An all-atom solution-equilibrated model for human extrinsic blood coagulation complex (sTF-VIIa-Xa): a protein-protein docking and molecular dynamics refinement study.

Tissue factor (TF)-bound factor (F)VIIa plays a critical role in activating FX, an event that rapidly results in blood coagulation. Despite recent advances in the structural information about soluble TF (sTF)-bound VIIa and Xa individually, the atomic details of the ternary complex are not known. As part of our long-term goal to provide a structural understanding of the extrinsic blood coagulation pathway, we built an all atom solution-equilibrated model of the human sTF-VIIa-Xa ternary complex using protein-protein docking and molecular dynamics (MD) simulations. The starting structural coordinates of sTF-VIIa and Xa were derived from dynamically equilibrated solution structures. Due to the flexible nature of the light-chain of the Xa molecule, a three-stage docking approach was employed in which SP (Arg195-Lys448)/EGF2 (Arg86-Arg139), EGF1 (Asp46-Thr85) and GLA (Ala1-Lys45) domains were docked in a sequential manner. The rigid-body docking approach of the FTDOCK method in conjunction with filtering based on biochemical knowledge from experimental site-specific mutagenesis studies provided the strategy. The best complex obtained from the docking experiments was further refined using MD simulations for 3 ns in explicit water. In addition to explaining most of the known experimental site-specific mutagenesis data pertaining to sTF-VIIa, our model also characterizes likely enzyme-binding exosites on FVIIa and Xa that may be involved in the ternary complex formation. According to the equilibrated model, the 140s loop of VIIa serves as the key recognition motif for complex formation. Stable interactions occur between the FVIIa 140s loop and the FXa -strand B2 region near the sodium-binding domain, the 160 s loop and the N-terminal activation loop regions. The helical-hydrophobic stack region that connects the GLA and EGF1 domains of VIIa and Xa appears to play a potential role in the membrane binding region of the ternary complex. The proposed model may serve as a reasonable structural basis for understanding the exosite-mediated substrate recognition of sTF-VIIa and to advance understanding of the TFPI-mediated regulatory pathway of the extrinsic blood coagulation cascade.