Proteins

Some proteins need to functionally associate with others as subunits in a multimeric structure. This is called the quaternary structure of the protein. This can also be stabilized by disulfide bonds and by noncovalent interactions with reacting substrates or cofactors. For example an antibody consists of two "light" polypeptide chains covalently linked to two longer "heavy" chains, forming a Y-shaped molecule with each branch able to bond to an identical antigen. The protein subunits of the single-stranded binding protein of Escherichia coli bind to DNA only as a tetramer (a multimeric form), acting to stabilize the separated DNA strands during replication.

Another excellent example of quaternary structure is that of hemoglobin. Adult hemoglobin consists of two alpha subunits and two beta subunits, held together by noncovalent interactions. Each of the four subunits contains a heme group that binds an oxygen molecule, O₂. This binding of oxygen is a cooperative process whereby the binding of one oxygen molecule occurs slowly, but once achieved then speeds the binding of the remaining three oxygen molecules. The fourth oxygen molecule binds 300 times faster than the first oxygen molecule. This cooperativity assures that maximum oxygen is captured and retained as it enters into the capillaries within the lungs.

The unloading of oxygen is also facilitated by cooperativity, such that after one oxygen molecule is released, the other three soon follow. This assures that the tissues will receive maximum oxygen once it is delivered. Alpha-hemoglobin by itself, or tetramers of all beta subunits, also bind oxygen, but not with the same cooperativity. Such evidence indicates that there is some form of molecular interaction between the subunits of the tetramer of adult hemoglobin.