the structure of a protein is usually described in terms of four hierarchical level of organization: the primary secondary tertiary and quaternary structures. primary structure refers to the amino acid sequence while the highest level of organization concerns the interactions between the amino acid r groups that give the protein its characteristic conformation or three-dimensional arrangement of atoms in space. the secondary structure involves local interactions between contiguous amino acids along the chain; the tertiary structure results from long-distance interactions between stretches of amino acids from different parts of the molecule; and quaternary structure concerns the interaction of two or more individual polypeptides to form a single multimeric protein