Engineering a membrane-independent human prothrombinase through parsimonious mutation of factor Xa

ABSTRACT Background Thrombin is generated from its precursor prothrombin by sequential cleavage at Arg320 and Arg271 by the prothrombinase complex, composed of factor (f) Xa and fVa on phospholipid (PL) membrane surfaces. The affinity of human fXa for fVa is low in the absence of PL. However, fXa orthologues from the venom of group D snakes bind to fVa with high affinity, forming an active complex without PL. We previously characterized the properties of the fXa orthologue Hopsarin D (HopD) from Hoplocephalus stephensii . Objectives Here we set out to create a PL-independent human prothrombinase by making mutations to fXa, guided by a model of the prothrombinase complex and the sequence differences between human fXa and HopD. Methods We assessed the contribution of individual domains of fXa to its binding to fVa by swapping each with the corresponding domain of HopD. We then chose three loops in the serine protease (SP) domain predicted to be in contact with fVa to swap to those of HopD. Eventually, 10 residues from the three loops in the SP domain and 7 from the EGF2 domain were selected for mutation. Results The resulting ‘M17’ fXa variant bound to fVa with a Kd of ~20 nM, similar to HopD, and together efficiently processed prothrombin through the meizothrombin intermediate in the absence of PL. Conclusions We conclude that the role of PL membranes in prothrombinase assembly and function is limited to improving the affinity of fXa for fVa. The M17-fVa complex is likely to be structurally equivalent to the human prothrombinase complex.