The specificity of enzyme—substrate interactions

Molecules interact with one another. There is continual competition between their kinetic energies to be independent and the forces of intermolecular attraction to bring them together. Kinetic energies may dominate the interaction, such that molecules exist as gases with individual mobilities only slightly impaired by their environment. But even weak forces of attraction, less than 5 kJ mol-1, may be sufficient to encourage the formation of a fragile ‘complex’ such as an inert gas dimer, held together in loose geometrical configuration by dispersion or Van der Waals forces. At the other extreme, the strong interactions of a chemical bond, with a stabilization of 250 kJ mol-1 or more, may hold groups together with a specific, relatively rigid, geometry. Between these two limits are various interactions of intermediate strength which account for the cohesion and yet mobility of liquids, and for the existence of specific biologically important complexes such as those of antibody-antigen, hormone-receptor and enzyme-substrate.