A membrane-mediated catalytic event in prothrombin activation

The role of procoagulant phospholipids on the surface of circulating blood cells in thrombosis and haemostasis

Phospholipids (PLs) are found in all cell types and are required for structural support and cell activation signalling pathways. In resting cells, PLs are asymmetrically distributed throughout the plasma membrane with native procoagulant aminophospholipids (aPLs) being actively maintained in the inner leaflet of the membrane. Upon platelet activation, aPLs rapidly externalize to the outer leaflet and are essential for supporting the coagulation cascade by providing binding sites for factors in the cell-based model. More recent work has uncovered a role for enzymatically oxidized PLs (eoxPLs) in facilitating coagulation, working in concert with native aPLs. Despite this, the role of aPLs and eoxPLs in thrombo-inflammatory conditions, such as arterial and venous thrombosis, has not been fully elucidated. In this review, we describe the biochemical structures, distribution and regulation of aPL externalization and summarize the literature on eoxPL generation in circulating blood cells. We focus on the currently understood role of these lipids in mediating coagulation reactions in vitro, in vivo and in human thrombotic disease. Finally, we highlight gaps in our understanding in how these lipids vary in health and disease, which may place them as future therapeutic targets for the management of thrombo-inflammatory conditions.

Stoichiometric analysis reveals a unique phosphatidylserine binding site in coagulation factor X

BACKGROUND

Cellular trauma or activation exposes phosphatidylserine (PS) and the substantially more abundant phospholipid, phosphatidylethanolamine (PE), on the outer layer of the plasma membrane, thereby allowing binding of many blood clotting proteins. We previously proposed the Anything But Choline (ABC) hypothesis to explain how PS and PE synergize to support binding of clotting proteins with gamma-carboxyglutamate (Gla)-rich domains, which posited that each Gla domain binds to a limited number of PS molecules and multiple PE molecules. However, the minimal number of PS molecules required to stably bind a Gla-domain-containing blood clotting protein in the presence of excess PE was unknown.

OBJECTIVE

To test the ABC hypothesis for factor X by determining the threshold binding requirement of PS molecules under conditions of PS-PE synergy.

METHODS

We used surface plasmon resonance to investigate the stoichiometry of factor X binding to nanoscale membrane bilayers (Nanodiscs) of varying phospholipid composition.

RESULTS AND CONCLUSIONS

We quantified 1.05 ± 0.2 PS molecules per bound factor X molecule in Nanodiscs containing a mixture of 10% PS, 60% PE, and 30% phosphatidylcholine. Hence, there appears to be one truly PS-specific binding site per Gla domain, while the remaining membrane binding interactions can be satisfied by PE.

Kinetic regulation of the binding of prothrombin to phospholipid membranes

A wide range of equilibrium and kinetic constants exist for the interaction of prothrombin and other coagulation factors with various model membranes from a variety of techniques. We have investigated the interaction of prothrombin with pure dioleoylphosphatidylcholine (DOPC) membranes and dioleoylphosphatidlyserine (DOPS)-containing membranes (DOPC:DOPS, 3:1) using surface plasmon resonance (SPR, with four different model membrane presentations) in addition to isotheral titration calorimetry (ITC, with suspensions of phospholipid vesicles) and ELISA methods. Using ITC, we found a simple low-affinity interaction with DOPC:DOPS membranes with a K(D) = 5.1 μM. However, ELISA methods using phospholipid bound to microtitre plates indicated a complex interaction with both DOPC:DOPS and DOPC membranes with K(D) values of 20 and 58 nM, respectively. An explanation for these discrepant results was developed from SPR studies. Using SPR with low levels of immobilised DOPC:DOPS, a high-affinity interaction with a K(D) of 18 nM was obtained. However, as phospholipid and prothrombin concentrations were increased, two distinct interactions could be discerned: (i) a kinetically slow, high-affinity interaction with K(D) in the 10(-8) M range and (ii) a kinetically rapid, low-affinity interaction with K(D) in the 10(-6 )M range. This low affinity, rapidly equilibrating, interaction dominated in the presence of DOPS. Detailed SPR studies supported a heterogeneous binding model in agreement with ELISA data. The binding of prothrombin with phospholipid membranes is complex and the techniques used to measure binding will report K D values reflecting the mixture of complexes detected. Existing data suggest that the weaker rapid interaction between prothrombin and membranes is the most important in vivo when considering the activation of prothrombin at the cell surface.

