HMP Shunt Pathway Notes

Pentose Pathway / HMP Shunt Pathway

Case Presentation

• A 32-year-old male of Mediterranean descent presents with jaundice, fatigue, and dark urine three days after taking trimethoprim-sulfamethoxazole for a urinary tract infection.

• He denies significant past medical history but recalls his father experiencing a similar reaction after taking an antimalarial medication.

• Examination findings:

  • Pale with icterus.

• Laboratory workup:

  • Low hemoglobin.

  • Increased reticulocyte count.

  • Elevated indirect bilirubin.

  • Decreased haptoglobin.

  • Peripheral blood smear reveals bite cells.

  • G6PD enzyme assay confirms low enzyme activity.

Introduction

• Instead of glucose going through the glycolytic pathway, it is shunted through the HMP shunt pathway.

• Approximately 10% of glucose molecules per day enter this pathway.

• The liver and RBCs metabolize about 30% of glucose via this pathway.

• The major purpose is:

  • Generation of reduced NADPH

  • Production of pentose phosphates for nucleotide synthesis.

Overview of the Shunt Pathway

• The HMP shunt pathway has two phases:

  • Oxidative phase

  • Non-oxidative phase

• Oxidative Phase:

  • Glucose-6-phosphate is oxidized, generating 2 molecules of NADPH.

  • One molecule of pentose phosphate is produced.

  • One molecule of $CO_2$ is liberated.

• Non-Oxidative Phase:

  • Pentose phosphates are converted to intermediates of glycolysis.

Oxidative Phase

• Reaction 1: Glucose-6-phosphate is oxidized by $NADP^+$-dependent Glucose-6-phosphate dehydrogenase (G6PD).

  • 6-phosphoglucono lactone is formed.

  • One molecule of NADPH is formed.

  • This is the rate-limiting step.

  • Regulation is affected by this enzyme.

Non-Oxidative Phase

• Reactions:

  • Ribulose-5-phosphate is converted to Ribose-5-phosphate by Isomerase.

  • Ribulose-5-phosphate is converted to Xylulose-5-phosphate by Epimerase.

  • Transketolase converts Ribose 5-phosphate and Xylulose 5-phosphate into Glyceraldehyde 3-phosphate and Sedoheptulose 7-phosphate.

  • Transaldolase converts Xylulose 5-phosphate and Erythrose 4-phosphate into Fructose 6-phosphate and Glyceraldehyde 3-phosphate.

  • These products then feed into Glycolysis.

Regulation of HMP Shunt Pathway

• The pathway is mainly regulated by the level of $NADP^+$.

• The first reaction catalyzed by GPD is the rate-limiting step and is inhibited by NADPH.

• The oxidative phase is controlled by the level of $NADP^+$.

• The non-oxidative phase is controlled by the requirement of pentoses.

• Insulin will induce GPD and therefore will increase the overall pathway.

• When more NADPH is needed, the pathway proceeds to completion with the equivalent of one molecule of glucose being completely oxidized to $CO_2$.

Summary of Shunt Pathway

• Suppose 6 molecules of glucose (6 × 6 = 36 carbons) enter this pathway.

• The first carbon atoms of all 6 glucose molecules are removed as 6 molecules of $CO_2$ (equivalent to complete oxidation of 1 molecule of glucose).

• In this process, 12 NADPH are generated.

• The remaining 6 molecules of 5-carbon pentoses (6 × 5 = 30C) are further metabolized.

• Simplified Stoichiometry:

  • 6 x glucose-6-P + 12 $NADP^+$ + 6 $H<em>2O$ --> 5 x glucose-6-P + 6 $CO</em>2$ + 12 NADPH + 12 $H^+$ + Pi

  • Net: 1 glucose-6-phosphate is completely oxidized to 6 $CO_2$

Significance of HMP Shunt Pathway

• Metabolic Importance:

  1. To produce NADPH and pentose phosphates

     • NADPH is required for:

       • Reductive biosynthesis (fatty acids, cholesterol, steroid hormones).

       • Free radical scavenging.

       • Maintaining RBC membrane integrity by keeping GSH in the reduced state.

