Activation of bovine factor IX (Christmas factor) by factor XIa (activated plasma thromboplastin antecedent) and a protease from Russell's viper venom

Snake venom components in medicine: From the symbolic rod of Asclepius to tangible medical research and application

Both mythologically and logically, snakes have always fascinated man. Snakes have attracted both awe and fear not only because of the elegant movement of their limbless bodies, but also because of the potency of their deadly venoms. Practically, in 2017, the world health organization (WHO) listed snake envenomation as a high priority neglected disease, as snakes inflict up to 2.7 million poisonous bites, around 100.000 casualties, and about three times as many invalidities on man. The venoms of poisonous snakes are a cocktail of potent compounds which specifically and avidly target numerous essential molecules with high efficacy. The individual effects of all venom toxins integrate into lethal dysfunctions of almost any organ system. It is this efficacy and specificity of each venom component, which after analysis of its structure and activity may serve as a potential lead structure for chemical imitation. Such toxin mimetics may help in influencing a specific body function pharmaceutically for the sake of man's health. In this review article, we will give some examples of snake venom components which have spurred the development of novel pharmaceutical compounds. Moreover, we will provide examples where such snake toxin-derived mimetics are in clinical use, trials, or consideration for further pharmaceutical exploitation, especially in the fields of hemostasis, thrombosis, coagulation, and metastasis. Thus, it becomes clear why a snake captured its symbolic place at the Asclepius rod with good reason still nowadays.

Expression and Characterization of Gly-317 Variants of Factor IX Causing Variable Bleeding in Hemophilia B Patients

We recently identified two hemophilia B patients who carried Gly-317 to Arg (FIX-G317R) or Gly-317 to Glu (FIX-G317E) substitutions in their FIX gene. The former mutation caused severe and the latter moderate bleeding in afflicted patients. To understand the molecular basis for the variable clinical manifestation of Gly-317 mutations, we prepared recombinant G317R and G317E derivatives of FIX and compared their kinetic properties to those of recombinant wild-type FIX in appropriate assay systems. Both physiological activators, factor XIa and extrinsic Tenase (factor VIIa-tissue factor), activated both zymogen variants with an ∼1.5-fold elevated K(m); however, extrinsic Tenase activated FIX-G317E with an ∼2-fold improved k(cat). By contrast to zymogen activation, the catalytic activities of both FIXa-G317R and FIXa-G317E enzymes toward the natural substrate, factor X, were dramatically (>4 orders of magnitude) impaired, but their apparent affinity for interaction with factor VIIIa was only slightly (<2-fold) decreased. Further studies revealed that the reactivity of FIXa-G317R and FIXa-G317E with antithrombin has been impaired 10- and 13-fold, respectively, in the absence and 166- and 500-fold, respectively, in the presence of pentasaccharide. As expected, the clotting activities of FIX variants could not be measured by the aPTT assay. These results implicate a critical role for Gly-317 in maintaining normal catalytic function for FIX/FIXa in the clotting cascade. The results further suggest that improved k(cat) of FIX-G317E activation in the extrinsic pathway together with dramatically impaired reactivity of FIXa-G317E with antithrombin may account for the less severe bleeding phenotype of a hemophilia B patient carrying the FIX-G317E mutation.

Factor VIII assay mimicking in vivo coagulation conditions

Under certain circumstances, the determination of coagulation factor VIII (FVIII) is hampered by assay discrepancies between clotting and chromogenic approaches. These are observed in certain patients' plasma as well as in certain concentrates. We intended to develop a novel assay for the quantification of coagulation FVIII which reflects the physiological situation better than the established assays. It is based on plasma without chelation of divalent cations and simultaneously minimizes the generation of activated factors which could function as uncontrolled triggers of coagulation. FVIII deficient plasma is prepared with the aid of biotinylated antibodies against FVIII from normal plasma in presence of inhibitors of contact activation. To start the assay only tiny amounts of activated FIX serve as trigger. The FVIII determination is performed in a kinetic experiment and is based on the cleavage of a fluorogenic substrate for activated FX. FVIII concentrations between 0.01 and 1 IU mL(-1) are easily determined. Plasma-derived and recombinant FVIII concentrates were compared. All plasma-derived concentrates were found to contain FVIII activities within the specification of the manufacturer. Recombinant concentrates yielded only 35-50% of the claimed potency. The novel in vivo-like assay avoids the undue advantage or disadvantage of certain product characteristics by eliminating unphysiological assay conditions. Its usefulness could turn out in future experiments with plasma from haemophilia A patients.

