Medical Biochemistry: Amino Acid Structure

Amino Acids and Peptides

• Proteins are polymers of amino acids, and all proteins are composed of a number of amino acids linked by peptide bonds.
• Although there are around 300 amino acids in nature, only 20 of them are found in the human body, as three-letter genetic codes provide codons for only these 20 L-α-amino acids.
• Most amino acids (except proline) are alpha amino acids, meaning the amino group is attached to the same carbon atom as the carboxyl group.

Classification of Amino Acids

• Amino acids can be classified in five ways: based on chemical structure, metabolic product, nutritional requirement, nature or polarity of the side chain, and chemical nature of the amino acid in solutions.
• Classification based on chemical structure of the side chain:
  • (no specific information provided)
• Classification based on metabolism:
  • Ketogenic amino acids: yield either acetoacetate (one of the ketone bodies) or one of its precursors (e.g., Leucine and Lysine).
  • Ketogenic and glucogenic amino acids: partially ketogenic and partially glucogenic.
  • Glucogenic amino acids: yield pyruvate or one of the intermediates of the citric acid cycle (e.g., used for gluconeogenesis in the liver and kidney).
• Classification based on nutritional requirements:
  • Essential amino acids: cannot be synthesized by human beings and must be obtained through food (e.g., Isoleucine, Leucine, Threonine, Lysine, Methionine, Valine, Phenylalanine, Tryptophan).
  • Partially essential (semi-essential) amino acids: required for growing children, but not essential for adult individuals (e.g., Histidine and Arginine).
  • Non-essential amino acids: can be synthesized by the body, and even if absent in the diet, the body can synthesize them (e.g., Alanine, Asparagine, Aspartic acid, Cysteine, Glutamine, Glutamic Acid, Glycine, Proline, Serine, and Tyrosine).
• Classification based on nature or polarity of the side chain:
  • Hydrophilic (polar) amino acids
  • Hydrophobic (nonpolar) amino acids

Optical Activity

• Amino acids having an asymmetric carbon atom have different groups attached to the same carbon atom, resulting in optical activity.
• (no additional information provided)