Amino Acids Chemistry and Metabolism

Key Topics

• General Structure of Amino Acids
• Classification of Amino Acids
• Significance of Amino Acids

General Structure of Amino Acids

• Amino acids are the building blocks of proteins.
• Structure: Contains an amino group (NH2) and a carboxyl group (COOH) attached to the same alpha carbon.
• Alpha carbon is also attached to a hydrogen atom and a side chain (R).
• Differ in the structure of their side chains.
• Exist in ionic state in the body, with amino group protonated and carboxyl group deprotonated at physiological pH.
• Alpha carbon acts as a chiral center, exhibiting optical isomerism (L and D forms based on glyceraldehyde configuration).
• L-amino acids are found in biological proteins.
• D-amino acids are found in brain tissues.

Numbering of Carbon Atoms

• Example: Lysine
  • Carboxyl carbon is C1.
  • Alpha carbon is C2.
  • Subsequent carbons: Beta (C3), Gamma (C4), Delta (C5), Epsilon (C6).

Types of Amino Acids

• Standard Amino Acids: 20 in number, used during protein synthesis. Two additional: selenocysteine (21st) and pyrrolysine (22nd).
• Non-Standard Amino Acids: Not incorporated during protein synthesis but have important biological functions (e.g., D-amino acids, non-protein amino acids, amino acid derivatives).

Classification of Amino Acids

1. Based on Structure and Chemical Nature: 7 Subgroups

   • Aliphatic Side Chain: Glycine (Gly, G), Alanine (Ala, A), Valine (Val, V), Leucine (Leu, L), Isoleucine (Ile, I).
   • Hydroxy Group: Serine (Ser, S), Threonine (Thr, T), Tyrosine (Tyr, Y).
   • Sulfur Containing: Cysteine (Cys, C), Methionine (Met, M).
   • Acidic Groups/Amines: Aspartic acid (Asp, D), Asparagine (Asn, N), Glutamic acid (Glu, E), Glutamine (Gln, Q).
   • Basic Groups: Arginine (Arg, R), Lysine (Lys, K), Histidine (His, H).
   • Aromatic Rings: Phenylalanine (Phe, F), Tyrosine (Tyr, Y), Tryptophan (Trp, W).
   • Other: Proline (Pro, P).

2. Based on Polarity (Solubility)

   • Polar (Hydrophilic):
     • Negatively charged: Aspartic acid, Glutamic acid.
     • Positively charged: Lysine, Arginine, Histidine.
     • Uncharged: Serine, Threonine, Tyrosine, Asparagine, Glutamine, Cysteine, Glycine.
   • Non-Polar (Hydrophobic):
     • Aliphatic: Alanine, Valine, Leucine, Isoleucine.
     • Aromatic: Phenylalanine, Tryptophan.
     • Others: Proline, Methionine.

3. Based on Metabolic Fate

   • Glucogenic and Ketogenic: Phenylalanine, Isoleucine, Tyrosine, Tryptophan.
   • Exclusively Ketogenic: Leucine.
   • Glucogenic: Most other amino acids (e.g., Alanine, Aspartate, Glycine, Methionine).

4. Based on Nutritional Requirement

   • Essential: Not synthesized by the body, must be obtained from diet. (Mnemonic: PVT TIM HALL) - Phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine.
   • Semi-Essential: Histidine, Arginine (essential during growth, pregnancy, lactation).
   • Non-Essential: Synthesized by the body - Glycine, Alanine, Serine, Cysteine, Aspartate, Asparagine, Glutamate, Glutamine, Tyrosine, Proline.

Significance of Amino Acids

• Involved in synthesis of important biological compounds:
  • Tyrosine: Thyroxine, Melanin, Epinephrine, Norepinephrine, Dopamine.
  • Glutamic Acid: GABA (neurotransmitter).
  • Tryptophan: Niacin, Serotonin.
  • Glycine, Arginine, Methionine: Creatine.
  • Glycine, Cysteine: Bile salts.
  • Aspartic Acid, Glutamic Acid: Pyrimidine bases.
  • Glycine, Glutamic Acid, Aspartic Acid: Purine bases.
  • Beta-Alanine: Coenzyme A.
  • Histidine: Histamine.

Disorders of Amino Acid Metabolism

• Phenylketonuria, Tyrosinemia, Albinism, Alkaptonuria, Homocystinuria, Pheochromocytoma, Carcinoid Syndrome, Hartnup Disease.

Summary

• Amino acids are essential for protein synthesis and various metabolic functions.
• Classified based on structure, polarity, metabolic fate, and nutritional requirement.
• Each class and type of amino acid plays unique and critical roles in the body.