Step 1 · Biochemistry

Pentose Phosphate Pathway & NADPH

Name: Pentose Phosphate Pathway & NADPH
Creator: Veli Bakalov, MD

One pathway, two products, and the diseases when it fails

Biochemistry·Carbohydrate Metabolism·4 min read·8 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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The HMP shunt (cytosol, no ATP used or made) produces NADPH and ribose-5-phosphate; it is most active in liver, adrenal cortex, RBCs, and lactating mammary gland.
G6PD is the rate-limiting, irreversible enzyme of the oxidative phase (2 NADPH per glucose-6-phosphate); the reversible nonoxidative phase uses transketolase (thiamine/B1) and transaldolase.
NADPH regenerates reduced glutathione (glutathione reductase), drives fatty acid/cholesterol/steroid synthesis, fuels cytochrome P450, and powers the NADPH oxidase respiratory burst.
Glutathione peroxidase (selenium) detoxifies H2O2; RBCs have no mitochondria, so the HMP shunt is their only NADPH source.
G6PD deficiency: X-linked recessive, ~400 million people, malaria-protective; hemolysis after infection (most common), fava beans, or dapsone, primaquine, sulfonamides, nitrofurantoin, methylene blue.
Heinz bodies (supravital crystal violet stain) are plucked out by splenic macrophages to leave bite cells; assay G6PD weeks after the crisis because reticulocytes give a falsely normal result.
CGD = defective NADPH oxidase (most often X-linked gp91phox): recurrent catalase-positive infections (S. aureus, Serratia, Burkholderia cepacia, Nocardia, Aspergillus) and granulomas; abnormal DHR flow or negative NBT.
NADPH = reducing power for biosynthesis and defense; NADH = carrier that feeds oxidative phosphorylation for ATP.

The HMP shunt and its two phases

What it does. The pentose phosphate pathway (hexose monophosphate, or HMP, shunt) branches off glucose-6-phosphate in the cytosol and neither uses nor makes ATP. It yields two products: NADPH, for reductive reactions, and ribose-5-phosphate, for nucleotide synthesis. It is most active in liver, adrenal cortex, RBCs, and lactating mammary gland.

Sites of NADPH use are the sites of the shunt: fatty acid and steroid synthesis (liver, adrenal cortex, lactating breast) and red cell antioxidant defense.

Oxidative phase (irreversible). Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme, makes the first NADPH; it is stimulated by NADP⁺ and insulin and inhibited by NADPH. After a lactonase step, 6-phosphogluconate dehydrogenase makes the second NADPH and releases CO₂, leaving ribulose-5-phosphate. Net: two NADPH per glucose-6-phosphate.

Glucose-6-phosphateEntry substrate
→ 6-phosphogluconolactoneG6PD; first NADPH; rate-limiting
→ 6-phosphogluconateLactonase; part of the irreversible phase
→ ribulose-5-phosphate + NADPH6-phosphogluconate dehydrogenase; CO₂ released

Oxidative phase

Nonoxidative phase (reversible). Ribulose-5-phosphate is isomerized to ribose-5-phosphate. Transketolase (which needs thiamine, B1) and transaldolase then reversibly interconvert ribose-5-phosphate with the glycolytic intermediates fructose-6-phosphate and glyceraldehyde-3-phosphate. This lets a cell that needs NADPH but little ribose recycle the sugars back into glycolysis, and a cell that needs ribose make it from glycolytic intermediates without the oxidative steps. Erythrocyte transketolase activity is used to assess thiamine status.

Two phases of the HMP shunt

Feature	Oxidative phase	Nonoxidative phase
Reversibility	Irreversible	Reversible
Key enzymes	G6PD (rate-limiting), 6-phosphogluconate dehydrogenase	Transketolase, transaldolase
Cofactor	NADP⁺ (reduced to NADPH)	Thiamine (B1) for transketolase
Products	2 NADPH, CO₂, ribulose-5-phosphate	Ribose-5-phosphate, fructose-6-phosphate, glyceraldehyde-3-phosphate

What NADPH is for

The cell's reducing currency. NADPH drives reductive biosynthesis (fatty acids, cholesterol, steroids), regenerates reduced glutathione, fuels the cytochrome P450 system, and powers the phagocyte respiratory burst through NADPH oxidase.

