Molecular interactions between coagulation factor IX and low density lipoprotein receptor-related protein

Abstract

Factor IX is an essential blood haemostatic protein, which is apparent from the observation that the absence of functional FIX is associated with the severe bleeding disorder haemophilia B. To achieve its full enzymatic activity, the serine protease precursor factor IX must first be activated into the active protease factor IXa, and then form a complex with factor VIIIa, its protein cofactor. The activation alters the structure of factor IX, leading to the exposure of the active site, which mediates its enzymatic activity. The endocytic receptor low density lipoprotein receptor-related protein (LRP) has the potential to contribute to the removal of factor IXa from blood. Remarkably, LRP binds factor IXa, but not non-activated factor IX. This suggests that the interaction between factor IXa and LRP involves an activation-dependent region in factor IXa. Our research shows that LRP indeed interacts with such a region in the protease domain of factor IXa. This positively charged region also binds the cofactor factor VIIIa and the polysaccharide heparin, an effective anticoagulant. Interestingly, LRP binding to this region affects the enzymatic activity of factor IXa. This happens not only in the presence of factor VIIIa, but also in its absence. As the region is relatively distant from the active site, LRP binding may affect the active site indirectly. Factor IXa surface loop 340-347 seems to play a particularly important role in this process. These findings suggest that LRP may regulate the activity of factor IXa via two mechanisms: by directly removing factor IXa from blood, and by inhibiting factor IXa enzymatic activity. Accordingly, in the future small recombinant LRP fragments could be used as effective antithrombotica.