Makindo Medical Notes

Related Subjects: |Protein metabolism |Protein Synthesis |Amino acids |Human Metabolism

🍖 Amino acids are the fundamental building blocks of proteins, with 20 standard types. Each has a unique side chain (R group) that defines its chemistry, polarity, and biological role.

🔬 Structure of Amino Acids

• All amino acids share:
  • An amino group (-NH₂)
  • A carboxyl group (-COOH)
  • A hydrogen atom
  • A variable side chain (R group)
• The central (alpha) carbon links these groups.

🧩 Classification of Amino Acids

• By Polarity & Charge:
  • 🌑 Nonpolar (hydrophobic): Gly, Ala, Val, Leu, Ile, Pro, Met, Phe, Trp
  • 💧 Polar (hydrophilic): Ser, Thr, Cys, Tyr, Asn, Gln
  • ⚡ Charged:
    • Acidic (–): Asp, Glu
    • Basic (+): Lys, Arg, His
• By Nutritional Requirement:
  • 🥦 Essential (must be obtained from diet): His, Ile, Leu, Lys, Met, Phe, Thr, Trp, Val
  • 🍳 Non-Essential (synthesised by body): Ala, Asn, Asp, Glu, Ser
  • ⚠️ Conditionally Essential (needed in illness/rapid growth): Arg, Cys, Gln, Gly, Pro, Tyr

⚙️ Functions of Amino Acids

• 🧱 Protein synthesis: structural + functional proteins.
• ⚡ Enzyme activity: catalytic proteins are amino acid polymers.
• 🧠 Neurotransmitter synthesis: Trp → serotonin, Tyr → dopamine/adrenaline.
• 🧪 Hormone production: Tyr → thyroid hormones; peptides (e.g. insulin, glucagon).
• 🛡️ Immune function: building blocks for antibodies + cytokines.
• 🔥 Energy production: especially during fasting, exercise, or catabolism.

🔄 Metabolism of Amino Acids

• 🔁 Transamination: swap amino group with keto acid → synthesises new amino acids.
• ➖ Deamination: removal of amino group → ammonia (→ urea cycle in liver).
• 💨 Decarboxylation: removes –COOH → biogenic amines (e.g. histamine, GABA).

🏥 Clinical Relevance

• ❌ Deficiencies: impaired growth, muscle wasting, poor wound healing, immune suppression.
• 🧬 Inborn errors of metabolism:
  • PKU (Phenylketonuria) → phenylalanine hydroxylase deficiency → neurotoxicity.
  • Maple Syrup Urine Disease → defective branched-chain amino acid metabolism.
• 🍚 Protein-energy malnutrition:
  • Kwashiorkor (protein-deficient diet → oedema, fatty liver).
  • Marasmus (severe calorie + protein deficiency → muscle wasting).
• 💊 Clinical use: Amino acid–based formulas in TPN, special diets for liver/renal disease, trauma recovery.

📌 Summary

Amino acids are essential for life — forming proteins, enzymes, hormones, neurotransmitters, and immune molecules. They are classified by chemistry (polar/nonpolar, acidic/basic) and nutrition (essential/non-essential). Their metabolism (transamination, deamination, decarboxylation) is tightly linked to nitrogen balance and energy production. Deficiency or metabolic defects (e.g. PKU, MSUD) can cause severe clinical consequences.