The glutathione loop. Reduced glutathione (GSH) neutralizes hydrogen peroxide and free radicals through glutathione peroxidase (which needs selenium), becoming oxidized GSSG. Glutathione reductase then uses NADPH to regenerate GSH. Red cells depend completely on this loop: they have no mitochondria, and the HMP shunt is their only source of NADPH.

NADPH is the REDUCING power for biosynthesis and antioxidant/immune defense; NADH is the CARRIER that feeds oxidative phosphorylation to make ATP. Different jobs, one letter apart.

G6PD deficiency

Genetics and scale. G6PD deficiency is X-linked recessive and the most common human enzyme defect, affecting about 400 million people; it persists because it protects against malaria. Low G6PD means low NADPH, so glutathione cannot be regenerated and oxidative stress causes episodic hemolytic anemia.

~400 millionpeople with G6PD deficiency worldwide

Triggers. Hemolysis follows oxidative stress:

Infection — the most common trigger (oxidants from activated phagocytes).
Fava beans — favism.
Drugs — classically dapsone, primaquine, sulfonamides, nitrofurantoin, and methylene blue.

Think of the offenders as sulfa drugs, antimalarials, and fava beans on a background of any infection.

Smear findings. Oxidized hemoglobin precipitates as Heinz bodies, seen with a supravital stain such as crystal violet. Splenic macrophages pluck them out, leaving bite cells (degmacytes). Patients develop anemia, jaundice, and dark urine a few days after exposure; hemolysis is episodic and self-limited in the common variant.

Heinz bodies, then Bite cells: the spleen takes a bite out of the Heinz ketchup.

Testing and variants. A G6PD assay can be falsely normal during acute hemolysis, because the oldest, most deficient cells have already lysed and reticulocytes and young RBCs carry more enzyme. Retest weeks later. The African A− variant is mild and self-limited; the Mediterranean variant is more severe.

A normal G6PD level drawn during a hemolytic episode does not exclude the diagnosis.

Respiratory burst and chronic granulomatous disease

Oxygen-dependent killing. In neutrophils and macrophages, NADPH fuels the first step of the respiratory (oxidative) burst.

O₂ → superoxideNADPH oxidase (defective in CGD)
Superoxide → H₂O₂Superoxide dismutase
H₂O₂ + Cl⁻ → HOClMyeloperoxidase makes hypochlorous acid (bleach)
Microbial killingOxygen-dependent

Respiratory burst

Chronic granulomatous disease (CGD). Defective NADPH oxidase, most commonly X-linked (gp91phox subunit): phagocytes engulf microbes but cannot kill them. Patients have recurrent bacterial and fungal infections, abscesses, and granulomas from early childhood. Diagnose with an abnormal dihydrorhodamine (DHR) flow cytometry test or a negative nitroblue tetrazolium (NBT) test (cells fail to turn blue). Management includes prophylactic TMP-SMX and an antifungal such as itraconazole.

Catalase-positive organisms. CGD infections are classically caused by catalase-positive organisms: Staphylococcus aureus, Serratia marcescens, Burkholderia cepacia, Nocardia, and Aspergillus, plus E. coli and Candida. The classic explanation: catalase-negative microbes release their own H₂O₂, which CGD phagocytes can borrow for myeloperoxidase, whereas catalase-positive microbes degrade it.

Defects linked to NADPH

Feature	G6PD deficiency	CGD
Defect	G6PD (HMP shunt)	NADPH oxidase (phagocytes)
Inheritance	X-linked recessive	Most commonly X-linked
Consequence	RBCs cannot regenerate glutathione	No superoxide, so no oxidative killing
Presentation	Episodic hemolysis after oxidant stress	Recurrent catalase-positive infections, granulomas
Key test	G6PD assay after the crisis; Heinz bodies, bite cells	Abnormal DHR flow; negative NBT