Membrane binding events in the initiation and propagation phases of tissue factor-initiated zymogen activation under flow

This study investigates the dynamics of zymogen activation when both extrinsic tenase and prothrombinase are assembled on an appropriate membrane. Although the activation of prothrombin by surface-localized prothrombinase is clearly mediated by flow-induced dilutional effects, we find that when factor X is activated in isolation by surface-localized extrinsic tenase, it exhibits characteristics of diffusion-mediated activation in which diffusion of substrate to the catalytically active region is rate-limiting. When prothrombin and factor X are activated coincident with each other, competition for available membrane binding sites masks the diffusion-limiting effects of factor X activation. To verify the role of membrane binding in the activation of factor X by extrinsic tenase under flow conditions, we demonstrate that bovine lactadherin competes for both factor X and Xa binding sites, limiting factor X activation and forcing the release of bound factor Xa from the membrane at a venous shear rate (100 s(-1)). Finally, we present steady-state models of prothrombin and factor X activation under flow showing that zymogen and enzyme membrane binding events further regulate the coagulation process in an open system representative of the vasculature geometry.

Modulation of prothrombinase assembly and activity by phosphatidylethanolamine

Constituents of platelet membranes regulate the activity of the prothrombinase complex. We demonstrate that membranes containing phosphatidylcholine and phosphatidylethanolamine (PE) bind factor Va with high affinity (K(d) = ∼10 nm) in the absence of phosphatidylserine (PS). These membranes support formation of a 60-70% functional prothrombinase complex at saturating factor Va concentrations. Although reduced interfacial packing does contribute to factor Va binding in the absence of PS, it does not correlate with the enhanced activity of the Xa-Va complex assembled on PE-containing membranes. Instead, specific protein-PE interactions appear to contribute to the effects of PE. In support of this, soluble C6PE binds to recombinant factor Va(2) (K(d) = ∼6.5 μm) and to factor Xa (K(d) = ∼91 μm). C6PE and C6PS binding sites of factor Xa are specific, distinct, and linked, because binding of one lipid enhances the binding and activity effects of the other. C6PE triggers assembly (K(d)(app) = ∼40 nm) of a partially active prothrombinase complex between factor Xa and factor Va(2), compared with K(d)(app) for C6PS ∼2 nm. These findings provide new insights into the possible synergistic roles of platelet PE and PS in regulating thrombin formation, particularly when exposed membrane PS may be limiting.

Dilutional control of prothrombin activation at physiologically relevant shear rates

The generation of proteolyzed prothrombin species by preassembled prothrombinase in phospholipid-coated glass capillaries was studied at physiologic shear rates (100-1000 s(-1)). The concentration of active thrombin species (α-thrombin and meizothrombin) reaches a steady state, which varies inversely with shear rate. When corrected for shear rate, steady-state levels of active thrombin species exhibit no variation and a Michaelis-Menten analysis reveals that chemistry of this reaction is invariant between open and closed systems; collectively, these data imply that variations with shear rate arise from dilutional effects. Significantly, the major products observed include nonreactive species arising from the loss of prothrombin's phospholipid binding domain (des F1 species). A numerical model developed to investigate the spatial and temporal distribution of active thrombin species within the capillary reasonably approximates the observed output of total thrombin species at different shears; it also predicts concentrations of active thrombin species in the wall region sufficient to account for observed levels of des FI species. The predominant feedback formation of nonreactive species and high levels of the primarily anticoagulant intermediate meizothrombin (∼40% of total active thrombin species) may provide a mechanism to prevent thrombus propagation downstream of a site of thrombosis or hemorrhage.

Thrombin physiology and pathophysiology

OBJECTIVES

To review the role of thrombin in physiology and clinical disease and to discuss the pharmacology of antithrombosis.

DATA SOURCES

Original research articles, scientific reviews, textbooks.

HUMAN DATA SYNTHESIS

Thrombin and thrombin receptors are involved in a variety of physiologic and pathologic processes resulting in a great deal of interest in thrombin-related pharmacologic intervention.

VETERINARY DATA SYNTHESIS

Although there is little clinical research data available on thrombin specifically in veterinary patients, some of the original research on protease activated receptors was performed at veterinary institutions and many of the human molecular biology studies have been done on animals including dogs.

