quaternary
Four interactions stabilize the tertiary structure of a protein: a ionic bonding, b hydrogen bonding, c disulfide linkages, and d dispersion forces. When a protein contains more than one polypeptide chain, each chain is called asubunit. The arrangement of multiple subunits represents a fourth level of structure, the quaternary structure of a protein. Hemoglobin, with four polypeptide chains or subunits, is the most frequently cited example of a protein having quaternary structure Saylor URL: http://www. saylor. org/books.