Pentose Phosphate Pathway

PPP Overview - The NADPH & Ribose Factory

• Also known as: Hexose Monophosphate (HMP) Shunt, Phosphogluconate Pathway.
• Cellular Location: Cytosol.
• Key Tissues: Liver, adipose tissue, adrenal cortex, erythrocytes, lactating mammary glands, gonads.
• Primary Functions:
  • NADPH Production: For reductive biosynthesis (e.g., fatty acids, steroids) and antioxidant defense (maintaining reduced glutathione).
  • Ribose-5-Phosphate (R5P) Synthesis: For nucleotide and nucleic acid formation.
• No direct ATP consumption or production.

  ⭐ In erythrocytes, PPP is the sole source of NADPH, essential for preventing hemolysis by reducing oxidative stress.

Oxidative Phase - NADPH Power‑Up

• Nature: Irreversible, cytosolic.
• Primary Goal: Produce $NADPH$ & Ribulose-5-Phosphate.
• Starting Molecule: Glucose-6-Phosphate (G6P).
• Key Steps & Enzymes:

- **1. G6P $\rightarrow$ 6-Phosphogluconolactone:**
    + Enzyme: **Glucose-6-Phosphate Dehydrogenase (G6PD)**.
    + **Rate-limiting step.** Irreversible.
    + Produces **1st $NADPH$**.
    + Reg: G6PD (↑$NADP^+$, ↓$NADPH$, Insulin).
- **2. 6-Phosphogluconolactone $\rightarrow$ 6-Phosphogluconate:**
    + Enzyme: **Gluconolactonase**. Hydrolysis.
- **3. 6-Phosphogluconate $\rightarrow$ Ribulose-5-Phosphate + $CO_2$:**
    + Enzyme: **6-Phosphogluconate Dehydrogenase (6PGD)**.
    + Produces **2nd $NADPH$** and $CO_2$. Irreversible.

• Overall Yield (per G6P): 2 $NADPH$, 1 Ribulose-5-P, 1 $CO_2$.
• Significance of $NADPH$:
  • Reductive biosynthesis (e.g., fatty acids, cholesterol, steroid hormones).
  • Antioxidant defense (regenerates reduced glutathione).

⭐ G6PD deficiency is the most common enzymopathy; causes hemolytic anemia triggered by oxidative stress (e.g., fava beans, certain drugs).

Non‑Oxidative Phase - Sugar Shuffle Dance

• Reversible interconversion of pentose phosphates (e.g., Ribulose-5-P) into glycolytic intermediates Fructose-6-P (F6P) & Glyceraldehyde-3-P (GAP).
• Purpose: Produces Ribose-5-P (R5P) for nucleotide synthesis if needed, or converts excess R5P to glycolytic/gluconeogenic intermediates. No ATP used or produced.
• Key Enzymes & Reactions:
  • Transketolase (TK): Transfers a 2-carbon ketol unit. Requires Thiamine Pyrophosphate (TPP - Vitamin B1).
    • $C5 (Xylulose\textit{-5-}P) + C5 (Ribose\textit{-5-}P) \rightleftharpoons C3 (GAP) + C7 (Sedoheptulose\textit{-7-}P)$
    • $C5 (Xylulose\textit{-5-}P) + C4 (Erythrose\textit{-4-}P) \rightleftharpoons C3 (GAP) + C6 (Fructose\textit{-6-}P)$
  • Transaldolase (TA): Transfers a 3-carbon dihydroxyacetone unit.
    • $C7 (Sedoheptulose\textit{-7-}P) + C3 (GAP) \rightleftharpoons C6 (Fructose\textit{-6-}P) + C4 (Erythrose\textit{-4-}P)$
• Overall (from 3 molecules of Ribulose-5-P): $3 \times C5 \rightarrow 2 \times C6 (F6P) + 1 \times C3 (GAP)$.
• Links PPP to glycolysis (F6P, GAP enter) & gluconeogenesis.

⭐ Transketolase activity in RBCs (with/without TPP; "TPP effect") is a key diagnostic index for thiamine (Vitamin B1) deficiency. 📌 TK needs TPP.

Regulation & Clinical Significance - G6PD & Friends

• Regulation of PPP:

  • Key Enzyme: Glucose-6-Phosphate Dehydrogenase (G6PD) - rate-limiting step.
  • Primary Regulator: NADP+ (substrate availability).
    • High NADP+ (low NADPH) $\rightarrow$ ↑ G6PD activity.
    • High NADPH $\rightarrow$ Allosteric inhibitor of G6PD.
  • Hormonal: Insulin upregulates G6PD gene expression (promotes NADPH for lipogenesis).

• G6PD Deficiency: Most common human enzyme defect.

  • Inheritance: X-linked recessive.
  • Pathophysiology: ↓ NADPH $\rightarrow$ ↓ reduced glutathione $\rightarrow$ ↑ RBC oxidative stress $\rightarrow$ hemolysis.
  • Clinical Features: Neonatal jaundice, episodic acute hemolytic anemia.
    • Usually asymptomatic between episodes.
  • Triggers:
    • Oxidant Drugs: Primaquine, sulfonamides, dapsone.
    • Infections (release oxidants).
    • Fava beans (Favism).
  • Blood Smear: Heinz bodies (denatured Hb), Bite cells.

  ⭐ G6PD deficiency confers partial protection against P. falciparum malaria.

High‑Yield Points - ⚡ Biggest Takeaways

• Key products: NADPH (for reductive biosynthesis, antioxidant defense) and ribose-5-phosphate (nucleotide precursor).
• Cellular site: Exclusively in the cytosol; no direct ATP production or consumption.
• Two phases: Oxidative (irreversible, NADPH generation by G6PD) and Non-oxidative (reversible, sugar interconversions).
• Rate-limiting step: Catalyzed by Glucose-6-Phosphate Dehydrogenase (G6PD); induced by NADP+, inhibited by NADPH.
• G6PD deficiency: X-linked disorder causing hemolytic anemia due to ↓NADPH, impairing protection against oxidative stress in RBCs.