There are 4 levels of protein structure: Primary: sequence of amino acids (also called residues) Secondary: refers to local folded structures that form within a polypeptide due to interactions between atoms of the backbone. (The backbone just refers to the polypeptide chain apart from the R groups – so all we mean here is that secondary structure does not involve R group atoms.) Tertiary: The overall three-dimensional structure of a polypeptide is called its tertiary structure. The tertiary structure is primarily due to interactions between the R groups of the amino acids that make up the protein. Quaternary: Some proteins are made up of multiple polypeptide chains. On chains: A protein can have multiple polypeptide chains. Each chain has its own set of amino acids, assembled in a particular order (a primary sequence). ...