A sequential mechanism for exosite-mediated factor IX activation by factor XIa

During blood coagulation, the protease factor XIa (fXIa) activates factor IX (fIX). We describe a new mechanism for this process. FIX is cleaved initially after Arg(145) to form fIXα, and then after Arg(180) to form the protease fIXaβ. FIXα is released from fXIa, and must rebind for cleavage after Arg(180) to occur. Catalytic efficiency of cleavage after Arg(180) is 7-fold greater than for cleavage after Arg(145), limiting fIXα accumulation. FXIa contains four apple domains (A1-A4) and a catalytic domain. Exosite(s) on fXIa are required for fIX binding, however, there is lack of consensus on their location(s), with sites on the A2, A3, and catalytic domains described. Replacing the A3 domain with the prekallikrein A3 domain increases K(m) for fIX cleavage after Arg(145) and Arg(180) 25- and ≥ 90-fold, respectively, and markedly decreases k(cat) for cleavage after Arg(180). Similar results were obtained with the isolated fXIa catalytic domain, or fXIa in the absence of Ca(2+). Forms of fXIa lacking the A3 domain exhibit 15-fold lower catalytic efficiency for cleavage after Arg(180) than for cleavage after Arg(145), resulting in fIXα accumulation. Replacing the A2 domain does not affect fIX activation. The results demonstrate that fXIa activates fIX by an exosite- and Ca(2+)-mediated release-rebind mechanism in which efficiency of the second cleavage is enhanced by conformational changes resulting from the first cleavage. Initial binding of fIX and fIXα requires an exosite on the fXIa A3 domain, but not the A2 or catalytic domain.

Conformational comparability of factor IX-Fc fusion protein, factor IX, and purified Fc fragment in the absence and presence of calcium

A long lasting recombinant factor IX -Fc fusion protein (rFIX-Fc) is being developed for the treatment of hemophilia B and is currently in late stage clinical investigation. By limiting injection frequency and maintaining efficacy, rFIX-Fc shows promise as a new therapeutic option for hemophilia B patients. However, before gaining regulatory approval, rFIX-Fc must undergo rigorous analytical and biological testing, in addition to clinical trials. Included in this testing is the need to understand this protein's higher-order structure and dynamics. In this study, we investigated and compared the biophysical properties of rFIX-Fc, rFIX, and Fc using hydrogen/deuterium exchange mass spectrometry and differential scanning calorimetry. Within the limits of these techniques, our results show that structural comparability exists between rFIX and the FIX region of rFIX-Fc. In addition, changes in the structure and dynamics of both proteins, in response to calcium binding, a requirement for FIX function, are also highly comparable. In the case of Fc and Fc region of rFIX-Fc, conformational comparability is also established. These biophysical results further support the conclusion that fusing an immunoglobulin gamma 1 Fc to rFIX does not significantly alter the higher-order structure of FIX or Fc, Ca binding to FIX, or Fc functionality.

Productive recognition of factor IX by factor XIa exosites requires disulfide linkage between heavy and light chains of factor XIa

In the intrinsic pathway of blood coagulation factor XIa (FXIa) activates factor IX (FIX) by cleaving the zymogen at Arg(145)-Ala(146) and Arg(180)-Val(181) bonds releasing an 11-kDa activation peptide. FXIa and its isolated light chain (FXIa-LC) cleave S-2366 at comparable rates, but FXIa-LC is a very poor activator of FIX, possibly because FIX undergoes allosteric modification on binding to an exosite on the heavy chain of FXIa (FXIa-HC) required for optimal cleavage rates of the two scissile bonds of FIX. However preincubation of FIX with a saturating concentration of isolated FXIa-HC did not result in any potentiation in the rate of FIX cleavage by FXIa-LC. Furthermore, if FIX binding via the heavy chain exosite of FXIa determines the affinity of the enzyme-substrate interaction, then the isolated FXIa-HC should inhibit the rate of FIX activation by depleting the substrate. However, whereas FXIa/S557A inhibited FIX activation of by FXIa, FXIa-HC did not. Therefore, we examined FIX binding to FXIa/S557A, FXIa-HC, FXIa-LC, FXIa/C362S/C482S, and FXIa/S557A/C362S/C482S. The heavy and light chains are disulfide-linked in FXIa/S557A but not in FXIa/C362S/C482S and FXIa/S557A/C362S/C482S. In an ELISA assay only FXI/S557A ligated FIX with high affinity. Partial reduction of FXIa/S557A to produce heavy and light chains resulted in decreased FIX binding, and this function was regained upon reformation of the disulfide linkage between the heavy and the light chains. We therefore conclude that substrate recognition by the FXIa exosite(s) requires disulfide-linked heavy and light chains.

