Pentose Phosphate Pathway Quiz

Pentose Phosphate Pathway (PPP)

• Also known as the hexose monophosphate shunt
• Occurs in the cytosol
• Includes two phases: irreversible oxidative and reversible nonoxidative
• Irreversible Oxidative Phase: Produces ribulose 5-phosphate, CO2, and 2 NADPH per glucose 6-phosphate
• Reversible Nonoxidative Phase: Interconverts sugar-phosphates (e.g., ribose 5-phosphate, fructose 6-phosphate) to meet cellular energy or metabolic needs
• No ATP is used or produced directly

Key Functions

• NADPH Production: A major source of reducing power for
  • Fatty acid and steroid biosynthesis
  • Detoxification of reactive oxygen species (ROS)
• Ribose 5-Phosphate Production: Required for nucleotide and nucleic acid synthesis
• Five-Carbon Sugar Utilization: Metabolizes sugars from the diet or carbohydrate breakdown

Irreversible Oxidative Phase

• Dehydrogenation of Glucose 6-Phosphate is the first step

Reversible Nonoxidative Phase

• Converts ribulose 5-phosphate to either
  • Ribose 5-phosphate for nucleotide synthesis
  • Glycolytic intermediates (fructose 6-phosphate, glyceraldehyde 3-phosphate) to meet cellular energy or metabolic needs
• Key enzymes: Transketolase (requires thiamine) and Transaldolase

Uses of NADPH

• Reductive Biosynthesis: Supports synthesis of fatty acids and steroids
• Detoxification of Hydrogen Peroxide: NADPH maintains reduced glutathione, critical for neutralizing ROS; RBCs depend solely on PPP for NADPH
• Antioxidant Support: Protects cells from oxidative damage caused by ROS

Clinical Relevance

• NADPH and antioxidant mechanisms play roles in minimizing oxidative stress linked to aging, inflammation, and diseases like cancer
• Despite correlations between dietary antioxidants and reduced chronic disease risk, clinical trials have shown mixed results, emphasizing the complexity of dietary influences on health

Cytochrome P450 Monooxygenase System

• Cytochrome P450 enzymes are a superfamily of heme-containing monooxygenases
• Catalyze the hydroxylation of various substrates (e.g., steroids, drugs, toxins) using NADPH and molecular oxygen.
• Reaction: R-H + O2 + NADPH + H+ → R-OH + H2O + NADP+
• Functions in two locations:
  • Mitochondrial System: Found in steroidogenic tissues (placenta, adrenal cortex, ovaries, testes) and liver. Hydroxylates intermediates in steroid hormone biosynthesis (from cholesterol)
• Bile acid synthesis and vitamin D activation

Microsomal System

• Located in the smooth endoplasmic reticulum, especially in the liver. Detoxifies xenobiotics (e.g., drugs, pesticides).
• Hydroxylation increases solubility of toxins for excretion and allows conjugation with polar molecules (e.g., glucuronic acid).
• Polymorphisms in cytochrome P450 genes (e.g., CYP3A4) influence drug metabolism.

Phagocytosis and Reactive Oxygen Species (ROS)

• Phagocytosis : Neutrophils and macrophages engulf pathogens via receptor-mediated endocytosis, utilizing
  • Oxygen-independent mechanisms: pH changes and lysosomal enzymes.
  • Oxygen-dependent mechanisms: ROS generation via NADPH oxidase and myeloperoxidase (MPO)
• Respiratory Burst : NADPH oxidase reduces O2 to superoxide (O2¯•),leading to ROS production (H2O2, HOCl, OH•); MPO converts H2O2 to hypochlorous acid (HOCl) a potent bactericidal agent
• Clinical Note: NADPH oxidase deficiency causes chronic granulomatous disease (CGD), leading to persistent infections. MPO deficiency does not significantly increase infection susceptibility due to bactericidal activity of H2O2

Nitric Oxide (NO) Synthesis and Functions

• Synthesis: Catalyzed by nitric oxide synthase (NOS), requiring arginine, O2, NADPH, and coenzymes (e.g., FMN, FAD, heme). Three types of NOS:
  • eNOS (endothelium): Vasodilation
  • nNOS (neural tissue): Neurotransmission
  • iNOS (inducible in macrophages): Immune defense
• Functions of NO:
  • Vasodilation: Activates guanylate cyclase in vascular smooth muscle, increasing cGMP levels and promoting relaxation. Clinical applications: Nitroglycerin and sildenafil enhance NO-mediated vasodilation.
  • Macrophage Defense: Combines with O2¯• to generate bactericidal radicals
  • Platelet Inhibition: Prevents adhesion and aggregation via the cGMP pathway
  • Neurotransmission: Acts as a signaling molecule in the nervous system

Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency

• Overview: Hereditary X-linked disorder characterized by hemolytic anemia due to the inability to detoxify oxidizing agents.
• Clinical manifestations:
  • Hemolytic anemia triggered by oxidative stress
  • Neonatal jaundice due to increased unconjugated bilirubin
• Evolutionary Note: G6PD deficiency provides resistance to Plasmodium falciparum malaria, similar to sickle cell trait and β-thalassemia minor.
• Role of G6PD in Red Blood Cells (RBCs) G6PD is critical in the pentose phosphate pathway (PPP), producing NADPH needed to maintain reduced glutathione (G-SH); NADPH and G-SH protect RBCs from oxidative damage caused by free radicals and peroxides. Oxidative damage leads to Heinz bodies and rigid RBCs, removed by macrophages.

Precipitating Factors in Hemolysis

• Common triggers include oxidant drugs (antibiotics, antimalarials, antipyretics), fava beans (favism), and infections.

Properties of Variant Enzymes

• G6PD mutations result in enzymatic variants with altered properties (decreased activity, reduced stability, altered substrate or coenzyme affinity).
• Severity correlates with residual enzymatic activity (Class I-III).

Molecular Biology of G6PD Deficiency

• Over 400 variants of G6PD have been identified, most caused by missense mutations in the coding region (examples: G6PD A¯, G6PD Mediterranean)
• Complete absence of G6PD activity is likely lethal.

Clinical Implications

• Understanding G6PD variants and triggers can help manage patients with the deficiency, avoiding oxidative stressors and monitoring for complications like neonatal jaundice or chronic hemolysis.