Summary
Upon binding to tissue factor, FVIIa triggers coagulation by activating vitamin K-dependent
zymogens, factor IX (FIX) and factor X (FX). To understand recognition mechanisms
in the initiation step of the coagulation cascade, we present a three-dimensional
model of the ternary complex between FVIIa:TF:FIX. This model was built using a full-space
search algorithm in combination with computational graphics. With the known crystallographic
complex FVIIa:TF kept fixed, the FIX docking was performed first with FIX Gla-EGF1
domains, followed by the FIX protease/EGF2 domains. Because the FIXa crystal structure
lacks electron density for the Gla domain, we constructed a chimeric FIX molecule
that contains the Gla-EGF1 domains of FVIIa and the EGF2-protease domains of FIXa.
The FVIIa:TF:FIX complex has been extensively challenged against experimental data
including site-directed mutagenesis, inhibitory peptide data, haemophilia B database
mutations, inhibitor antibodies and a novel exosite binding inhibitor peptide. This
FVIIa:TF:FIX complex provides a powerful tool to study the regulation of FVIIa production
and presents new avenues for developing therapeutic inhibitory compounds of FVIIa:TF:substrate
complex.
Keywords
Protein-protein interaction - haemostasis - coagulation cascade - haemophilia - extrinsic
pathway - comparative modeling - homology modelling