Prothrombin circulates in equilibrium between closed and open conformations. Under normal conditions, ≈80% of all prothrombin is in the closed conformation, in which the kringle-1 domain (red) interacts with the protease domain (yellow). Type I antiprothrombin antibodies found in people with antiphospholipid syndrome recognize an epitope on the kringle-1 domain of prothrombin that is only exposed in the open conformation of prothrombin. Binding of model type I antiprothrombin antibody POmAb (prothrombin open monoclonal antibody) to kringle-1 stabilizes the open conformation, resulting in reduced conversion to thrombin by prothrombinase.