Interactions that occur between folded polypeptides in a multimeric complex.
Many proteins consist of multiple subunits and those subunits interact in specific ways, generally involving weak intermolecular bonds. Consideration of those interactions as well as communication that occurs between subunits is considered under the heading of protein quaternary structure. Below quaternary structure is tertiary structure, holding together random coils of polypeptide as well as protein secondary structure, and all of the above is specified by protein primary structure (which, in turn, is determined by the sequence of genes).
Cooperativity is one consequence of protein quaternary structure, which represents an activation of one protein subunit by another subunit associated with the same protein.
Another consideration is that proteins which fit together either within multimeric proteins or multiprotein complexes often have evolved to physically (and chemically) fit together quite well. This tight fitting represents both a form of intragenomic coevolution (i.e., as between different genes) and serves as a constraint on the evolution of individual genes since any changes in a protein's primary structure could result in changes in conformation that interferes with otherwise well-established quaternary structure. See too the idea of epistasis as well as that of pseudorevesion.
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