Proteins associate with each other to form quaternary structures. Many proteins consist of more than one subunit. For example, hemoglobin has a molecular weight of 64,000 and is composed of four subunits, each of molecular weight 16,000. Two of the subunits are alike, and two are different. The enzyme tryptophan synthetase from Escherichia coli, which catalyzes the final two steps in the biosynthesis of that amino acid, consists of two nonidentical subunits, each of which catalyzes one reaction. Other enzymes contain regulatory and catalytic subunits. Still other enzymes consist of aggregates of two, three, or more identical subunits. The specific, noncovalent association of protein subunits is termed the quaternary structure of a protein. If the subunits are not identical, the association is called heterotypic. The association of identical subunits is termed homotypic.
The same forces that contribute to the structure of a single polypeptide also contribute to subunit interactions. Salt bridges, hydrogen bonds, hydrophobic, and van der Waal's interaction act in an additive fashion to specifically associate subunits.