Proteins contain 20 different types of monomers, also known as amino acids. All 20 monomers have a basic structure of a central carbon, an amino group, a carboxyl or acid group, a hydrogen and a unique side chain.
The side chain for all 20 monomers is different, giving rise to the different types of amino acids. The human body makes 11 of the 20 monomers, which are referred to as non-essential amino acids. Nine of the 20 monomers are considered essential amino acids and must be obtained from food. Essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.