CONCLUSION

Thrombin plays a significant role in coagulation, anticoagulation, and fibrinolysis. Antithrombotic treatment is focused on preventing thrombosis while maintaining hemostasis. Pharmaceutical agents are selected for the specific component of the coagulation pathway associated with a specific disease process, for a proven prophylactic benefit with procedures that carry a risk of thromboembolism, for rapidity of onset and ease of reversibility, for limited monitoring requirements, and for oral formulation and bioavailablity. Recent insight into other aspects of thrombin physiology presents an opportunity for pharmacologic intervention in a variety of other processes such as inflammation and sepsis, peripheral blood cell activation and chemotaxis, vascular endothelial and smooth muscle activity, cellular development and tissue repair, mitogenesis, neoplasia, and the function of nervous tissue following injury.

Cibles des médicaments antithrombotiques

New antithrombotic agents are being developed not only to improve efficacy, but also to increase safety in comparison with widely used conventional agents such as the oral anticoagulants. New anticoagulant, antiplatelet, and profibrinolytic compounds are currently under study in drug development programs, and most of those in phase II or III of development are derived from the observation of natural phenomena and merely mimic processes developed by mammalians, including humans, to avoid thrombosis, or by blood-sucking insects or animals to prevent coagulation of the blood their are feeding on. By contrast, drug candidates identified by means of rigorous research and designed to target new pathways and achieve direct and specific inhibition of factors that are presumed to play an important role in thrombogenesis have generally failed to show any benefit and sometimes even induce deleterious effects. The clinical development of new drugs, even those mimicking natural phenomena, improves our knowledge of the pathogenesis of thrombosis and sheds light, retrospectively, on previous conceptual errors. The improvement in our basic knowledge and the development of new types of drugs suggest that, in contrast to the current antithrombotic compounds that are used in a broad range of clinical settings, use of new drugs should be restricted to specific situations in which their mechanisms of action are predicted to deliver the highest medical benefit. A major obstacle resides in the fact that current drug development programs are still required to comply with long obsolete guidelines based on the characteristics of first-generation antithrombotic agents, and that do not take into account the specific mechanisms of action of new drugs. This situation should change, however, and new antithrombotic drugs should soon be able to benefit from adapted development programs that will make it possible to determine their optimal risk-benefit ratio.

The Factor V Activation Paradox

The prothrombinase complex consists of the protease factor Xa, Ca2+, and factor Va assembled on an anionic membrane. Factor Va functions both as a receptor for factor Xa and a positive effector of factor Xa catalytic efficiency and thus is key to efficient conversion of prothrombin to thrombin. The activation of the procofactor, factor V, to factor Va is an essential reaction that occurs early in the process of tissue factor-initiated blood coagulation; however, the catalytic sequence leading to formation of factor Va is a subject of disagreement. We have used biophysical and biochemical approaches to establish the second order rate constants and reaction pathways for the activation of phospholipid-bound human factor V by native and recombinant thrombin and meizothrombin, by mixtures of prothrombin activation products, and by factor Xa. We have also reassessed the activation of phospholipid-bound human prothrombin by factor Xa. Numerical simulations were performed incorporating the various pathways of factor V activation including the presence or absence of the pathway of factor V-independent prothrombin activation by factor Xa. Reaction pathways for factor V activation are similar for all thrombin forms. Empirical rate constants and the simulations are consistent with the following mechanism for factor Va formation. alpha-Thrombin, derived from factor Xa cleavage of phospholipid-bound prothrombin via the prethrombin 2 pathway, catalyzes the initial activation of factor V; generation of factor Va in a milieu already containing factor Xa enables prothrombinase formation with consequent meizothrombin formation; and meizothrombin functions as an amplifier of the process of factor V activation and thus has an important procoagulant role. Direct activation of factor V by factor Xa at physiologically relevant concentrations does not appear to be a significant contributor to factor Va formation.

Contribution of regions distal to glycine-160 to the anticoagulant activity of tissue factor pathway inhibitor

