The Leu252-Thr254 region is responsible for the effect of FVL on prothrombinase inhibition by TFPIα. FV810 (filled circle) or FVL810 (open circle; 0.5 nM) was incubated with phospholipid vesicles (20 μM), the thrombin inhibitor DAPA (3 μM), and the indicated concentrations of TFPI-AAKA (A) or TFPIΔK2 (B), and thrombin generation was measured as in Figure 2A. The initial rate of thrombin generation is shown as a percentage of control (mean ± standard deviation; n ≥ 3). Lines represent best-fit inhibition curves.