Abstract
BACKGROUND: All current X-ray structures of factor (F)Xa are devoid of the γ-carboxyglutamate (Gla) domain and fail to reveal the overall conformation of the free protein. The recent cryogenic electron microscopy (cryo-EM) structure of FXa in the prothrombinase complex is the only structure of full-length FXa and shows that the Gla domain is positioned at an angle relative to the epidermal growth factor 1 domain. OBJECTIVES: Establish if the curved conformation of FXa revealed by cryo-EM is also present in solution. METHODS: The conformation of FXa in solution was studied by single-molecule Förster resonance energy transfer. RESULTS: The conformation of full-length FXa in solution is resolved for the first time. The conformation is curved and extremely sensitive to Ca(2+). It does not differ significantly from its zymogen form or from that present in the prothrombinase complex free or bound to the physiologic substrates prothrombin and meizothrombin. CONCLUSION: Measurements by single-molecule Förster resonance energy transfer reveal that FXa has a curved conformation in solution, free or bound to physiologic ligands, and validate the recent cryo-EM structures of prothrombinase. The drastic conformational changes observed in the absence of Ca(2+) suggest that the structural architecture of FXa changes upon administration of vitamin K antagonists that perturb the interaction of the Gla domain with divalent cations.