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 activation of factor X by a complex of tissue factor and factor VIIa. This complex usually is studied using phospholipid vesicles into which tissue factor 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 from which Michaelis-Menten parameters are estimated. Simulated Km values 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. Simulations with different modes of enzyme-substrate complex assembly show that if the true substrate is lipid-bound, under certain conditions, the true Kcat is not measured; rather, the product "leaving rate" from the complex is the rate-limiting step that is measured as substrate is taken to infinity. This model is applicable to any surface-bound enzyme reaction.