aspartic acid (əspärˈtĭk) [key], organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l -stereoisomer participates in the biosynthesis of proteins. Its acidic side chain adds a negative charge and hence a greater degree of water-solubility to proteins in neutral solution and has been shown to be near the active sites of some enzymes (see pepsin). Aspartic acid is not essential to the human diet. It was discovered in protein in 1868.