Biochemistry Chapter 13 Summary PDF

Summary: Pentose Phosphate Pathway (PPP) Overview: The Pentose Phosphate Pathway (PPP), also known as the hexose monophosphate shunt, occurs in the cytosol and includes: 1. Irreversible Oxidative Phase: Produces ribulose 5-phosphate, CO₂, and 2 NADPH per glucose 6-phosphate. 2. Re...

Summary: Pentose Phosphate Pathway (PPP) Overview: The Pentose Phosphate Pathway (PPP), also known as the hexose monophosphate shunt, occurs in the cytosol and includes: 1. Irreversible Oxidative Phase: Produces ribulose 5-phosphate, CO₂, and 2 NADPH per glucose 6-phosphate. 2. Reversible Nonoxidative Phase: Interconverts sugar-phosphates (e.g., ribose 5-phosphate, fructose 6-phosphate) to meet cellular needs. No ATP is directly used or produced. Key Functions: 1. NADPH Production: ○ A major source of reducing power for: Fatty acid and steroid biosynthesis. Detoxification of reactive oxygen species (ROS). 2. Ribose 5-Phosphate Production: Required for nucleotide and nucleic acid synthesis. 3. Five-Carbon Sugar Utilization: Metabolizes sugars from the diet or carbohydrate breakdown. Irreversible Oxidative Phase: 1. Dehydrogenation of Glucose 6-Phosphate:  ○ Enzyme: Glucose 6-phosphate dehydrogenase (G6PD). ○ Regulated by NADPH/NADP⁺ ratio and upregulated by insulin. 2. Formation of Ribulose 5-Phosphate: ○ Catalyzed by 6-phosphogluconolactone hydrolase and 6-phosphogluconate dehydrogenase, releasing CO₂ and generating a second NADPH. 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: 1. Reductive Biosynthesis: ○ Supports synthesis of fatty acids and steroids. 2. Detoxification of Hydrogen Peroxide: ○ NADPH maintains reduced glutathione, critical for neutralizing ROS. ○ RBCs depend solely on PPP for NADPH. 3. 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. Summary: Cytochrome P450 Monooxygenase System and Related Pathways 1. Cytochrome P450 Monooxygenase System Cytochrome P450 enzymes are a superfamily of heme-containing monooxygenases that 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 (e.g., 25-hydroxycholecalciferol in liver; 1,25-dihydroxycholecalciferol in kidney). 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. 2. 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. 3. Nitric Oxide (NO) Synthesis and Functions Synthesis: 1. Catalyzed by nitric oxide synthase (NOS), requiring arginine, O₂, NADPH, and coenzymes (e.g., FMN, FAD, heme). 2. Three types of NOS: eNOS (endothelium): Vasodilation. nNOS (neural tissue): Neurotransmission. iNOS (inducible in macrophages): Immune defense. Functions of NO: 1. Vasodilation: Activates guanylate cyclase in vascular smooth muscle, increasing cGMP levels and promoting relaxation. Clinical applications: Nitroglycerin and sildenafil enhance NO-mediated vasodilation. 2. Macrophage Defense: Combines with O₂⁻ to generate bactericidal radicals. 3. Platelet Inhibition: Prevents adhesion and aggregation via the cGMP pathway. 4. Neurotransmission: Acts as a signaling molecule in the nervous system. Note: NO is a short-lived gas that decomposes to nitrates and nitrites, including reactive nitrogen species like peroxynitrite (O=NOO⁻). Summary: Glucose-6-Phosphate Dehydrogenase (G6PD) Deficiency 1. Overview G6PD deficiency is a hereditary X-linked disorder characterized by hemolytic anemia due to the inability to detoxify oxidizing agents. It is the most common enzyme deficiency worldwide, affecting over 400 million people, particularly in the Middle East, Africa, Asia, and the Mediterranean. Clinical manifestations include: 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. 2. 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: Insoluble denatured hemoglobin that damages RBC membranes. ○ Rigid RBCs, which are removed by macrophages in the spleen and liver. RBC vulnerability: ○ Lack of alternative NADPH sources (e.g., malic enzyme). ○ Inability to synthesize new enzymes due to the absence of a nucleus or ribosomes. 3. Precipitating Factors in Hemolysis Most individuals are asymptomatic unless exposed to oxidative stress. Common triggers include: 1. Oxidant Drugs: ○ Remembered by the mnemonic AAA: Antibiotics (e.g., sulfamethoxazole, chloramphenicol). Antimalarials (e.g., primaquine but not chloroquine). Antipyretics (e.g., acetanilid but not acetaminophen). 2. Fava Beans (Favism): ○ Common in individuals with the Mediterranean variant of G6PD deficiency. 3. Infections: ○ The inflammatory response generates free radicals in macrophages, causing oxidative damage to RBCs. 4. Properties of Variant Enzymes  G6PD mutations (often point mutations) result in enzymatic variants with altered properties: ○ Decreased activity. ○ Reduced stability. ○ Altered substrate or coenzyme affinity (e.g., NADP⁺, NADPH, glucose-6-phosphate). Severity correlates with residual enzymatic activity: ○ Class I: Rare, severe, chronic hemolysis even without oxidative stress. ○ Class II: Severe hemolysis with oxidative stress (e.g., Mediterranean variant). ○ Class III: Moderate hemolysis triggered by stress (e.g., G6PD A⁻ variant). 5. Molecular Biology of G6PD Deficiency Over 400 variants of G6PD have been identified, with most caused by missense mutations in the coding region. Examples: ○ G6PD A⁻: A moderate variant with normal activity in younger RBCs but reduced activity in older cells. ○ G6PD Mediterranean: A severe variant with unstable enzyme and decreased activity. Complete absence of G6PD activity is likely lethal, as no large deletions or frameshift mutations have been observed. 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.