Aspartic acid (abbreviated as Asp or D; encoded by the codons [GAU and GAC]) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+
3 form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO form under biological conditions), and a side chain CH₂COOH. Under physiological conditions in proteins the sidechain usually occurs as the negatively charged aspartate form, −COO. It is semi-essential in humans, meaning the body can synthesize it from oxaloacetate.