Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid contains an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R-group) attached to a central alpha-carbon. Twenty standard amino acids are encoded by the genetic code.

Classification by Side Chain Properties

Nonpolar, Hydrophobic Amino Acids

These amino acids have side chains that are hydrophobic and tend to cluster in the interior of proteins. They include glycine, alanine, valine, leucine, isoleucine, methionine, proline, phenylalanine, and tryptophan. Proline is unique because its side chain forms a ring with the amino group.

Polar, Uncharged Amino Acids

These amino acids have side chains that can form hydrogen bonds with water. They include serine, threonine, cysteine, tyrosine, asparagine, and glutamine. Cysteine can form disulfide bonds that stabilize protein structure.

Positively Charged (Basic) Amino Acids

These amino acids have side chains that are positively charged at physiological pH. They include lysine, arginine, and histidine. Histidine has a pKa near physiological pH, allowing it to act as a proton donor or acceptor in enzyme active sites.

Negatively Charged (Acidic) Amino Acids

These amino acids have side chains that are negatively charged at physiological pH. They include aspartic acid and glutamic acid. Their carboxyl groups are often involved in salt bridges and metal binding.

Special Properties

Isoelectric Point

Each amino acid has a characteristic isoelectric point (pI) — the pH at which it carries no net charge. At pH values below the pI, the amino acid is positively charged; above the pI, it is negatively charged.

Optical Activity

All standard amino acids except glycine are chiral, existing as L- and D-enantiomers. Only L-amino acids are incorporated into proteins by the ribosome.