Proteins are made up of chains of small organic molecules called amino acids such as glycine, alanine, lysine or tryptophan. Amino acids contain an amine functional group, which contains a nitrogen atom, possibly one or two atoms of hydrogen, and a carboxylic acid group, which has the formula -COOH.
The proteins in most living organisms are composed of 20 amino acids; however, more than 500 amino acids are known. Different proteins contain different sequences of amino acids, and these sequences dictate the shape and the function of the protein. Of the 20 common amino acids, nine are considered essential. The human body cannot synthesize amino acids, so they need to be obtained from food.