The functions of the first two Kunitz domains of tissue factor pathway inhibitor (TFPI) are well defined as active site-directed inhibitors of factor VIIa and factor Xa. The anticoagulant properties of the third Kunitz domain and C-terminal region were probed using altered forms of TFPI. TFPI-160 contains the first two Kunitz domains. K1K2C contains the first two Kunitz domains and the basic C-terminus. Neither TFPI-160 nor K1K2C contains the third Kunitz domain. In amidolytic assays containing calcium, TFPI-160 is a less potent inhibitor of factor Xa than TFPI. However, addition of the C-terminus in K1K2C nearly restores inhibitory activity to that of TFPI, indicating that the third Kunitz domain is not required for direct inhibition of factor Xa. When compared in assays containing phospholipids and factor Va, K1K2C and TFPI-160 are poor inhibitors compared to TFPI, demonstrating that the third Kunitz domain is required for the full anticoagulant activity of TFPI. TFPI was further characterized in amidolytic assays performed with Gla-domainless factor Xa and in prothrombin activation assays using submicellar concentrations of short-chain phospholipids (C6PS). TFPI and K1K2C are worse inhibitors of Gla-domainless factor Xa, compared to wild-type factor Xa, while TFPI-160 inhibits both forms of factor Xa equally, suggesting a C-terminus/Gla domain interaction. TFPI is a potent inhibitor of thrombin generation by prothrombinase assembled with C6PS, while TFPI-160 and K1K2C are not. Conversely, TFPI does not inhibit prothrombin activation by prothrombinase assembled on a two-dimensional lipid bilayer. Together, the data indicate that the region between Gly-160 and the end of the third Kunitz domain contributes to TFPI function by orienting the second Kunitz domain so that it can bind the active site of phospholipid-associated factor Xa prior to prothrombinase assembly and/or by slowing formation of the prothrombinase complex.

Blood coagulation at the site of microvascular injury: effects of low-dose aspirin

The sequence of coagulant reactions in vivo following vascular injury is poorly characterized. Using quantitative immunoassays, the time courses were evaluated for activation of prothrombin, factor (F)V, FXIII, fibrinogen (Fbg) cleavage, and FVa inactivation in bleeding-time blood collected at 30-second intervals from 12 healthy subjects both before and after aspirin ingestion. Prothrombin decreased at a maximum rate of 14.2 +/- 0.6 nM per second to 10% of initial values at the end of bleeding. Significant amounts of alpha-thrombin B chain appeared rapidly at 90 seconds of bleeding and increased at a maximum rate of 0.224 +/- 0.03 nM per second to a peak value of 38 nM. Kinetics of prethrombin 2 generation was almost identical. Prothrombinase concentration reached a peak value of 22 pM at 150 seconds and then decreased to 9 pM at the end of bleeding. Prothrombin fragment 1.2 (F1.2) was produced explosively (0.673 +/- 0.05 nM per second), whereas thrombin-antithrombin III (TAT) complexes were generated at a much slower rate (0.11 +/- 0.008 nM per second; P =.002). FVa light chain was detectable 30 seconds later than the heavy chain (150 seconds) and was produced at a slightly slower rate (0.027 +/- 0.001 nM per second) when compared with the heavy chain (0.032 +/- 0.002 nM per second; P =.041). The 30 000 fragment (residues 307-506) of FVa heavy chain produced by activated protein C appeared as early as at 90 seconds and increased with time. Fbg was removed from the blood shed with a high rate of 0.047 +/- 0.02 microM/s and became undetectable at approximately 180 seconds of bleeding. The velocity of FXIII activation correlated with thrombin B-chain formation. A 7-day aspirin administration (75 mg/d) resulted in significant reductions in maximum rates of (1) prothrombin removal (by 29%; P =.008); generation of alpha-thrombin B-chain (by 27.2%; P =.022), and prethrombin 2 (by 26%; P =.014); formation of F1.2 (by 31.4%; P =.009) and TAT (by 30.3%; P = 0.013); (2) release of FVa heavy chain (by 25%; P =.003) and FVa light chain (by 29.6%; P =.007); (3) Fbg depletion from solution (by 30.5%; P =.002); and (4) FXIII activation (by 28.6%; P =.003). Total amounts of the proteins studied, collected at every interval, also significantly decreased following aspirin ingestion. These results indicate that low-dose aspirin impairs thrombin generation and reactions catalyzed by this enzyme at the site of the injury.

Lipid oxidation enhances the function of activated protein C

Although lipid oxidation products are usually associated with tissue injury, it is now recognized that they can also contribute to cell activation and elicit anti-inflammatory lipid mediators. In this study, we report that membrane phospholipid oxidation can modulate the hemostatic balance. Oxidation of natural phospholipids results in an increased ability of the membrane surface to support the function of the natural anticoagulant, activated protein C (APC), without significantly altering the ability to support thrombin generation. Lipid oxidation also potentiated the ability of protein S to enhance APC-mediated factor Va inactivation. Phosphatidylethanolamine, phosphatidylserine, and polyunsaturation of the fatty acids were all required for the oxidation-dependent enhancement of APC function. A subgroup of thrombotic patients with anti-phospholipid antibodies specifically blocked the oxidation-dependent enhancement of APC function. Since leukocytes are recruited and activated at the thrombus or sites of vessel injury, our findings suggest that after the initial thrombus formation, lipid oxidation can remodel the membrane surface resulting in increased anticoagulant function, thereby reducing the thrombogenicity of the thrombus or injured vessel surface. Anti-phospholipid antibodies that block this process would therefore be expected to contribute to thrombus growth and disease.

