Basics of Amino Acids

Amino acids (AAs) are composed of a central carbon atom attached to a carboxyl group, an amino group, a hydrogen atom, and a side chain (R group). There are hundreds of AAs found in nature, but only 20 are predominantly used as the building blocks of proteins in humans (proteinogenic). Nine of these 20 are “essential,” as they cannot be synthesized. In addition to the 20 standard AAs, 2 rare proteinogenic amino acids are also recognized, selenocysteine and pyrrolysine. Amino acids differ from one another only in the chemical nature of the R group. They are most commonly classified according to their interaction with water as hydrophobic, hydrophilic, or ionic.