ORIENTATIONAL STEERING IN ENZYME-SUBSTRATE ASSOCIATION - IONIC-STRENGTH DEPENDENCE OF HYDRODYNAMIC TORQUE EFFECTS

The effect of hydrodynamic torques on the association rate constants f or enzyme-ligand complexation is investigated by Brownian dynamics sim ulations. Our hydrodynamic models of the enzyme and ligand are compose d of spherical elements with friction forces acting at their centers. A quantitative measure of hydrodynamic torque orientational effects is introduced by choosing, as a reference system, an enzyme-ligand model with the same average hydrodynamic interactions but without orientati onal dependence. Our simple models show a 15% increase in the rate con stant caused by hydrodynamic torques at physiological ionic strength. For more realistic hydrodynamic models, which are not computationally feasible at present, this effect is probably higher. The most importan t finding of this work is that hydrodynamic complementarity in shape ( i.e. like the fitting together of pieces of a puzzle) is most effectiv e for interactions between molecules at physiological ionic strength.