Elucidating the interaction between the Fragment 2 domain of Prothrombin and Factor Va
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The prothrombinase (IIase) complex is an essential component of the coagulation cascade and is composed of a serine protease, Factor Xa (FXa), its non-enzymatic cofactor, Factor Va (FVa), calcium and a phospholipid membrane surface. It activates prothrombin (FII) to thrombin, the principal stimulator of clot formation in vivo. FII activation by IIase is mediated by specific interactions between FII and FVa. Preliminary NMR and peptidyl mimicry studies identified six residues within the FII Fragment 2 (F2) domain (S160, Q177, R181, L182, V184 and T185) that likely mediate an interaction between it and the heavy chain of FVa. Therefore, six recombinant FII derivatives were prepared whereby each of the aforementioned residues was mutated to alanine. FII activation kinetics by FXa in the presence or absence of FVa was measured by DAPA-thrombin complex formation. The results show that FII-S160A, -R181A, -L182A, -V184A and -T185A had no significant effect on the catalytic efficiency of the reaction in the presence of FVa. In the absence of FVa, the catalytic efficiency of FII-R181A, -L182A, -V184A, and -T185A derivatives decreased by 17-27% compared with wildtype, while FII-S160A had no effect. FII-Q177A, however, showed a significant increase of 17% in catalytic efficiency in the presence of FVa but no change in its absence. Two double (FII-Q177A/R181A and FII-R181A/T185A) and one triple (FII-Q177A/R181A/T185) mutants were generated to determine if multiple mutations would have an additive effect. These derivatives were indistinguishable from wildtype in the presence of FVa. In the absence of FVa, however, their catalytic efficiency values decreased 12-25% compared with wildtype. Further comparison of these values showed that FII-R181A and -Q177A/R181A both decreased by 25%, while FII-R181A/T185A and -Q177A/R181A/T185A decreased by 12% and 24% with respect to the wildtype, respectively. Both comparisons, where the only difference was an additional mutation at Q177, suggest that Q177 does not affect the activation kinetics of FII in the absence of FVa. Taken together, our data suggest that Q177 in the F2 domain of FII is likely involved in interacting with IIase through a FVa-dependent mechanism while residues R181, L182, V184 and T185 may be involved through a FVa-independent mechanism.