Amino Acid and Protein Metabolism Notes

Overview of Amino Acid Metabolism

  • Amino acids: Macronutrients sourced mainly from dietary protein.
  • Energy provided: 17 kJ/g (4 kcal/g), similar to carbohydrates; lipids: 38 kJ/g (9 kcal/g).
  • Typical intake: 10-15% of daily calories from protein (in terms of energy).

Functions of Amino Acids

  • Sources: Dietary proteins, body protein synthesis, energy.
  • Breakdown: Amino acids are metabolized, nitrogen is excreted as urea.
  • Importance during exercise: Amino acids as fuel sources, roles in muscle recovery.
  • Overview includes: Metabolism, nitrogen disposal, catabolism of carbon skeletons.

Amino Acid Pool and Turnover

  • Amino Acid Pool: Small fraction in blood, enters from diet or cellular breakdown.
  • Proteins: Adult approx. 10 kg protein, 170 g free amino acids.
  • Turnover example: 1 million billion hemoglobin molecules synthesized/sec.

Amino Acid Categories

  • Essential Amino Acids: Cannot be synthesized by the body.
    • Examples: Arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine.
  • Nonessential Amino Acids: Synthesized by the body.
    • Examples: Alanine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, tyrosine.

Metabolic Regulation by the Liver

  • Main site for amino acid metabolism, synthesizes some amino acids.
  • Essential amino acids must be obtained from food.
  • Regulation of amino acid pool composition by the liver.

Amino Acids in Muscle and Hormonal Regulation

  • Skeletal muscle: Largest reservoir of amino acids (45% body weight).
  • Hormones: Insulin, growth hormone promote growth; cortisol increases protein breakdown.

Amino Acid Degradation and Energy Production

  • Excess amino acids are oxidized; amino groups are removed.
  • Converted into glucose (gluconeogenesis) or fats (fat storage).
  • Amino acids undergo transamination (transfer of amino groups).
Key processes:
  • Deamination: Removal of amino groups; significant tissue: liver.
  • Transamination: Conversion of one amino acid to another; involves aminotransferases, utilizing vitamin B6.

Urea Cycle

  • Converts toxic ammonia to urea for excretion.
  • Urea synthesized from nitrogen in ammonia and aspartate.
  • Steps include: Carbon dioxide + Ammonia -> Carbamoyl phosphate -> Urea
  • Regulation: Allosteric regulation of carbamoyl phosphate synthetase by N-acetylglutamate.

Fate of Carbon Skeletons

  • Removed amino groups leave carbon skeletons for multiple pathways:
    • Glucogenic Amino Acids: Can produce glucose.
    • Ketogenic Amino Acids: Produce ketone bodies.
  • Examples: Leucine and lysine are primarily ketogenic.

Exercise and Amino Acid Metabolism

  • Increased usage in both low/moderate and high-intensity exercise.
Low to Moderate-Intensity Exercise:
  • Muscle in a catabolic state; amino acids are released (especially alanine and glutamine).
  • Glucose-Alanine Cycle: Transfers nitrogen from muscle to liver for urea synthesis.
High-Intensity Exercise:
  • Less release of glutamine and alanine; focus on ATP regeneration through adenylate deaminase reaction.
  • Increased intensity leads to more ammonia release.

Additional Roles of Amino Acids

  • Beyond protein synthesis, amino acids play roles in producing neurotransmitters, hormones, and redox-regulating compounds (e.g., glutathione).
  • Hydroxylation of proline and lysine critical for collagen synthesis; requires vitamin C.
  • Tyrosine: Precursor for dopamine and norepinephrine; histidine to histamine.

Protein Degradation and Synthesis Influences

  • Resistance training affects muscle hypertrophy via increased Muscle Protein Synthesis (MPS).
  • Timely protein intake and the type of protein are crucial for maximizing MPS post-exercise.