Preparation, characterization, and the crystal structure of the inhibitor ZK-807834 (CI-1031) complexed with factor Xa

Factor Xa plays a critical role in the formation of blood clots. This serine protease catalyzes the conversion of prothrombin to thrombin, the first joint step that links the intrinsic and extrinsic coagulation pathways. There is considerable interest in the development of factor Xa inhibitors for the intervention in thrombic diseases. This paper presents the structure of the inhibitor ZK-807834, also known as CI-1031, bound to factor Xa and provides the details of the protein purification and crystallization. Results from mass spectrometry indicate that the factor Xa underwent autolysis during crystallization and the first EGF-like domain was cleaved from the protein. The crystal structure of the complex shows that the amidine of ZK-807834 forms a salt bridge with Asp189 in the S1 pocket and the basic imidazoline fits snugly into the S4 site. The central pyridine ring provides a fairly rigid linker between these groups. This rigidity helps minimize entropic losses during binding. In addition, the structure reveals new interactions that were not found in the previous factor Xa/inhibitor complexes. ZK-807834 forms a strong hydrogen bond between an ionized 2-hydroxy group and Ser195 of factor Xa. There is also an aromatic ring-stacking interaction between the inhibitor and Trp215 in the S4 pocket. These interactions contribute to both the potency of this compound (K(I) = 0.11 nM) and the >2500-fold selectivity against homologous serine proteases such as trypsin.

Prothrombinase acceleration by oxidatively damaged phospholipids

The optimally efficient production of thrombin by the prothrombinase complex relies on suitable positioning of its component factors and substrate on phosphatidylserine-containing lipid membranes. The presence of oxidatively damaged phospholipids in a membrane disrupts the normal architecture of a lipid bilayer and might therefore be expected to interfere with prothrombinase activity. To investigate this possibility, we prepared phosphatidylserine-containing lipid vesicles containing oxidized arachidonoyl lipids, and we examined their ability to accelerate thrombin production by prothrombinase. Oxidized arachidonoyl chains caused dose-dependent increases in prothrombinase activity up to 6-fold greater than control values. These increases were completely attenuated by the presence of alpha-tocopherol, gamma-tocopherol, or ascorbate. Over the course of a 300-min oxidation, the ability of arachidonoyl lipids to accelerate prothrombinase peaked at 60 min and then declined to base-line levels. These results suggest that instead of being impeded by oxidative membrane damage, prothrombinase activity is enhanced by one or more products of nonenzymatic lipid oxidation.

A model describing the inactivation of factor Va by APC: bond cleavage, fragment dissociation, and product inhibition

The inactivation of factor Va is a complex process which includes bond cleavage (at three sites) and dissociation of the A2N.A2C peptides, with intermediate activity in each species. Quantitation of the functional consequences of each step in the reaction has allowed for understanding of the presentation of disease in individuals possessing the factor V polymorphism factor VLEIDEN. APC cleavage of membrane-bound bovine factor Va (Arg306, Arg505, Arg662) leads to the dissociation of fragments of the A2 domain, residues 307-713 (A2N.A2C + A2C-peptide), leaving behind the membrane-bound A1.LC species. Evaluation of the dissociation process by light scattering yields invariant mass loss estimates as a function of APC concentration. The rate constant for A2 fragment dissociation varies with [APC], reaching a maximal value of k = 0.028 s-1, the unimolecular rate constant for A2 domain fragment dissociation. The APC binding site resides in the factor Va light chain (LC) (Kd = 7 nM), suggesting that the membrane-bound LC.A1 product would act to sequester APC. This inhibitory interaction (LC.A1.APC) is demonstrated to exist with either purified factor Va LC or the products of factor Va inactivation. Utilizing these experimental data and the reported rates of bond cleavage, binding constants, and product activity values for factor Va partial inactivation products, a model is developed which describes factor Va inactivation and accounts for the defect in factor VLEIDEN. The model accurately predicts the rates of inactivation of factor Va and factor VaLEIDEN, and the effect of product inhibition. Modeled reaction progress diagrams and activity profiles (from either factor Va or factor VaLEIDEN) are coincident with experimentally derived data, providing a mechanistic and kinetic explanation for all steps in the inactivation of normal factor Va and the pathology associated with factor VLEIDEN.

