Lysine is an essential amino acid that has a net positive charge at physiological pH values making it one of the three basic (with respect to charge) amino acids. This polar amino acid is commonly found on the surfaces of proteins and enzymes, and sometimes appears in the active site. Sources of lysine include meats, fish, poultry, and dairy products. Lysine is incorporated into proteins at the rate of 7 percent on a molar basis compared to the other amino acids.