Functional roles of natural products: The involvement of extended arrays of weak interactions in cooperative binding phenomena
A factorisation of the free energy of binding into various "costs" and "benefits" has provided the basis for a semi-quantitation of some weak interactions in solution. It has become clear that the entropic cost of motional restriction on binding (A + B + A.B) increases with increasing exothermicity of the association; this exothermicity must of course reflect not only interactions at the interface between A and B, but also the change in bonding throughout B (if B represents the receptor). We illustrate cooperativity and anti-cooperativity by reference to effects of ligand binding (as models of classical agonists and antagonists) on the dimerisation of vancomycin-group antibiotics. Since dimerization of receptors (promoted by ligand binding) is a common theme in biological signalling, it must presumably have an advantage in natural selection. We suggest that concurrent demands of ligand binding and receptor dimerization may permit a more specific control of those ligand structures which can cause signal transmission. In this way, specificity in biological signalling might be aided.