Effect of C1 domain mutations on parameters of the factor Xase complex. (A) V_max was obtained with saturating factor IXa and factor X. (B) Apparent factor IXa affinities were measured by various factor IXa concentrations in the presence of factor VIII (0.15 U/mL) and excess phospholipid containing 4% or 15% PS. (C) Apparent factor X affinities were determined by various factor X concentrations in the presence of excess phospholipid vesicles and factor IXa. The apparent K_D values were determined using nonlinear least squares analysis with the software GraphPad Prism, Version 5.0. Data are mean ± SEM for 2 to 4 experiments, each performed in duplicate.