SURFACE-MEDIATED ENZYMATIC-REACTIONS - SIMULATIONS OF TISSUE FACTOR ACTIVATION OF FACTOR-X ON A LIPID SURFACE

Blood coagulation proceeds via reactions in which zymogen coagulation factors are activated to proteases. An essential step is the activatio n of factor X by a complex of tissue factor and factor VIIa. This comp lex usually is studied using phospholipid vesicles into which tissue f actor is inserted. Because factor X exists free in solution and bound to the lipid-surface, it is difficult to establish experimentally the kinetic contribution of surfaces. We therefore developed a stochastic model to simulate such reactions and generate initial velocity data fr om which Michaelis-Menten parameters are estimated. Simulated K-m valu es decrease slightly when substrate binding to lipid is increased and by a factor of four when the rates of surface diffusion are increased to that of fluid phase-diffusion. Simulations with various size planar surfaces established an enzyme capture radius of 32-64 nm. Simulation s with different modes of enzyme-substrate complex assembly show that if the true substrate is lipid-bound, under certain conditions, the tr ue k(cat) is not measured; rather, the product ''leaving rate'' from t he complex is the rate-limiting step that is measured as substrate is taken to infinity. This model is applicable to any surface-bound enzym e reaction.