PP13

Like that of nucleic acids, the molecular structure of proteins has several levels of the organization. The primary structure of a protein is its sequence of amino acids. Through interactions between neighboring amino acids, a polypeptide chain folds and twists into a secondary structure. Two common secondary structures found in proteins are the beta (β) pleated sheet and the alpha (α) helix. Secondary structures interact and fold further to form a tertiary structure, which is the overall, three-dimensional shape of the protein. The secondary and tertiary structures of a protein are largely determined by the primary structure—the amino acid sequence—of the protein. Finally, some proteins consist of two or more polypeptide chains that associate to produce a quaternary structure. Many proteins have an additional level of organization defined by domains. A domain is a group of amino acids that forms a discrete functional unit within the protein. For example, there are several different types of protein domains that function in DNA binding.