

⭐ In G6PD deficiency, ↓ NADPH impairs glutathione reduction, leaving RBCs susceptible to oxidative damage. This leads to hemolytic anemia and Heinz bodies after exposure to triggers like sulfa drugs or fava beans.

G6PD Deficiency - Bite Cells & Bad Beans

• Pathophysiology: X-linked recessive defect in G6PD → ↓NADPH → ↓glutathione reduction. RBCs are susceptible to oxidative damage, leading to hemolysis.
• Triggers: Oxidative stress from infections, fava beans, or drugs (e.g., sulfonamides, primaquine, dapsone).
• Clinical/Labs:
  • Acute hemolytic anemia, jaundice, dark urine.
  • Heinz bodies: Precipitated, denatured hemoglobin.
  • Bite cells: Phagocytic removal of Heinz bodies by splenic macrophages.

⭐ The defect confers a survival advantage against Plasmodium falciparum malaria.

High‑Yield Points - ⚡ Biggest Takeaways

• The primary functions are producing NADPH for reductive biosynthesis and antioxidant defense, and ribose-5-phosphate for nucleotide synthesis.
• The pathway occurs in the cytosol.
• Glucose-6-phosphate dehydrogenase (G6PD) is the rate-limiting, irreversible step.
• G6PD deficiency (X-linked) leads to hemolytic anemia triggered by oxidative stress.
• The non-oxidative reactions, catalyzed by transketolase and transaldolase, are reversible.
• High activity in RBCs, liver, and adrenal cortex.