Step by Step
Two
Two branches — different products
The pentose phosphate pathway (PPP/hexose monophosphate shunt) has two branches: Oxidative branch: glucose-6-phosphate → NADPH + ribulose-5-phosphate + CO₂. Produces 2 NADPH per glucose-6-P. Rate-limiting enzyme: glucose-6-phosphate dehydrogenase (G6PD). Non-oxidative branch: interconverts sugar phosphates, producing ribose-5-phosphate (for nucleotide synthesis) and returning carbons to glycolysis.
Memory trick: PPP = NADPH factory (oxidative) + pentose factory (non-oxidative).
NADPH
NADPH — why it matters
NADPH is the primary reducing agent for: Fatty acid synthesis (FAS needs NADPH). Cholesterol synthesis. Glutathione reductase (maintains GSH — antioxidant defense in RBCs). Cytochrome P450 reactions (drug metabolism in liver). NADPH oxidase (makes reactive oxygen species for immune killing). Without NADPH, RBCs cannot regenerate GSH → oxidative damage → hemolysis.
G6PD
G6PD deficiency — the clinical connection
G6PD deficiency is the most common enzyme deficiency worldwide (X-linked, affects males). Without G6PD, NADPH cannot be made → GSH cannot be regenerated → RBCs are vulnerable to oxidative stress. Triggers (oxidative stressors): primaquine, dapsone, fava beans, infections. Result: hemolytic anemia — RBCs burst. Heinz bodies (denatured hemoglobin) on blood smear.
Memory trick: G6PD = 'Got 6 Problems Daily' — the RBC can't handle oxidative stress without it.
Rib
Ribose-5-phosphate — for nucleotide synthesis
The non-oxidative branch produces ribose-5-phosphate (R5P) — the sugar backbone of all nucleotides (DNA, RNA, ATP, NADH, CoA, etc.). Tissues that need lots of nucleotides (bone marrow, skin, gut) run the non-oxidative branch actively. Transketolase and transaldolase are the key enzymes — both require B1 (TPP).
Applied Walkthrough
1
A red blood cell is exposed to primaquine (oxidative drug). G6PD is needed to make NADPH → regenerate GSH → neutralize oxidative stress.
2
In G6PD-deficient RBCs: no NADPH → no GSH → oxidative stress denatures hemoglobin → Heinz bodies form → RBC membrane ruptures → hemolytic anemia.
3
In a rapidly dividing cancer cell: PPP runs at high flux to provide ribose-5-P for nucleotide synthesis (DNA replication) AND NADPH for biosynthesis and antioxidant defense.
4
In the liver: high PPP activity provides NADPH for fatty acid synthesis and cytochrome P450 drug metabolism.
Exam Application
Exams test the two products (NADPH and ribose-5-phosphate), the rate-limiting enzyme (G6PD), G6PD deficiency and its triggers/consequences, and the distinction between NADPH (anabolic/antioxidant) and NADH (ETC). The clinical scenario of hemolytic anemia after primaquine or fava beans in G6PD-deficient patients is extremely high yield.
⚠ Common Trap
Students confuse NADPH with NADH — NADPH goes to biosynthesis and antioxidant defense; NADH goes to the ETC. The PPP makes NADPH, not NADH. Also: G6PD deficiency is X-linked recessive — males are primarily affected. The heterozygous state in females provides partial protection against malaria (selection advantage — explains its prevalence).
✓ Quick Self-Check
1. What are the two main products of the pentose phosphate pathway?
NADPH (from the oxidative branch) and ribose-5-phosphate (from the non-oxidative branch).
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2. What is the rate-limiting enzyme of the PPP?
Glucose-6-phosphate dehydrogenase (G6PD) — the first enzyme of the oxidative branch.
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3. Why does G6PD deficiency cause hemolytic anemia?
Without G6PD, RBCs cannot make NADPH → cannot regenerate reduced glutathione (GSH) → oxidative damage denatures hemoglobin → Heinz bodies → RBC lysis.
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4. Name three uses of NADPH.
Fatty acid synthesis, cholesterol synthesis, regenerating glutathione (antioxidant defense), cytochrome P450 reactions, NADPH oxidase (immune killing).
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5. What enzyme in the non-oxidative branch requires thiamine (B1)?
Transketolase — B1 (TPP) is an essential cofactor.
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