Role of the acidic hirudin-like COOH-terminal amino acid region of factor Va heavy chain in the enhanced function of prothrombinase

Blood coagulation factor Va's key interactive residues and regions for prothrombinase assembly and prothrombin binding

Blood coagulation factor Va serves an indispensable role in hemostasis as cofactor for the serine protease factor Xa. In the presence of an anionic phospholipid membrane and calcium ions, factors Va and Xa assemble into the prothrombinase complex. Following formation of the ternary complex with the macromolecular zymogen substrate prothrombin, the latter is rapidly converted into thrombin, the key regulatory enzyme of coagulation. Over the years, multiple binding sites have been identified in factor Va that play a role in the interaction of the cofactor with factor Xa, prothrombin, or the anionic phospholipid membrane surface. In this review, an overview of the currently available information on these interactive sites in factor Va is provided, and data from biochemical approaches and 3D structural protein complex models are discussed. The structural models have been generated in recent years and provide novel insights into the molecular requirements for assembly of both the prothrombinase and the ternary prothrombinase-prothrombin complexes. Integrated knowledge of functionally important regions in factor Va will allow for a better understanding of factor Va cofactor activity.

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.

The Dual Regulatory Role of Amino Acids Leu480 and Gln481 of Prothrombin

Prothrombin (FII) is activated to α-thrombin (IIa) by prothrombinase. Prothrombinase is composed of a catalytic subunit, factor Xa (fXa), and a regulatory subunit, factor Va (fVa), assembled on a membrane surface in the presence of divalent metal ions. We constructed, expressed, and purified several mutated recombinant FII (rFII) molecules within the previously determined fVa-dependent binding site for fXa (amino acid region 473-487 of FII). rFII molecules bearing overlapping deletions within this significant region first established the minimal stretch of amino acids required for the fVa-dependent recognition exosite for fXa in prothrombinase within the amino acid sequence Ser(478)-Val(479)-Leu(480)-Gln(481)-Val(482). Single, double, and triple point mutations within this stretch of rFII allowed for the identification of Leu(480) and Gln(481) as the two essential amino acids responsible for the enhanced activation of FII by prothrombinase. Unanticipated results demonstrated that although recombinant wild type α-thrombin and rIIa(S478A) were able to induce clotting and activate factor V and factor VIII with rates similar to the plasma-derived molecule, rIIa(SLQ→AAA) with mutations S478A/L480A/Q481A was deficient in clotting activity and unable to efficiently activate the pro-cofactors. This molecule was also impaired in protein C activation. Similar results were obtained with rIIa(ΔSLQ) (where rIIa(ΔSLQ) is recombinant human α-thrombin with amino acids Ser(478)/Leu(480)/Gln(481) deleted). These data provide new evidence demonstrating that amino acid sequence Leu(480)-Gln(481): 1) is crucial for proper recognition of the fVa-dependent site(s) for fXa within prothrombinase on FII, required for efficient initial cleavage of FII at Arg(320); and 2) is compulsory for appropriate tethering of fV, fVIII, and protein C required for their timely activation by IIa.

Amino acid region 1000-1008 of factor V is a dynamic regulator for the emergence of procoagulant activity

Single chain factor V (fV) circulates as an M_r 330,000 quiescent pro-cofactor. Removal of the B domain and generation of factor Va (fVa) are vital for procoagulant activity. We investigated the role of the basic amino acid region 1000–1008 within the B domain of fV by constructing a recombinant mutant fV molecule with all activation cleavage sites (Arg⁷⁰⁹/Arg¹⁰¹⁸/Arg¹⁵⁴⁵) mutated to glutamine (fV^Q3), a mutant fV molecule with region 1000–1008 deleted (fV^ΔB9), and a mutant fV molecule containing the same deletion with activation cleavage sites changed to glutamine (fV^ΔB9/Q3). The recombinant molecules along with wild type fV (fV^WT) were transiently expressed in COS-7L cells, purified, and assessed for their ability to bind factor Xa (fXa) prior to and following incubation with thrombin. The data showed that fV^Q3 was severely impaired in its interaction with fXa before and after incubation with thrombin. In contrast, K_D(app) values for fV^ΔB9 (0.9 nm), fVa^ΔB9 (0.4 nm), and fV^ΔB9/Q3 (0.7 nm) were similar to the affinity of fVa^WT for fXa (0.3 nm). Two-stage clotting assays revealed that although fV^Q3 was deficient in its clotting activity, fV^ΔB9/Q3 had clotting activity comparable with fVa^WT. The k_cat value of prothrombinase assembled with fV^ΔB9/Q3 was minimally affected, whereas the K_m value of the reaction was increased 57-fold compared with the K_m value obtained with prothrombinase assembled with fVa^WT. These findings strongly suggest that amino acid region 1000–1008 of fV is a regulatory sequence protecting the organisms from spontaneous binding to fXa and unnecessary prothrombinase complex formation, which in turn results in catastrophic physiological consequences.

