SOLUTION STRUCTURE OF THE N-TERMINAL EGF-LIKE DOMAIN FROM HUMAN FACTOR-VII

Blood coagulation is initiated by Ca2+-dependent binding of coagulatio n factor VIIa (FVIIa) to its cofactor, tissue factor(TF), The TF:FVIIa complex activates factors IX and X, ultimately leading to the formati on of thrombin and the coagulation of blood. FVII consists of an N-ter minal gamma-carboxy-glutamic-acid-containing (Gla) domain followed by two epidermal growth factor (EGF) like domains, the first of which can bind one Ca2+ ion (K-d approximate to 150 mu M) and a C-terminal seri ne protease domain. Using H-1 nuclear magnetic resonance spectroscopy, we have determined the solution structure of a synthetic N-terminal E GF-like domain (EGF1) of human FVII (residues 45-85) in the absence of Ca2+. A comparison of this structure of apo EGF1 with the Ca2+-bound EGF1 in the complex of FVIIa and TF [Banner, D. W., et al. (1996) Natu re 380, 41-46] suggests that the structural changes in the EGF1 domain upon Ca2+ binding are minor and are concentrated near the Ca2+-bindin g site, which is facing away from the TF interaction surface. Amino ac id side chains that are crucial for the binding of FVII to TF show a s imilar conformation in both structures and are therefore unlikely to d irectly influence the Ca2+-dependent binding of FVII to TF. As Ca2+ bi nding to EGF1 does not lead to a conformational change in the residues constituting the interaction surface for binding to TF, our results a re consistent with the idea that the altered orientation between the G la and EGF1 domains that result from Ca2+ binding is responsible for t he increased affinity of FVII/FVIIa for TF.