HMP Shunt

Overview

Every cell in the body constantly walks a tightrope between building new molecules and defending itself against damage, and the HMP shunt is one of the main tools that makes both possible. Rather than producing energy the way glycolysis does, this pathway diverts glucose 6-phosphate down a side route to generate two things a cell cannot survive without: NADPH, the reducing power behind fatty acid synthesis, steroid hormone production, and the neutralization of reactive oxygen species, and ribose 5-phosphate, the sugar backbone of every nucleotide the cell will ever build.

• Function 
  • generate NADPH
    • required for FA synthesis, steroid synthesis, reduction of oxidizing agents (H₂O₂ see figure) 
  • provide ribose 5-phosphate
    • required for nucleotide synthesis
• Pathway
  • occurs in cytoplasm of all cells
  • no ATP consumed or generated
  • 2 phases
    • oxidative
      • produces NADPH
      • glucose 6-phosphate (G6P) → 6-phosphogluconate
        • catalyzed by glucose 6-phosphate dehydrogenase (G6PDH)
          • rate limiting step
          • activated by NADP+, insulin
          • inhibited by NADPH
      • irreversible
    • nonoxidative
      • exchanging intermediate substrates between glycolysis and HMP shunt
        • catalyzed by transketolase 
          • requires thiamine
      • reversible
• Clinical relevance
  • glucose-6-phosphate dehydrogenase (G6PDH) deficiency
    • pathophysiology
      • ↓ NADPH production
        • cells (specifically RBCs) lose protection against oxidizing agents
          • cannot regenerate glutathione
      • XR
      • most common human enzyme deficiency
      • ↑ prevalence among blacks
        • ↑ malarial resistance
          • by shortening the circulation life of RBCs
            • Plasmodium does not have enough time for life span
          • plasmodium does not have defense against free radicals
            • ↑ in free radicals kills parasite
    • presentation
      • episodic hemolytic anemia
        • intravascular hemolysis
        • normocytic
        • 2-3 days post precipitating stress
          • foods
            • fava beans
              • common in Mediterranean foods
              • presentation
                • pallor, hemoglobinuria 24-48 post ingestion
          • drugs
            • sulfonamides, primaquine, antituberculosis drugs
          • infection
            • free radicals generated by the immune system
      • Heinz bodies 
        • oxidized hemoglobin that precipitates within RBCs
      • bite cells 
        • result from the phagocytic removal of Heinz bodies by macrophages 
      • back pain
    • test
      • active hemolysis screen
• Heinz body prep 

Introduction

The HMP shunt, also known as the pentose phosphate pathway, is a metabolic pathway that occurs in the cytoplasm of cells in most organisms. It is a parallel pathway to glycolysis, and its primary function is to generate NADPH and ribose-5-phosphate, which are important for various cellular processes, including biosynthesis and antioxidant defense.

The HMP shunt consists of two phases: the oxidative phase and the non-oxidative phase. In the oxidative phase, glucose-6-phosphate is oxidized and decarboxylated to generate NADPH and ribulose-5-phosphate. This phase also produces intermediate metabolites that can enter glycolysis or be used for biosynthesis. In the non-oxidative phase, the intermediates of the oxidative phase are rearranged and converted to other sugars, including ribose-5-phosphate, which is an important component of nucleotides and nucleic acids.

Clinical Significance Beyond G6PD Deficiency

NADPH from the HMP shunt also supports processes outside RBC survival. Increased pathway activity has been observed in some cancer cells, where it supplies the NADPH and ribose-5-phosphate needed for rapid proliferation. G6PD deficiency itself has been linked in limited studies to altered oxidative stress responses in conditions such as hypertension and viral infection, though evidence in these areas remains preliminary compared to its well-established role in hemolytic anemia.

Key References

G6PD deficiency is the most common human enzyme defect, affecting an estimated 400 million people worldwide, historically concentrated in populations with endemic malaria exposure.

Insufficient NADPH impairs glutathione reductase’s ability to regenerate reduced glutathione, causing oxidative damage that denatures hemoglobin into Heinz bodies and stiffens the RBC membrane.

Over 186 distinct G6PD gene mutations have been catalogued worldwide, producing a range of clinical severity from asymptomatic to chronic hemolysis. (Source: PMC, ncbi.nlm.nih.gov/pmc/articles/PMC5187869)

Emerging research is examining G6PD’s role in cardiovascular disease and redox-related conditions beyond hemolysis, though clinical evidence here is still limited.

The Nonoxidative Phase

This phase is reversible and does not produce NADPH. Its job is to shuffle sugar intermediates back and forth between the HMP shunt and glycolysis, depending on what the cell needs at the moment.

Transketolase drives this phase and needs thiamine as a cofactor, which is why thiamine deficiency can affect this pathway too.

Depending on the cell’s needs, the nonoxidative phase can either produce more ribose 5-phosphate for nucleotide synthesis, or convert leftover sugars back into fructose 6-phosphate and glyceraldehyde 3-phosphate, which then feed back into glycolysis.

G6PD Deficiency

This is the clinical condition almost every student associates with the HMP shunt, and for good reason. It is the most common enzyme deficiency in humans and follows an X linked inheritance pattern.

When G6PD is deficient, cells make less NADPH. Less NADPH means less reduced glutathione, and without enough reduced glutathione, cells lose their main defense against oxidizing agents. Red blood cells feel this the hardest, since they have no nucleus and cannot replace damaged proteins the way other cells can.

Interestingly, this deficiency is more common in people with ancestry from malaria endemic regions, including parts of Africa, the Mediterranean, and Southeast Asia. The reason ties back to malaria itself. Red blood cells with less protection against oxidative stress break down faster, which shortens the amount of time the malaria parasite has to complete its life cycle inside them, and the parasite also has a harder time surviving the buildup of free radicals in a G6PD deficient cell.

How It Shows Up

Patients typically present with episodic hemolytic anemia, meaning the hemolysis is not constant but triggered by specific exposures. The hemolysis is intravascular and the anemia is usually normocytic, showing up two to three days after whatever triggered it.

Common triggers include fava beans, which cause a reaction known as favism and are more often linked to a Mediterranean variant of the deficiency, oxidant drugs such as sulfonamides, primaquine, and certain antituberculosis medications, and infections, since the immune response itself generates free radicals.

Look for pallor, dark urine from hemoglobinuria roughly one to two days after the trigger, and sometimes back pain during an active episode.

On a blood smear, you may see Heinz bodies, which are clumps of oxidized, precipitated hemoglobin sitting inside the red cells, and bite cells, which form when macrophages in the spleen bite off and remove those Heinz bodies as the cell passes through.

Why This Pathway Matters Beyond G6PD Deficiency

The HMP shunt is not just a topic for one disease. Because rapidly dividing cells need large amounts of both NADPH and ribose 5-phosphate to support biosynthesis and DNA replication, this pathway tends to run faster in fast growing tissue, including many cancer cells. That connection is one reason the pathway shows up again in physiology and pathology discussions beyond hematology.

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