Structural basis of thrombin-mediated factor V activation: the Glu666-Glu672 sequence is critical for processing at the heavy chain-B domain junction

Thrombin-catalyzed activation of coagulation factor V (FV) is an essential positive feedback reaction within the blood clotting system. Efficient processing at the N- (Arg(709)-Ser(710)) and C-terminal activation cleavage sites (Arg(1545)-Ser(1546)) requires initial substrate interactions with 2 clusters of positively charged residues on the proteinase surface, exosites I and II. We addressed the mechanism of activation of human factor V (FV) using peptides that cover the entire acidic regions preceding these cleavage sites, FV (657-709)/ (FVa2) and FV(1481-1545)/(FVa3). FVa2 appears to interact mostly with exosite I, while both exosites are involved in interactions with the C-terminal linker. The 1.7-Å crystal structure of irreversibly inhibited thrombin bound to FVa2 unambiguously reveals docking of FV residues Glu(666)-Glu(672) to exosite I. These findings were confirmed in a second, medium-resolution structure of FVa2 bound to the benzamidine-inhibited proteinase. Our results suggest that the acidic A2-B domain linker is involved in major interactions with thrombin during cofactor activation, with its more N-terminal hirudin-like sequence playing a critical role. Modeling experiments indicate that FVa2, and likely also FVa3, wrap around thrombin in productive thrombin·FV complexes that cover a large surface of the activator to engage the active site.

Contribution of amino acid region 659-663 of Factor Va heavy chain to the activity of factor Xa within prothrombinase

Factor Va, the cofactor of prothrombinase, is composed of heavy and light chains associated noncovalently in the presence of divalent metal ions. The COOH-terminal region of the heavy chain contains acidic amino acid clusters that are important for cofactor activity. In this work, we have investigated the role of amino acid region 659−663, which contains five consecutive acidic amino acid residues, by site-directed mutagenesis. We have generated factor V molecules in which all residues were mutated to either lysine (factor V^5K) or alanine (factor V^5A). We have also constructed a mutant molecule with this region deleted (factor V^Δ659−663). The recombinant molecules along with wild-type factor V (factor V^WT) were transiently expressed in mammalian cells, purified, and assessed for cofactor activity. Two-stage clotting assays revealed that the mutant molecules had reduced clotting activities compared to that of factor Va^WT. Kinetic analyses of prothrombinase assembled with the mutant molecules demonstrated diminished k_cat values, while the affinity of all mutant molecules for factor Xa was similar to that for factor Va^WT. Gel electrophoresis analyses of plasma-derived and recombinant mutant prothrombin activation demonstrated delayed cleavage of prothrombin at both Arg³²⁰ and Arg²⁷¹ by prothrombinase assembled with the mutant molecules, resulting in meizothrombin lingering throughout the activation process. These results were confirmed after analysis of the cleavage of FPR-meizothrombin. Our findings provide new insights into the structural contribution of the acidic COOH-terminal region of factor Va heavy chain to factor Xa activity within prothrombinase and demonstrate that amino acid region 659−663 from the heavy chain of the cofactor contributes to the regulation of the rate of cleavage of prothrombin by prothrombinase.

