Competitive effect of cZPI on the PZ-dependent ZPI–factor Xa reaction. (A) Progress curves for the reaction of 5.5nM ZPI with 0.1nM factor Xa in the presence of 5.5nM PZ, 25μM lipid, 5mM Ca²⁺, and increasing concentrations of cZPI. Solid lines are fits to a single exponential function from which k_obs was determined. (B) Percentage decrease in k_obs for the PZ-accelerated reactions of wild-type ZPI (●) and M71A ZPI (▴) with factor Xa plotted as a function of the competitor cZPI concentration. Solid lines are fits by the cubic equation for competitive binding of cZPI and wild-type or M71A ZPI to PZ after fixing the K_D for the M71A ZPI-PZ interaction at the value determined from the data shown in Figure 4.