Definition for Quaternary structure
From Biology Forums Dictionary
1) The association of two or more polypeptides to form a protein. One of four levels of protein structure.
2) The highest level of the hierarchical description of protein structure, applying to proteins composed of more than one polypeptide chain. In this case, the individual chains are called subunits, and the quaternary structure of the protein is the manner in which subunits assemble to form the complete complex or holoprotein. Associations of identical (or very similar) subunits (monomers) in a symmetric arrangement is a particularly common motif of quaternary structure. Two subunits constitute a dimer, three a trimer, four a tetramer, and so on. Quaternary structure can give rise to more complex functional properties in proteins, as compared to their monomeric counterparts. The classic example of this is provided by the oxygen-binding proteins, myoglobin (monomer) and hemoglobin (tetramer). The latter shows oxygen binding properties characteristic of an intersubunit cross-talk called cooperativity.
See also: tertiary structure.