A protein is a Quaternary structure. It is made up of two or more polypeptide chains. Complex proteins are made up of various protein molecules which are linked by non-covalent bonds. Amino acids are the basic units of the proteins. They link together to form a primary structure, polypeptide chain. Secondary structure is made up of beta sheets or alpha helices. This structure then folds into a three-dimensional structure called as tertiary structure. The protein structure binds with a non-protein moiety to form a Quaternary structure called as complex protein.