Differences in prethrombin-1 activation with human or bovine factor Va can be attributed to the heavy chain

Identification and characterization of a factor Va-binding site on human prothrombin fragment 2

The fragment 2 domain (F2) of prothrombin and its interaction with factor (F) Va is known to contribute significantly to prothrombinase-catalyzed activation of prothrombin. The extent to which the F2-FVa interaction affects the overall thrombin generation, however, is uncertain. To study this interaction, nuclear magnetic resonance spectroscopy of recombinant F2 was used to identify seven residues within F2 that are significantly responsive to FVa binding. The functional role of this region in interacting with FVa during prothrombin activation was verified by the FVa-dependent inhibition of thrombin generation using peptides that mimic the same region of F2. Because six of the seven residues were within a 9-residue span, these were mutated to generate a prothrombin derivative (PT6). These mutations led to a decreased affinity for FVa as determined by surface plasmon resonance. When thrombin generation by an array of FXa containing prothrombinase components was monitored, a 54% decrease in thrombin generation was observed with PT6 compared with the wild-type, only when FVa was present. The functional significance of the specific low-affinity binding between F2 and FVa is discussed within the context of a dynamic model of molecular interactions between prothrombin and FVa engaging multiple contact sites.

Spellbinding Effects of the Acidic COOH-Terminus of Factor Va Heavy Chain on Prothrombinase Activity and Function

Human factor Va (hfVa) is the important regulatory subunit of prothrombinase. Recent modeling data have suggested a critical role for amino acid Arg⁷⁰¹ of hfVa for human prothrombin (hPro) activation by prothrombinase. Furthermore, it has also been demonstrated that hfVa has a different effect than that of bovine fVa on prethrombin-1 activation by prothrombinase. The difference between the two cofactor molecules was also found within the Asn⁷⁰⁰-Arg⁷⁰¹ dipeptide in the human factor V (hfV) molecule, which is replaced by the Asp-Glu sequence in bfV. As a consequence, we produced a recombinant hfV (rhfV) molecule with the substitution ⁷⁰⁰NR⁷⁰¹→DE. rhfV^NR→DE together with the wild-type molecule (rhfV^WT) were expressed in COS7 cells, purified, and tested for their capability to function within prothrombinase. Kinetic studies showed that the K_d of rhfVa^NR→DE for human fXa as well as the k_cat and K_m of prothrombinase made with rhfVa^NR→DE for hPro activation were similar to the values obtained following hPro activation by prothrombinase made with rhfVa^WT. Remarkably, sodium dodecyl sulfate polyacrylamide gel electrophoresis analyses of hPro activation time courses demonstrated that the rate of cleavage of hPro by prothrombinase reconstituted with rhfVa^NR→DE was significantly delayed with substantial accumulation of meizothrombin, and delayed thrombin generation, when compared to activation of hPro by prothrombinase made with rhfVa^WT. These unanticipated results provide significant insights on the role of the carboxyl-terminal end of the heavy chain of hfVa for hPro cleavage and activation by prothrombinase and show that residues ⁷⁰⁰NR⁷⁰¹ regulate at least in part the enzyme-substrate/product interaction during fibrin clot formation.

Down regulation of prothrombinase by activated protein C during prothrombin activation

Activated protein C (aPC) proteolytically inactivates factor Va (FVa) and thereby downregulates prothrombinase. Although FVa inactivation by aPC has been studied extensively, the inactivation of prothrombinase during prothrombin activation has not. Therefore, prothrombin activation initiated both without and with aPC (5.0, 7.5 or 10.0 nM) was monitored over time by fluorescence. The experiments were performed with 0.075 nM FVa and 1.0 nM FXa, and with these concentrations reversed. The time courses of the residual prothrombinase activity with aPC, determined from the slopes of fluorescence over time, were pseudo first order with both limiting and excess FVa. With FVa limiting or in excess, the second rate constants for inactivation of prothrombinase were 1.98 +/- 0.09 x 10(5) M(-1)s(-1) and 2.54 +/- 0.13 x 10(5) M(-1)s(-1), respectively. The former value is 101-fold smaller than that for FVa inactivation by aPC alone. Since with limiting FVa the second order rate constants for prothrombinase inactivation and FVa inactivation are equal, FVa is protected 101-fold, presumably by both FXa and prothrombin. In contrast, with excess FVa, the calculated rate constant for FVa inactivation exceeds that for prothrombinase inactivation 17.3-fold, which reflects a loss of protection by FXa. Since the protective effects of the two proteins are theoretically multiplicative, FXa protected 17.3-fold and prothrombin protected 5.8-fold. With 150 nM protein S and limiting FVa, prothrombinase inactivation was two-fold faster, yet it was still protected 91-fold. These studies show that FVa is down-regulated by aPC during prothrombin activation, but both FXa and prothrombin protect FVa in a multiplicative way, with or without protein S.