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

Human and bovine factor Va (FVa) function similarly in the activation of prothrombin but differently in the activation of prethrombin-1 (Pre-1). Pre-1 activation with human FVa proceeds at about 22 percent of the rate with bovine FVa. The dependencies of initial rates on the FVa and Pre-1 concentrations indicate that the differential activity is expressed in kcat differences, rather than differences in the assembly of prothrombinase or the K(m) value of the substrate. The heavy and light chains of both species of FVa were separated and interspecies hybrids were constructed in the presence of Ca(++). Studies of the activation of Pre-1 with these hybrids indicate that the species difference can be attributed specifically to the heavy chain of FVa. Analyses of the reactions by SDS-PAGE indicated that cleavage at Arg271 occurs at about the same rate with both species of FVa, but cleavage at Arg320 with human FVa is specifically retarded. A major difference in primary structure between the human and bovine FVa heavy chains comprises 10 residues at COOH-terminus, adjacent to the negatively charged hirudin-like DYDYQ sequence. These residues have pI values of 12.5 and 4.26 in human and bovine FVa, respectively. The lower value would complement the negatively charged DYDYQ sequence but the higher value would counteract it. Thus, we suggest that the differences in the COOH-terminus of the heavy chain are responsible for the differences in Pre-1 activation, and that it specifically influences cleavage at Arg320 in Pre-1.

Membrane-dependent interaction of factor Xa and prothrombin with factor Va in the prothrombinase complex

Because all three protein components of prothrombinase, factors (f) Xa and Va and prothrombin, bind to negatively charged membrane phospholipids, the exact role of the membrane in the prothrombinase reaction has not been fully understood. In this study, we prepared deletion derivatives of fXa and prothrombin in which both the Gla and first EGF-like domains of the protease (E2-fXa) as well as the Gla and both kringle domains of the substrate (prethrombin-2) had been deleted. The fVa-mediated catalytic activity of E2-fXa toward prethrombin-2 was analyzed in both the absence and presence of phospholipids composed of 80% phosphatidylcholine (PC) and 20% phosphatidylserine (PS). PCPS markedly accelerated the initial rate of prethrombin-2 activation by E2-fXa, with the cofactor exhibiting saturation only in the presence of phospholipids (apparent K(d) of approximately 60 nM). Competitive kinetic studies in the presence of the two exosite-1-specific ligands Tyr(63)-sulfated hirudin(54-65) and TM456 suggested that while both peptides are highly effective inhibitors of the fVa-mediated activation of prethrombin-2 by E2-fXa in the absence of PCPS, they are ineffective competitors in the presence of phospholipids. Since neither E2-fXa nor prethrombin-2 can interact with membranes, these results suggest that interaction of fVa with PCPS improves the affinity of the activation complex for proexosite-1 of the substrate. Direct binding studies employing OG(488)-EGR-labeled fXa and E2-fXa revealed that the interaction of the Gla domain of fXa with PCPS also induces conformational changes in the protease to facilitate its high-affinity interaction with fVa.

Cooperative regulation of the activity of factor Xa within prothrombinase by discrete amino acid regions from factor Va heavy chain

The prothrombinase complex catalyzes the activation of prothrombin to alpha-thrombin. We have repetitively shown that amino acid region (695)DYDY(698) from the COOH terminus of the heavy chain of factor Va regulates the rate of cleavage of prothrombin at Arg(271) by prothrombinase. We have also recently demonstrated that amino acid region (334)DY(335) is required for the optimal activity of prothrombinase. To assess the effect of these six amino acid residues on cofactor activity, we created recombinant factor Va molecules combining mutations at amino acid regions 334-335 and 695-698 as follows: factor V(3K) ((334)DY(335) --> KF and (695)DYDY(698) --> KFKF), factor V(KF/4A) ((334)DY(335) --> KF and (695)DYDY(698) --> AAAA), and factor V(6A) ((334)DY(335) --> AA and (695)DYDY(698) --> AAAA). The recombinant factor V molecules were expressed and purified to homogeneity. Factor Va(3K), factor Va(K4/4A), and factor Va(6A) had reduced affinity for factor Xa, when compared to the affinity of the wild-type molecule (factor Va(Wt)) for the enzyme. Prothrombinase assembled with saturating concentrations of factor Va(3K) had a 6-fold reduced second-order rate constant for prothrombin activation compared to the value obtained with prothrombinase assembled with factor Va(Wt), while prothrombinase assembled with saturating concentrations of factor Va(KF/4A) and factor Va(6A) had approximately 1.5-fold reduced second-order rate constants. Overall, the data demonstrate that amino acid region 334-335 together with amino acid region 695-698 from factor Va heavy chain are part of a cooperative mechanism within prothrombinase regulating cleavage and activation of prothrombin by factor Xa.