Carbohydrate Metabolism Overview

CHO Metabolism

• CHO metabolism involves the fate of food molecules after digestion and absorption.
• It encompasses chemical enzymatic reactions inside the body, focusing on synthesis and breakdown of substances.

Metabolic Pathways

• Anabolic pathways: Synthesize complex molecules from simpler ones, requiring energy (e.g., protein synthesis).
• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy (e.g., oxidative processes).
• Amphibolic pathways: Act as links between anabolic and catabolic pathways (e.g., citric acid cycle).

Carbohydrate Metabolism

• CHO contributes 50% of daily caloric intake.
• Complete oxidation of 1 gram of CHO yields 4 kcal.
• Sources of CHO in food:
  • Starch (50%, e.g., potatoes)
  • Sucrose and lactose
  • Fructose and glucose (fruits, honey)

Digestion of CHO

• Polysaccharides and disaccharides must be broken down into monosaccharides for absorption.
• Enzymes involved:
  • Salivary amylase: Converts starch and glycogen into dextrins.
  • Pancreatic amylase: Converts dextrins into maltose.
  • Intestinal disaccharidases:
    • Maltase: Converts maltose into glucose.
    • Sucrase: Converts sucrose into glucose and fructose.
    • Lactase: Converts lactose into glucose and galactose.

Lactose Intolerance

• Definition: A condition stemming from lactase deficiency, potentially congenital or acquired.
• Cause: Insufficient lactase enzyme.
• Effects: Undigested lactose accumulates in the intestine.
• Symptoms: Abdominal distension, cramps, diarrhea.
• Treatment: Lactose-free milk or formula.

Metabolic Pathways of Carbohydrates

• Catabolic pathways:
  • Glycolysis
  • Hexose monophosphate shunt
  • Uronic acid pathway
  • Glycogenolysis
• Anabolic pathways:
  • Gluconeogenesis
  • Glycogenesis
• Amphibolic pathway:
  • Citric acid cycle

Glycolysis

• Definition: Oxidation of glucose to pyruvate (in presence of oxygen) or lactate (in absence of oxygen).
• Site: Cytoplasm of all cells, crucial for tissues lacking mitochondria (e.g., red blood cells).
• Steps:
  • Stage 1 (energy requiring): Glucose conversion to glyceraldehyde-3-phosphate (requires energy).
  • Stage 2 (energy producing): Glyceraldehyde-3-phosphate conversion to pyruvate or lactate (releases energy).
• Importance:
  • Energy production (2 ATP anaerobically, 6-8 ATP aerobically)
  • Main pathway for fructose and galactose metabolism
  • Tissue oxygenation (by formation of 2,3-biphosphoglycerate)
  • Production of intermediates for other metabolic processes (e.g., glycerol-3-P for lipogenesis, pyruvate for amino acid synthesis)

Glycolysis in Red Blood Cells

• Mature red blood cells lack mitochondria, entirely dependent on glycolysis for energy.
• Lactate is the end product.
• Net energy production: 2 ATP.
• Glucose uptake is independent of insulin.
• Production of 2,3-biphosphoglycerate occurs.

Regulation of Glycolysis

• Key enzymes: Hexokinase/glucokinase, Phosphofructokinase-1 (PFK-1), Pyruvate kinase.
• Hormonal regulation: Insulin stimulates synthesis; glucagon inhibits synthesis.
• Allosteric regulation:
  • G-6-P inhibits hexokinase (not glucokinase).
  • Fructose 2,6 biphosphate stimulates PFK-1.
  • Citrate inhibits PFK-1.
  • Fructose 1,6 biphosphate stimulates pyruvate kinase.
• Covalent modification: Pyruvate kinase inactivation by phosphorylation.
• Energy regulation: ATP and AMP affect PFK-1 and pyruvate kinase activity.

In Vitro Inhibition of Glycolysis

• Arsenate: Competes with inorganic phosphate in glycolysis.
• Iodoacetate: Inhibits glyceraldehyde-3-phosphate dehydrogenase.
• Fluoride: Inhibits enolase, used in clinical labs.

Important Note

• Deficiencies in glycolytic enzymes, particularly pyruvate kinase, can lead to hemolytic anemia.