Proteins, Quaternary Structure
Many proteins are composed of more than one folded polypeptide chain, which are known as protein subunits. Quaternary structure describes the organization of multiple subunits into a complex. Some examples of proteins with quaternary structure include hemoglobin and many ion channel proteins. Changes in quaternary structure can occur through conformational changes within individual subunits or through reorientation of the subunits relative to each other. These conformational changes, which may be cooperative or allosterically (induced by binding of other molecule) effected, often govern the mechanism by which proteins are regulated or perform their catalytic functions.