Lysine is an amino acid found in proteins. In mammals, lysine is an essential amino acid, meaning it must be present in the diet. Lysine's nonpolar side chain classifies it as a basic amino acid.

The basic amino acids are strongly polar, and as a consequence, they are usually found on the exterior surfaces of proteins, where they can be hydrated by the surrounding aqueous environment.

One Letter Code	Three Letter Code	Molecular Wt. (Daltons)	Genetic Code Codons
K	LYS	128.18	AAA, AAG