General Veterinary Biochemistry

CLASSIFICATION OF AMINO ACIDS BASED ON POLARITY

• There are 4 main classes of amino acids based on polarity, i.e. the interaction of the R group with water molecules at physiological pH.

Amino acids with non-polar (hydrophobic) side chain

• These amino acids have a non–polar hydrophobic side chain. They do not provide protons or participate in hydrogen or ionic bonding. E.g.: alanine, valine, leucine, isoleucine, proline, phenylalanine, tryptophan and methionine.

Amino acids with uncharged polar side chains (- R group)

• These amino acids are more soluble in water. They are hydrophilic in nature. The functional groups can make hydrogen bonds with water. E.g.: glycine, serine, threonine, tyrosine, cysteine, asparagine and glutamine.
• The polarity of serine, threonine is due to the presence of hydroxyl groups; the polarity of asparagine and glutamine is due to the amide group and the sulfhydryl group (thiol group) is responsible for cysteine.

Amino acids with polar, negatively charged side chain

• These amino acids will have a net negative charge at neutral pH. E.g.: aspartic acid and glutamic acid.
• These amino acids will have one more carboxyl group, which contribute to negative charge at neutral pH.

Amino acids with polar, positively charged side chain

• These amino acids will have a net positive charge at pH 7. They accept protons. E.g.: lysine, arginine and histidine.
• Lysine contains a second amino group at ε - position on the aliphatic side chain. Arginine contains a positively charged guanidino group and histidine contains an imidazole group.