They are composed of long chains of amino acids.
The building blocks of protein are amino acids. Amino acids are organic molecules made up an alpha (center) carbon atom which is linked to an amino group, a carboxyl group, a hydrogen atom and a variable component known as a side chain.
Amino acids are joined to each other by peptide bonds to form either a short chain called an oligopeptide or a long chain called a polypeptide. These chains are what forms the protein.