53 - Nutrient metabolism. Carbohydrate metabolism. Level and regulation of Glucose. Protein metabolism and its regulation. Lipid metabolism and its regulation.

Metabolism

• Anabolism: synthesis of larger molecules from smaller units (e.g., protein synthesis, glycogenesis, lipogenesis)

• Catabolism: breakdown of large molecules into smaller ones (e.g., glycolysis, lipolysis, proteolysis)

All three energy-yielding nutrients — carbohydrates, lipids, and proteins — can be oxidized to produce ATP.

🍞 Carbohydrate Metabolism

• Main end products of digestion: glucose, fructose, galactose

• Glucose is the major energy source, especially for the brain (needs ~120g/day)

Key Pathways:

• Glycolysis: glucose → pyruvate → ATP (anaerobic or aerobic)

• Glycogenesis: excess glucose stored as glycogen (mainly in liver and muscles)

• Glycogenolysis: glycogen → glucose (when needed)

• Gluconeogenesis: new glucose synthesized from non-carb sources (e.g., amino acids, lactate)

Regulation of Blood Glucose

• Insulin (from β-cells of pancreas):

  • Lowers blood glucose (↑ glucose uptake, ↑ glycogenesis, ↓ gluconeogenesis)

• Glucagon (from α-cells):

  • Raises blood glucose (↑ glycogenolysis, ↑ gluconeogenesis)

• Adrenaline, cortisol, GH also raise glucose during stress/fasting

🥩 Protein Metabolism

Steps:

1. Digestion: proteins → amino acids (absorbed into blood)

2. Amino acid utilization:

   • Used for protein synthesis, enzymes, hormones, etc.

3. Deamination (mainly in liver):

   • Removal of amino group → forms ammonia (NH₃)

   • NH₃ → converted to urea → excreted via kidneys (urine)

   • Carbon skeleton used for ATP, gluconeogenesis, or lipogenesis

Regulation of Protein Metabolism

Regulation of Protein Metabolism

• Anabolic hormones:

  • Growth hormone, insulin, thyroid hormones, testosterone → promote protein synthesis

• Catabolic influence:

  • Glucocorticoids (e.g., cortisol) → increase protein breakdown

• Autonomic nervous system:

  • Parasympathetic: promotes anabolism

  • Sympathetic: promotes catabolism (via cortisol)

🧈 Lipid Metabolism

Digestion Products:

• Triglycerides (TAGs) → broken down into:

  • Free fatty acids (FFAs)

  • Glycerol

Utilization:

• Lipolysis (catabolism): TAG → FFA + glycerol → ATP via β-oxidation

• Lipogenesis (anabolism): excess glucose or protein → converted to fatty acids → stored as TAGs in adipose tissue

Regulation of Lipid Metabolism

Regulation of Lipid Metabolism

• Lipogenesis: stimulated by insulin

• Lipolysis: stimulated by:

  • Glucagon

  • Catecholamines (adrenaline, noradrenaline)

  • Glucocorticoids

  • Growth hormone

  • Sympathetic stimulation