Lipid-protein interactions in blood coagulation

It has long been appreciated that lipids, particularly anionic phospholipids, promote blood coagulation. The last two decades have seen an increasing insight into the kinetic and mechanistic aspects regarding the mode of action of phospholipids in blood coagulation. This essay attempts to review these developments with particular emphasis on the structure of lipid-binding domains of blood coagulation proteins, and the variable effect of phospholipid composition on the interaction with these proteins. Some examples are discussed of how lipid membranes direct the pathway of enzymatic conversions in blood coagulation complexes, also illustrating that the membrane lipid surface is more than an inert platform for the assembly of coagulation factors. Finally, the controlled exposure of procoagulant lipid on the surface of blood cells is shortly reviewed, and an example is discussed of how interference with lipid-protein interactions in blood coagulation may result in pathological phenomena.

Unsaturated phospholipid acyl chains are required to constitute membrane binding sites for factor VIII

Membranes containing phosphatidyl-L-serine (PS) and phosphatidylethanolamine (PE) greatly enhance the function of the enzymatic cofactor factor VIII. The mechanisms of enhanced function involve condensation of enzyme (factor IXa), activated cofactor (factor VIIIa), and substrate (factor X) at a common location and, most dramatically, activation of the assembled enzyme-cofactor complex. We asked whether unsaturated phospholipid (PL) acyl chains are necessary to constitute factor VIII binding sites or to activate the factor VIIIa-factor IXa complex. We found that membranes composed of saturated, dimyristoyl phospholipids had 20-fold fewer factor VIII binding sites and that these sites supported less than 5% normal activity of the factor VIIIa-factor IXa complex. Thrombin-activated factor VIII bound to a similar number of membrane sites, and thrombin activation did not reduce the affinity for saturated membranes more than 2-fold so that the loss of functional activity is due to a requirement of the factor VIIIa-factor IXa complex for unsaturated acyl chains that exceeds the requirement for factor VIII binding alone. Replacement of dimyristoyl-PS, -PE, or -PC individually with the corresponding unsaturated phospholipid restored 75%, 60%, and 15%, respectively, of factor VIII binding sites but less than 10% of factor VIIIa-factor IXa activating activity. Lyso-PS did not support binding of factor VIII or function of the factor VIIIa-factor IXa complex even when PE and phosphatidylcholine contained unsaturated acyl chains. We conclude that the sn-2 acyl chain of PS and unsaturated phospholipid acyl chains are chemical requirements for constitution of fully functional factor VIII binding sites on phospholipid membranes.

Plasma lipoproteins support prothrombinase and other procoagulant enzymatic complexes

The prothrombinase complex (factor [F]Xa, FVa, calcium ions, and lipid membrane) converts prothrombin to thrombin (FIIa). To determine whether plasma lipoproteins could provide a physiologically relevant surface, we determined the rates of FIIa production by using purified human coagulation factors, and isolated fasting plasma lipoproteins from healthy donors. In the presence of 5 nmol/L FVa, 5 nmol/L FXa, and 1.4 micromol/L prothrombin, physiological levels of very low density lipoprotein (VLDL) (0.45 to 0.9 mmol/L triglyceride, or 100 to 200 micromol/L phospholipid) yielded rates of 2 to 8 nmol Flla x L(-1) x s(-1) in a donor-dependent manner. Low density lipoprotein (LDL) and high density lipoprotein (HDL) also supported prothrombinase but at much lower rates (< or =1.0 nmol FIIa x L(-1) x s[-1]). For comparison, VLDL at 2 mmol/L triglyceride yielded approximately 50% the activity of 2X10(8) thrombin-activated platelets per milliliter. Although the FIIa production rate was slower on VLDL than on synthetic phosphatidylcholine/phosphatidylsenne vesicles (approximately 50 nmol FIIa x L(-1) x s[-1]), the prothrombin Km values were similar, 0.8 and 0.5 micromol/L, respectively. Extracted VLDL lipids supported rates approaching those of phosphatidylcholine/phosphatidylserine vesicles, indicating the importance of the intact VLDL conformation. However, the presence of VLDL-associated, factor-specific inhibitors was ruled out by titration experiments, suggesting a key role for lipid organization. VLDL also supported FIIa generation in an assay system comprising 0.1 nmol/L FVIIa; 0.55 nmol/L tissue factor; physiological levels of FV, FVIII, FIX, and FX; and prothrombin (3 nmol/L FIIa x L(-1) x s[-1]). These results indicate that isolated human VLDL can support all the components of the extrinsic coagulation pathway, yielding physiologically relevant rates of thrombin generation in a donor-dependent manner. This support is dependent on the intact lipoprotein structure and does not appear to be regulated by specific VLDL-associated inhibitors. Further studies are needed to determine the extent of this activity in vivo.

