Proteins are made up of amino acids. All proteins made in living organisms consist of combinations of 20 amino acids. These contain carbon, hydrogen, oxygen, nitrogen and sometimes sulfur and phosphorus.
The unique sequence of amino acids that makes up a protein is called the protein's primary structure. Amino acids connect with one another through chemical bonds at the molecular level. Each amino acid has a general formula attached to a side chain. Amino acids differ from one another based on the type of side chain they possess. Interactions between side chains on different amino acids cause looping and folding, which determine the shape of a protein.