Amino Acids

An amino acid is a small, organic molecule consisting of an amine group, a carboxylic acid and a side chain specific to each individual amino acid.

There are twenty naturally occurring (L) amino acids that generally make up the primary structure of a protein. These vary in structure by the side chain R and fall into three general categories;

• hydrophobic amino acids
• charged
• polar

Glycine (Gly), is usually considered by itself owing to the fact its side chain consists simply of a single hydrogen. Sulfur-containing cysteine and secondary amine proline can also be considered special cases.

The exact nature of side chain R—structure, pK${}_a$ , hydrophobicity etc.—has an enormous impact on the way the peptide chain folds into the three-dimensional protein.

Hydrophobic amino acids

• Alanine
• Valine
• Isoleucine
• Leucine
• Methionine
• Phenylalanine
• Tyrosine
• Tryptophan

Charged amino acids

• Positively Charged
  • Arginine
  • Histidine
  • Lysine
• Negatively Charged
  • Aspartic Acid
  • Glutamic Acid

Polar amino acids

• Serine
• Threonine
• Asparagine
• Glutamine