Amino Acids: Structure and Classification

Study Notes

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.

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

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.

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.

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

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