Partial activation of the factor VIIIa-factor IXa enzyme complex by dihexanoic phosphatidylserine at submicellar concentrations

Phosphatidylserine (PS)-containing membranes increase the kcat of the factor VIIIa-factor IXa enzyme complex by more than 1000-fold. While PS supports specific, high-affinity membrane binding of factor VIIIa and factor IXa, it is not known whether PS is the lipid that activates the membrane-bound complex. It is also not known whether PS or other activating lipids must reside in the two-dimensional membrane matrix for efficacy. We have found that submicellar concentrations of dihexanoic phosphatidylserine (C6PS) increase the activity of the factor VIIIa-factor IXa complex in a biphasic manner with half-maximal concentrations of 0.2 and 1.6 mM while the micelle-forming concentration is 4.0 mM. Increased cleavage of factor X at 0.25 mM C6PS was due to a 25-fold enhancement of the kcat and a 30-fold increase in the affinity of factor VIIIa for factor IXa. C6 phosphatidylethanolamine and C6 phosphatidic acid, but not C6 phosphatidylcholine, also accelerated the Xase complex, indicating that kcat enhancement has less structural specificity than membrane binding. Submicellar C6PS enhanced activity of factor IXa in the absence of factor VIIIa, but the effect was due to a decreased KM rather than an increased kcat. These results suggest that activation of the factor VIIIa-factor IXa complex can result from binding of individual C6PS molecules or small aggregates in the absence of a membrane bilayer. They provide a model system in which the phospholipid-induced activation may be distinguished from membrane-binding of the enzyme complex.

Contribution of the prothrombin fragment 2 domain to the function of factor Va in the prothrombinase complex

The prothrombinase complex assembles through reversible interactions between factor Xa, factor Va and acidic phospholipid-containing membranes in the presence of calcium ions. This complex catalyses the conversion of prothrombin to thrombin through two proteolytic steps. We have used prethrombin 2 as a substrate analog for the first cleavage reaction of prothrombin activation (cleavage at Arg323-Ile324) catalyzed by the prothrombinase complex and have also relied on the known ability of prethrombin 2 to interact tightly but reversibly with fragment 2 or fragment 1.2. The kinetics of cleavage at Arg323-Ile324 have been assessed with these substrate analogs to investigate the contribution of cofactor-substrate interactions mediated by the fragment 2 domain to the ability of factor Va to enhance the catalytic efficiency of factor Xa within the prothrombinase complex. Initial velocity measurements indicated that the rate of prethrombin 2 cleavage by the factor Xa-PCPS binary complex was increased by a factor of approximately 1300 upon the addition of saturating concentrations of factor Va to assemble prothrombinase. Although the measured initial velocity was higher when either fragment 2 or fragment 1.2 was present, the factor Va-dependent enhancement in initial rate (2600- and 1500-fold) was comparable in each case. Steady state kinetic constants were obtained using prethrombin 2, prethrombin 2 plus fragment 2, and prethrombin 2 plus fragment 1.2 as substrates. For each substrate, the addition of saturating concentrations of factor Va to the Xa-PCPS binary complex led to increases in catalytic efficiency of between 1000 and 9000-fold. The kcat/Km for prethrombin 2 cleavage by prothrombinase was essentially identical to that obtained for prethrombin 2 saturated with fragment 2. Thus, comparable accelerating effects of factor Va are observed independent of the presence of the fragment 2 domain in the substrate. The results indicate that interactions between factor Va and the substrate mediated by the fragment 2 domain do not contribute in a dominant way to the ability of factor Va to enhance the catalytic efficiency of factor Xa within the prothrombinase complex.

