The 20 Amino Acids — Complete Guide to Properties and Functions

Building blocks of life

Every protein in nature is built from the same set of 20 standard amino acids. Each has the same backbone — an amino group (NH₂), a carboxyl group (COOH), a hydrogen, and a central alpha carbon — but differs in its side chain (R group). The side chain determines each amino acid's chemical properties: whether it's hydrophobic or hydrophilic, charged or neutral, small or bulky.

Classification by properties

Nonpolar (hydrophobic) amino acids

Glycine (G), Alanine (A), Valine (V), Leucine (L), Isoleucine (I), Proline (P), Phenylalanine (F), Methionine (M), and Tryptophan (W) have nonpolar side chains. These residues tend to be found in the interior of proteins, away from water. They drive protein folding through the hydrophobic effect — the tendency of nonpolar molecules to cluster together in aqueous solution.

Polar (uncharged) amino acids

Serine (S), Threonine (T), Asparagine (N), Glutamine (Q), Tyrosine (Y), and Cysteine (C) have polar but uncharged side chains at physiological pH. They can form hydrogen bonds with water and other residues. Cysteine is special — it can form disulfide bonds (S-S bridges) with other cysteines, covalently linking different parts of a protein or different protein chains.

Positively charged amino acids

Lysine (K), Arginine (R), and Histidine (H) carry positive charges at pH 7. Lysine and arginine are almost always positively charged, while histidine (pKa ~6.0) can be either charged or neutral at physiological pH, making it crucial for enzyme catalysis where proton transfer is needed.

Negatively charged amino acids

Aspartate (D) and Glutamate (E) carry negative charges at pH 7. They're commonly found on protein surfaces where they interact with water and positively charged residues. Glutamate is the most abundant amino acid in the human body.

Essential vs non-essential

Nine amino acids are "essential" — the human body cannot synthesize them, so they must come from diet. These are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The remaining 11 can be synthesized by the body (though arginine, cysteine, and tyrosine are "conditionally essential" during growth, illness, or stress).

Special roles in structure

Proline is the only amino acid whose side chain cyclizes back to the backbone nitrogen, creating a rigid kink. This restricts backbone flexibility and often breaks alpha helices — proline is sometimes called a "helix breaker." Glycine, with the smallest possible side chain (a single hydrogen), has the most conformational freedom and is found in tight turns where other residues would cause steric clashes.