One aspect of the inflammatory response is the activation of the coagulation protease cascade resulting from the expression of tissue factor (TF) on vascular cells. TF is the cell-surface receptor for the coagulation serine protease factor VIIa, providing cofactor function by "switching on" the catalytic site of the bound enzyme and by contributing to the assembly with macromolecular substrate. The recently determined crystal structure of the TF extracellular domain shows two beta-strand modules of C2 immunoglobulin-like topology that align at a 125 degrees angle with an extensive intermodule interface. Mutagenesis studies have identified residues in both modules that are important for the binding of ligand. The deduced ligand interface extends from the convex side of the molecule into the concave side of the elbow angle. Specific binding residues control the catalytic activity of the bound protease. At the lower end of the carboxyl-terminal module, basic residues form part of a region that is important for both recognition and activation of macromolecular substrate and, potentially, for modulation of proteolytic function. After combining the biochemical data with the crystal structure, a model of TF function can be proposed in which the catalytic activity of the active site of the protease and the extended recognition of macromolecular substrates are separately controlled by distinct structural sites of the cofactor.