Amino Acids: Structure and Classification

Amino Acids: Structure and Classification

• Amino acids are the building units of proteins, with around 300 occurring in nature, but only 20 of them are involved in protein synthesis.
• Each amino acid has four different groups attached to the α-carbon (C-atom next to COOH): amino group, COOH group, hydrogen atom, and side chain (R).
• At physiological pH (7.4), the COOH group is dissociated, forming a negatively charged carboxylate ion (COO-), and the amino group is protonated, forming a positively charged ion (NH3+), creating a Zwitter ion.

Classification of Amino Acids

• Amino acids can be classified into four categories:
  • By R group (side chain)
  • Chemical classification
  • Nutritional classification
  • Metabolic classification

Classification by R Group (Polarity of Side Chain)

• Polar amino acids: have an R group that contains a polar hydrophilic group, allowing them to form hydrogen bonds with water.
  • Examples: serine, threonine, tyrosine, cysteine, glutamine, aspargine, lysine, arginine, histidine, aspartic, and glutamic.
• Non-polar amino acids: have an R group that is an alkyl hydrophobic group, unable to form hydrogen bonds.
  • Examples: glycine, alanine, valine, leucine, isoleucine, phenylalanine, tryptophan, proline, and methionine.

Nutritional Classification

• Essential amino acids: 10 amino acids that cannot be synthesized in the body and must be obtained through the diet.
  • Deficiency in essential amino acids can affect growth, health, and protein synthesis.
• Semi-essential amino acids: formed in the body but not in sufficient amounts, especially in children.
  • Examples: arginine and histidine.
• Non-essential amino acids: can be synthesized in the body.

Non-Standard Amino Acids

• Amino acids that occur naturally in cells but do not participate in peptide synthesis.
  • Examples: ornithine and citrulline, which are intermediates in urea biosynthesis.

Amino Acid Derivatives

• Chemical derivatives of amino acids have important biological functions.
  • Examples: catecholamines, GABA, dopamine, and histamine.

Functions of Amino Acids

• Amino acids serve various functions beyond being protein constituents.
  • Some amino acids are converted to carbohydrates (glucogenic amino acids).
  • Specific amino acids give rise to specialized products, such as:
    • Tyrosine: thyroid hormones, epinephrine, norepinephrine, and melanin.
    • Tryptophan: niacin.
    • Glycine, arginine, and methionine: creatine.
    • Glycine and cysteine: bile salts.

Learn about the building blocks of proteins, amino acids, including their structure, classification and properties at physiological pH.