       • Prevention of methemoglobin formation.

       • Detoxification by hydroxylation.

       • Maintaining the transparency of the lens.

       • Bactericidal activity of macrophages.

     • Ribose-5-phosphate is required for nucleic acid synthesis.

• Clinical Importance:

  1. Glucose-6-phosphate dehydrogenase deficiency

  2. Drug-induced hemolytic anemia

  3. Met-hemoglobinemia

  4. Thiamine deficiency leads to reduced transketolase activity

Pathway Location

• Pathway operates in the following organs:

  • Liver

  • Adipose tissue

  • Adrenal cortex

  • Mammary glands

  • Testes and ovaries

  • RBCs

  • Lens of the eye

• The oxidative phase of the pathway is seen in organs where NADPH generation is required for lipid or steroid synthesis.

• The non-oxidative phase is present in all tissues, so synthesis of ribose is possible in all tissues of the body.

Erythrocyte Membrane Integrity

• NADPH is required by the RBC to keep glutathione in the reduced state.

• Reduced glutathione detoxifies peroxides and free radicals formed within the RBC.

• NADPH, glutathione, and glutathione reductase together preserve the integrity of the RBC membrane.

G6PD Deficiency

• The enzyme glucose-6-phosphate dehydrogenase (G6PD) may be deficient in some persons.

• It is the most common enzyme deficiency seen in clinical practice.

• The defect is transmitted as an X-linked recessive trait.

Role of G-6-PD in RBC membrane stability

• G6PD enzyme helps generate NADPH which is needed for reduced glutathione

• Reduced glutathione maintains the RBCs membrane.

G6PD Deficiency and Drug-Induced Hemolytic Anemia

• G6PD deficiency leads to drug-induced hemolytic anemia.

• The deficiency is manifested only when exposed to certain drugs or toxins (e.g., antimalarial drugs like primaquine).

• Primaquine stimulates peroxide formation inside RBCs.

• In GPD deficient cells, the level of NADPH is low; hence, further production of peroxides will lead to cell lysis.

• Ingestion of toxic glycosides present in fava beans may have a similar effect (Favism).

• Sulfa drugs and furadantin may also precipitate hemolysis, leading to jaundice and severe anemia.

Availability of Ribose

• Ribose and deoxyribose are required for DNA and RNA synthesis.

• Ribose is also necessary for nucleotide co-enzymes.

• Reversal of the non-oxidative phase is present in all tissues, by which ribose could be made available.

ATP Production

• ATP is neither utilized nor produced by the HMP shunt pathway.

• Cells do not use the shunt pathway for energy production.

Detoxification of Drugs

• Most drugs and other foreign substances are detoxified by liver microsomal P450 enzymes with the help of NADPH.

Lens of Eye

• Maximum concentration of NADPH is seen in the lens of the eye.

• NADPH is required for preserving the transparency of the lens.

Prevention of Met-Hemoglobinemia

• NADPH is also required to keep the iron of hemoglobin in the reduced (ferrous) state and to prevent the accumulation of met-hemoglobin.

• Met-hemoglobin cannot carry oxygen.

Macrophage Bactericidal Activity

• NADPH is required for the production of reactive oxygen species (ROS) (superoxide anion radical) by macrophages to kill bacteria.

Free Radical Scavenging

• Free radicals (superoxide, hydrogen peroxide) are continuously produced in all cells.

• These can destroy DNA, proteins, fatty acids, and other biomolecules, leading to cell destruction.

• The free radicals are inactivated by enzyme systems containing superoxide dismutase (SOD), peroxidase (POD), and glutathione reductase (GR).

• Reduced GR is regenerated with the help of NADPH.

• Reactions:

  • $O<em>2^-$ + $O</em>2^-$ + 2$H^+$ $\underrightarrow{SOD}$ $H<em>2O</em>2$ + $O_2$

  • $H<em>2O</em>2$ $\underrightarrow{POD}$ $2H_2O$

  • 2GSH (reduced) $\underrightarrow{GR}$ GS--SG (oxidized)

Key Investigations for G6PD Deficiency