Leu252, Ile253, and Thr255 each contribute to prothrombinase inhibition. (A) Prothrombinase activity was measured as in Figure 2A, in the presence of the indicated concentrations of AIKTKRKRKK (AIKT; open diamond), LAKTKRKRKK (LAKT; open circle), or LIKAKRKRKK (LIKA; filled triangle). The initial rate of thrombin generation is shown as a percentage of control (no peptide; mean ± standard deviation; n ≥ 3). Lines represent best-fit inhibition curves. Inhibition of prothrombinase by LIKTKRKRKK (LIKT; filled square) is reproduced from Figure 3 for reference. (B-D) Thrombin generation was measured in PRP as in Figure 2D, using the indicated concentrations of AIKT (B), LAKT (C), or LIKA (D). Shown are the average thrombin generation curves from experiments using 3 donors.