Protein Metabolism and Metabolic States of the Body

Protein Metabolism

  • Proteins are continually broken down and replaced.
  • Amino acids are recycled or converted to other compounds.
  • Excess dietary proteins are oxidized for energy or converted to fat for storage.
  • Amino acid degradation:
    • Deamination: NH2 removal, converted to pyruvic acid or keto acid intermediates.
    • Transamination: Amine group transfer to α-ketoglutaric acid, forming glutamic acid; reversible.
    • Oxidative deamination: In liver, glutamic acid's amine group becomes ammonia (NH3), converted to urea and excreted.
    • Keto acid modification: Keto acids become metabolites for the citric acid cycle, contributing to gluconeogenesis.
  • Protein Synthesis
    • Amino acids are anabolic nutrients for proteins.
    • Synthesis is hormonally controlled, requiring a complete set of amino acids (essential ones from diet).

Metabolic States of the Body

  • Catabolic-Anabolic Steady State
    • Organic molecules are continuously broken down and rebuilt.
    • Nutrient pools (amino acids, carbohydrates, fats) are interconvertible.
    • Amino acid pool: source for protein resynthesis, derivatives, and gluconeogenesis.
    • Carbohydrate and fat pools: easily interconverted; excess stored; oxidized for energy.
  • Absorptive State (Fed State)
    • Anabolism exceeds catabolism; lasts ~4 hours after eating.
    • Excess nutrients stored as fats.
    • Carbohydrates: Glucose is main fuel, converted to glycogen or fat.
    • Triglycerides: Hydrolyzed to fatty acids and glycerol.
    • Amino acids: Deaminated or used for protein synthesis.
    • Hormonal control: Insulin promotes glucose uptake, glycogenesis, lipogenesis, and protein synthesis; inhibits gluconeogenesis.
  • Postabsorptive State (Fasting State)
    • Catabolism exceeds anabolism; energy from body reserves.
    • Goal: Maintain blood glucose, promote fat use for energy.
    • Sources of blood glucose: Glycogenolysis (liver, muscle), lipolysis, catabolism of cellular protein.
    • Glucose sparing: Conserves glucose for brain by using noncarbohydrate sources.
    • Hormonal control: Glucagon promotes glycogenolysis and gluconeogenesis; sympathetic nervous system triggers fat mobilization.