HMP Shunt (Hexose Monophosphate Shunt / Pentose Phosphate Pathway)

Synonyms

Also called the Pentose Phosphate Pathway (PPP) or Phosphogluconate Pathway.

Location and Basic Features

Occurs entirely in the cytosol
Runs in all cells (since all cells need NADPH for reductive detoxification, and most need ribose-5-P for nucleotides)
Does not produce or consume ATP
About 10% of glucose is metabolized via this route in most tissues; the rest goes through glycolysis

Two Main Products

Product	Primary Use
NADPH	Fatty acid synthesis, cholesterol synthesis, steroid hormone synthesis, glutathione regeneration, CYP450 reactions
Ribose 5-phosphate	Nucleotide and nucleic acid biosynthesis

Phase 1: Irreversible Oxidative Phase

Glucose 6-phosphate → → → Ribulose 5-phosphate + 2 NADPH + CO2

Step-by-step:

Glucose 6-phosphate dehydrogenase (G6PD) - the committed, rate-limiting step
- Oxidizes glucose 6-P → 6-phosphogluconolactone
- Reduces NADP+ → NADPH (#1)
- Regulated: NADPH is a potent competitive inhibitor of G6PD; when NADPH/NADP+ falls, flux increases
- Insulin upregulates G6PD expression (active in the fed/absorptive state)
6-Phosphogluconolactone hydrolase
- Hydrolyzes the lactone → 6-phosphogluconate
6-Phosphogluconate dehydrogenase
- Oxidative decarboxylation: releases C1 as CO2
- Reduces NADP+ → NADPH (#2)
- Product: Ribulose 5-phosphate

Net from oxidative phase per glucose 6-P: 2 NADPH + CO2 + ribulose 5-P

Pentose phosphate pathway - oxidative and nonoxidative reactions with enzymes labeled

Phase 2: Reversible Nonoxidative Phase

Ribulose 5-P is interconverted among various sugar phosphates by four enzymes:

Enzyme	Action	Coenzyme
Isomerase	Ribulose 5-P → Ribose 5-P	-
Epimerase	Ribulose 5-P → Xylulose 5-P	-
Transketolase	Transfers 2-carbon fragments (Δ2C)	Thiamine pyrophosphate (TPP)
Transaldolase	Transfers 3-carbon fragments (Δ3C)	-

Sugar interconversions: 5C + 5C → 7C + 3C → 4C + 6C

The reversible phase feeds fructose 6-phosphate and glyceraldehyde 3-phosphate back into glycolysis.

Nonoxidative phase interconversions and connection to glycolysis

Overall Equation (for 3 glucose 6-P through the full pathway)

3 Glucose 6-P + 6 NADP+ → 3 CO2 + 6 NADPH + 2 fructose 6-P + glyceraldehyde 3-P

Compared to glycolysis of 3 glucose 6-P: the HMP shunt yields more NADPH (6 mol) but less ATP (8 vs 9 mol) and less pyruvate.

Regulation

The pathway direction depends on the cell's metabolic needs:

Situation	What happens
Need NADPH + ribose 5-P	Oxidative phase runs fully
Need ribose 5-P only (NADPH adequate)	Inhibit oxidative phase; use glycolytic intermediates via nonoxidative phase in reverse
Need NADPH only (ribose 5-P adequate)	Run nonoxidative phase to recycle ribose 5-P back to glucose 6-P
Adequate NADPH, adequate ribose 5-P	Return pentose phosphates to glycolysis

Tissue Distribution (Importance)

Tissue	Why HMP shunt is active
Liver, adipose, lactating mammary gland	NADPH for fatty acid synthesis
Adrenal cortex, testes, ovaries, placenta	NADPH for steroid hormone synthesis
Red blood cells	NADPH to regenerate reduced glutathione (antioxidant protection)
All proliferating cells	Ribose 5-P for nucleotide biosynthesis

Critical Role in Erythrocytes and G6PD Deficiency

In mature RBCs (no mitochondria), the HMP shunt is the only source of NADPH. NADPH is required to regenerate reduced glutathione (GSH) via glutathione reductase. GSH detoxifies H2O2 and other reactive oxygen species (ROS) via glutathione peroxidase.

The cycle: Oxidant stress → GSSG forms → G6PD activated (NADPH/NADP+ falls) → more NADPH → GSH regenerated

G6PD deficiency (most common human enzyme defect, X-linked) disrupts this cycle:

NADPH cannot be made → GSH cannot be regenerated → RBCs susceptible to oxidative hemolysis
Triggered by: oxidant drugs (primaquine, dapsone, nitrofurantoin), infections, fava beans
Causes episodic hemolytic anemia

Thiamine Deficiency Link

Transketolase requires thiamine pyrophosphate (TPP). Transketolase activity in RBCs is used as a functional marker of thiamine (vitamin B1) nutritional status - low activity that is dramatically stimulated by adding TPP in vitro indicates thiamine deficiency (as seen in beriberi and Wernicke's encephalopathy).

Sources: Biochemistry (Lippincott Illustrated Reviews) 8e, p. 424-435; Basic Medical Biochemistry: A Clinical Approach 6e, p. 959-964; Goldman-Cecil Medicine, p. 1700-1701; Henry's Clinical Diagnosis and Management by Laboratory Methods

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