Crystal structure of a humanized Fab fragment of anti-tissue-factor antibody in complex with tissue factor

Abstract

Tissue factor (TF) is a membrane-anchored protein that initiates the extrinsic cascade of blood coagulation. TF forms a complex with serine protease Factor VIIa, and then activates Factor X zymogen to Factor Xa, leading to the blood coagulation. Humanized anti-TF antibody hATR-5 strongly inhibits TF-initiated blood coagulation, and is of potential use for various thrombotic diseases. The Fab fragment of antibody hATR-5 is obtained for crystallization. The crystal structure of the complex of the Fab with extracellular domains of human TF was determined with the molecular replacement method, and refined to an R factor of 0.196 at 2.1 Å resolution. All the complementarity-determining regions (CDRs) of the Fab are involved in interaction with the C-terminal-side extracellular domain of TF through 19 hydrogen bonds. The interface between the Fab and TF molecules contains 15 water molecules, and yields buried surface areas as wide as 2000 Å 2. The TF surface in the interface is possibly involved in the activation of Factor X, by forming a transient ternary complex of Factor X-TF-Factor VIIa. Electrostatic interactions are predominantly observed between the heavy-chain CDRs and TF. These hydrogen-bonding and electrostatic interactions together with the wide buried areas contribute to the high affinity of the antibody toward TF, leading to the effective inhibition of the TF-initiated blood coagulation.