Lecture Notes: Amino Acids

Structure of Amino Acids

• Carbon atom with four valences:
  • Amino group (NH2)
  • Acid group (COOH)
  • Hydrogen (H)
  • Variable group (R, can be Sulfur, Alcohol, etc.)

Key Definitions

Alpha Amino Acid

• Alpha Carbon: Carbon next to the functional group
• Function Group: COOH
• Most amino acids in the body are in the alpha form
• Exceptions:
  • Beta-alanine
  • Beta-aminoisobutyrate
  • Gamma-aminoisobutyrate

Chiral Carbon

• Carbon with four different groups attached
• Important for optical activity (Mirror images, optical isomers)
• Example: Glycine (No chiral carbon, optically inactive)

Immino Acid

• Amino group is not free, bound in a ring structure
• Example: Proline
• Ring name: Pyrrolidine

Classification of Amino Acids

Based on R Group

• Simple Amino Acids:
  • R = Hydrogen or Carbon
  • Examples: Glycine, Alanine
• Branched Chain Amino Acids:
  • R = Branched carbon chain
  • Examples: Leucine, Isoleucine, Valine
  • Mnemonic: LIV (Live light-hearted)
• Sulfur-Containing Amino Acids:
  • R contains Sulfur
  • Examples: Methionine, Cysteine
  • Mnemonic: Meaty cyst
• Hydroxyl Group Containing Amino Acids:
  • R contains OH group
  • Examples: Serine, Threonine (Tyrosine contains OH but is hydrophobic)
  • Mnemonic: Siro tero
• Acidic Amino Acids:
  • R with excess acid (Negative charge)
  • Examples: Aspartate, Glutamate
• Amide Group Containing Amino Acids:
  • R contains CONH2
  • Examples: Asparagine, Glutamine
• Basic Amino Acids:
  • R with excess base (Positive charge)
  • Examples: Histidine, Lysine, Arginine
  • Mnemonic: His Last Urge (Most basic: Arginine)

Based on Water Solubility

• Water-Soluble (Hydrophilic/Polar):
  • Categories: Alcohol, Acid, Base, Amide
  • Examples:
    • Acidic: Aspartate, Glutamate
    • Basic: Histidine, Lysine, Arginine
    • Amide: Asparagine, Glutamine
    • Alcohol: Serine, Threonine
    • Others: Glycine, Cysteine
• Non-Water-Soluble (Hydrophobic):
  • Remaining amino acids
• Most Water-Soluble: Arginine
• Least Water-Soluble: Glycine
• Controversial: Tyrosine (Least hydrophobic among hydrophobic)

Based on Metabolism

• Ketogenic: Can generate ketone bodies
  • Examples: Leucine (Most), Lysine
• Glucogenic: Can generate glucose
  • Examples: Remaining 16 amino acids, Most: Alanine
• Keto + Glucogenic: Can generate both
  • Examples: Tryptophan, Phenylalanine, Tyrosine, Isoleucine

Based on Nutritional Requirement

• Essential: Must be taken in diet
  • Examples: TV Till 9 PM H (Tryptophan, Valine, Threonine, Isoleucine, Leucine, Lysine, Phenylalanine, Methionine, Histidine)
  • Non-Essential: Tyrosine
• Semi-Essential: Required during growth
  • Example: Arginine
• Non-Essential:
  • Remaining 12 amino acids
• Controversy: Histidine (Regarded as essential in some books, semi-essential in others)

Special Properties

• Aromatic Amino Acids: Have a peculiar odor or smell
  • Examples: Tryptophan, Phenylalanine, Tyrosine, Histidine
  • UV Absorption: Maximum in Tryptophan
• Derived Amino Acids: Cannot be represented by a codon
  • Examples: Hydroxyproline, Hydroxylysine, Desmosine, Cystine, Ornithine, Citrulline
  • Important: Cystine has disulfide bonds, found in insulin and immunoglobulin

New Amino Acids

• 21st Amino Acid: Selenocysteine
  • Codon: UGA
  • Precursor: Serine
• 22nd Amino Acid: Pyrrolysine
  • Codon: UAG

This concludes the classification and key points of amino acids.