Dynamic features of prothrombin interaction with phospholipid vesicles of different size and composition: implications for protein--membrane contact

The dynamics of prothrombin interaction with membrane vesicles of different size and composition was investigated to ascertain the impact of membrane surface characteristics and particle size on this interaction. Dissociation rates were highly sensitive to membrane composition and varied from about 20/s for membranes of 10% PS to 0.1/s for membranes of 50% PS. Overall affinity also varied by more than two orders of magnitude. Very small differences between prothrombin binding to SUV versus LUV were found. Association with large unilamellar vesicles (LUV of 115 nm diameter) was about 4-fold slower, when expressed on the basis of binding sites, than association with small unilamellar vesicles (SUV, 30 nm diameter) of the same composition. Both reactions proceeded at less than 25% of the collisional limit so that the differences were largely due to intrinsic binding properties. Vesicles of 325 nm diameter showed even slower association velocities. Dissociation rates from LUV were about 2-fold slower than from SUV. Again, these differences arose primarily from intrinsic binding properties. Dissociation conformed to a single first order reaction over a wide range of protein occupancy on the membrane. At very high packing density, the dissociation rate increased by about 2-fold. At equilibrium, prothrombin preferred binding to SUV over LUV by about 2-fold. This very small difference, despite substantial differences in phospholipid headgroup packing and hydrocarbon exposure, appeared inconsistent with an important role for protein insertion into the hydrocarbon region of the membrane. However, prothrombin-membrane interaction may arise from a series of interaction forces that have compensating features at equilibrium. The small differences in prothrombin binding to SUV versus LUV, together with differences in the number of protein binding sites per vesicle, were important to identify mechanisms of substrate delivery to the active site of the prothrombinase enzyme [Lu, Y., & Nelsestuen, G. L. (1996) Biochemistry 35, 8201-8209].

Activation of the factor VIIIa-factor IXa enzyme complex of blood coagulation by membranes containing phosphatidyl-L-serine

Factor IXa, a serine protease of blood coagulation, functions at least 100,000 times more efficiently when bound to factor VIIIa on a phospholipid membrane than when free in solution. We have utilized the catalytic activity of the factor VIIIa-factor IXa complex to report the effect of phospholipid membranes on binding of factor IXa to factor VIIIa and on enzymatic cleavage of the product. The apparent affinity of factor IXa for factor VIIIa was 10-fold lower in the absence of phospholipid membranes with a KD of 46 nM versus 4.3 nM with phospholipid membranes. The Km for activation of factor X by the factor VIIIa-factor IXa complex was 1700 nM in solution, 70-fold higher than the value of 28 nM when bound to membranes containing phosphatidyl-L-serine, phosphatidylethanolamine, and phosphatidylcholine at a ratio of 4:20:76. The largest effect of phosphatidyl-L-serine-containing membranes on the factor VIIIa-factor IXa complex was the accelerated rate of peptide bond cleavage, with the k(cat) increased by 1,500-fold from 0.022 to 33 min-1. Membranes in which phosphatidyl-L-serine was replaced by phosphatidyl-D-serine, phosphatidic acid, or phosphatidylglycerol were at least 10-fold less effective for enhancing the k(cat). Thus, while membranes containing phosphatidyl-L-serine enhance condensation of the enzyme with its cofactor and substrate, their largest effect is activation of the assembled factor VIIIa-factor IXa enzyme complex.

Prothrombin contributes to the assembly of the factor Va-factor Xa complex at phosphatidylserine-containing phospholipid membranes

The activation of prothrombin is catalyzed by prothrombinase, a complex of factor Xa and factor Va assembled on a negatively charged phospholipid membrane. We used a tubular flow reactor to identify the relative contributions of factor Va, prothrombin, and the negatively charged phosphatidylserine to the assembly of prothrombinase. Perfusion of phospholipid-coated capillaries with a mixture of factor Xa, factor Va, and prothrombin resulted in a steady-state rate of thrombin production that increased with (i) the phosphatidylserine content of the phospholipid bilayer, (ii) the factor Va concentration, and, most interestingly, (iii) the prothrombin concentration of the perfusion solution. Incorporation of 20 mol % phosphoatidylethanolamine, a phospholipid with poor ability to promote prothrombinase activity, into a 5 mol % phosphatidylserine membrane also increased the steady-state rate of thrombin production. Direct measurements of the amount of prothrombinase in the flow reactor demonstrated that increased catalytic activities were the result of an increased steady-state amount of membrane-associated prothrombinase. Thus, similar turnover numbers of prothrombin activation (3100 min-1) were calculated, irrespective of the phosphatidylserine content of the membrane. We established for membranes with low phosphatidylserine content (< 10 mol%) a linear relationship between the prothrombinase activity and the arithematical product of the factor Va concentration in the perfusion solution and the prothrombin concentration near the catalytic surface. Our results indicate that, in addition to factor Va, prothrombin also is essential to the assembly of prothrombinase at macroscopic surfaces with low phosphatidylserine content. The data further suggest that the prothrombin concentration near the surface, controlled by the prothrombinase activity and mass transfer, is an important regulator of the prothrombinase surface density.