CHO Metabolism Overview

CHO Metabolism

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

Metabolic Pathways

• Anabolic pathways: These pathways synthesize complex molecules from simpler ones. They require energy. A key example is protein synthesis.
• Catabolic pathways: These pathways break down complex molecules into simpler ones. They release energy. An example is oxidative processes.
• Amphibolic pathways: These act as links between anabolic and catabolic pathways. The citric acid cycle is an example.
• Food molecules undergo digestion and absorption. Then they enter amphibolic pathways. Further breakdown goes to catabolic pathways to produce energy. Conversely, anabolic pathways use energy to build complex molecules.

Carbohydrate Metabolism

• Carbohydrates provide 50% of daily calories.
• Complete oxidation of 1 gram of carbohydrate yields 4 kcal.
• Sources of carbohydrates in food:
  • Starch (e.g., potatoes) makes up about 50%.
  • Sucrose and lactose comprise most of the rest.
  • Fructose and glucose come from fruits and honey.

Digestion of Carbohydrates

• Polysaccharides and disaccharides must be broken down into monosaccharides (e.g., glucose) to be absorbed.
• Key enzymes involved in this process include:
  • Salivary amylase: breaks down 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 disease (congenital or acquired) caused by lactase enzyme deficiency.
• Cause: Insufficient lactase enzyme.
• Effects: Undigested lactose accumulates in the intestine, leading to fermentation by intestinal bacteria, producing acids and gases.
• Symptoms: Abdominal distension, cramps, and diarrhea (due to increased intestinal osmotic pressure).
• Treatment: Lactose-free milk formulas.

Metabolic Pathways of Carbohydrates

• Catabolic pathways:
  • Glycolysis (glucose oxidation to pyruvate or lactate)
  • Pentose phosphate shunt
  • Uronic acid pathway
  • Glycogenolysis (glycogen breakdown)
• Anabolic pathways:
  • Gluconeogenesis (gluconeogenesis)
  • Glycogenesis (glycogen synthesis)
• Amphibolic pathways:
  • Citric acid cycle

Glycolysis

• Definition: The oxidation of glucose to pyruvate (in the presence of oxygen) or lactate (in the absence of oxygen).
• Site: Occurs in the cytoplasm of all tissue cells, crucial for tissues without mitochondria (e.g., red blood cells) and those with frequent oxygen deficiency (e.g., muscles during exercise).
• Steps:
  • Stage 1 (energy-requiring): Glucose is converted into glyceraldehyde-3-phosphate. Energy is consumed.
  • Stage 2 (energy-producing): Glyceraldehyde-3-phosphate is converted to pyruvate or lactate. Energy is produced.

Importance of Glycolysis

• Key energy production process: anaerobically (2 ATP) and aerobically (6-8 ATP).
• Major pathway for fructose and galactose metabolism.
• Good oxygenation of tissues: 2,3-biphosphoglycerate decreases hemoglobin's affinity for oxygen.
• Source of important intermediates such as dihydroxyacetone phosphate (for lipogenesis) and pyruvate (for amino acid synthesis).

Energy Production in Glycolysis

• Aerobic glycolysis:
  • 4 ATP from substrate-level phosphorylation.
  • Additional 6-8 ATP from the electron transport chain (from oxidizing NADH).
• Anaerobic glycolysis:
  • 2 ATP from substrate-level phosphorylation.

Glycolysis in Red Blood Cells

• Mature red blood cells lack mitochondria, thus relying entirely on glycolysis for energy.
• The end product is lactate.
• The net energy yield is 2 ATP.
• Glucose uptake by red blood cells is independent of insulin.
• 2,3-BPG is produced.

Regulation of Glycolysis

• Key enzymes (hexokinase/glucokinase, phosphofructokinase-1, and pyruvate kinase) are regulated hormonally and allosterically.
• Hormonal regulation: Insulin stimulates enzyme synthesis; glucagon inhibits it.
• Allosteric regulation: G-6-P inhibits hexokinase, fructose 2,6-bisphosphate stimulates phosphofructokinase-1, and citrate inhibits phosphofructokinase-1. Fructose 2,6-biphosphate stimulates pyruvate kinase.
• Covalent modification: Pyruvate kinase is inactivated by phosphorylation.
• Energy regulation: ADP and AMP stimulate phosphofructokinase-1, and ATP inhibits phosphofructokinase-1 and pyruvate kinase.

Inhibition of Glycolysis

• In vitro:
  • Arsenate competes with inorganic phosphate.
  • Iodoacetate inhibits glyceraldehyde 3-phosphate dehydrogenase.
  • Fluoride inhibits enolase.
• Important: Hemolytic anemia is often caused by pyruvate kinase deficiency.