The role of factor XIa (FXIa) catalytic domain exosite residues in substrate catalysis and inhibition by the Kunitz protease inhibitor domain of protease nexin 2

To select residues in coagulation factor XIa (FXIa) potentially important for substrate and inhibitor interactions, we examined the crystal structure of the complex between the catalytic domain of FXIa and the Kunitz protease inhibitor (KPI) domain of a physiologically relevant FXIa inhibitor, protease nexin 2 (PN2). Six FXIa catalytic domain residues (Glu(98), Tyr(143), Ile(151), Arg(3704), Lys(192), and Tyr(5901)) were subjected to mutational analysis to investigate the molecular interactions between FXIa and the small synthetic substrate (S-2366), the macromolecular substrate (factor IX (FIX)) and inhibitor PN2KPI. Analysis of all six Ala mutants demonstrated normal K(m) values for S-2366 hydrolysis, indicating normal substrate binding compared with plasma FXIa; however, all except E98A and K192A had impaired values of k(cat) for S-2366 hydrolysis. All six Ala mutants displayed deficient k(cat) values for FIX hydrolysis, and all were inhibited by PN2KPI with normal values of K(i) except for K192A, and Y5901A, which displayed increased values of K(i). The integrity of the S1 binding site residue, Asp(189), utilizing p-aminobenzamidine, was intact for all FXIa mutants. Thus, whereas all six residues are essential for catalysis of the macromolecular substrate (FIX), only four (Tyr(143), Ile(151), Arg(3704), and Tyr(5901)) are important for S-2366 hydrolysis; Glu(98) and Lys(192) are essential for FIX but not S-2366 hydrolysis; and Lys(192) and Tyr(5901) are required for both inhibitor and macromolecular substrate interactions.

Complex assemblies of factors IX and X regulate the initiation, maintenance, and shutdown of blood coagulation

Blood hemostasis is accomplished by a complex network of (anti-)coagulatory and fibrinolytic processes. These physiological processes are implemented by the assembly of multiprotein complexes involving both humoral and cellular components. Coagulation factor X, and particularly, factor IX, exemplify the dramatic enhancement that is obtained by the synergistic interaction of cell surface, inorganic and protein cofactors, protease, and substrate. With a focus on structure-function relationship, we review the current knowledge of activity modulation principles in the coagulation proteases factors IX and X and indicate future challenges for hemostasis research. This chapter is organized by describing the principles of hierarchical activation of blood coagulation proteases, including endogenous and exogenous protease activators, cofactor binding, substrate specificities, and protein inhibitors. We conclude by outlining pharmaceutical opportunities for unmet needs in hemophilia and thrombosis.

Coagulation procofactor activation by factor XIa

SUMMARY BACKGROUND

In the extrinsic pathway, the essential procofactors factor (F) V and FVIII are activated to FVa and FVIIIa by thrombin. In the contact pathway and its clinical diagnostic test, the activated partial thromboplastin time (APTT) assay, the sources of procofactor activation are unknown. In the APTT assay, FXII is activated on a negatively charged surface and proceeds to activate FXI, which activates FIX upon the addition of Ca(2+). FIXa feeds thrombin generation through activation of FX. FIXa is an extremely poor catalyst in the absence of its FVIIIa cofactor, which, in the intrinsic FXase complex, increases FXa generation by approximately 10(7). One potential APTT procofactor activator in this setting is FXIa.

OBJECTIVE

To test the hypothesis that FXIa can activate FVIII and FV.

METHODS

Recombinant FVIII and plasma FV were treated with FXIa, and the activities and integrities of each procofactor were measured using commercial clotting assays and sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

Kinetic analyses of FXIa-catalyzed activation and inactivation of FV and FVIII are reported, and the the timing and sites of cleavage are defined.

CONCLUSIONS

FXIa activates both procofactors at plasma protein concentrations, and computational modeling suggests that procofactor activation during the preincubation phase of the APTT assay is critical to the performance of the assay. As the APTT assay is the primary tool for the diagnosis and management of hemophilias A and B, as well as in the determination of FVIII inhibitors, these findings have potential implications in the clinical setting.

Activation mechanisms of coagulation factor IX

Blood haemostasis is accomplished by a complex network of coagulatory and fibrinolytic processes. These processes have to be delicately balanced, as clinically manifested by bleeding disorders, such as haemophilia A and B. These disorders are caused by defects in coagulation factor VIII and factor IX, respectively. Following a dual strategy, we emphasise on the one hand principles conserved in most coagulation enzymes, thus mirroring much of the underlying complexity in haemostasis; on the other hand, we identify enzymatic properties of the factor IXa-factor VIIIa system (Xase) that distinguish this proteolytic machine from other components of the coagulation system. While the exact mechanisms of its activity modulation remain baffling until today, superactive factor IX mutants significantly improve our current understanding and serve as a specific and testable model of Xase action.

New insights into the functions and N-glycan structures of factor X activator from Russell's viper venom

The coagulation factor X activator from Russell's viper venom (RVV-X) is a heterotrimeric glycoprotein. In this study, its three subunits were cloned and sequenced from the venom gland cDNAs of Daboia siamensis. The deduced heavy chain sequence contained a C-terminal extension with four additional residues to that published previously. Both light chains showed 77-81% identity to those of a homologous factor X activator from Vipera lebetina venom. Far-western analyses revealed that RVV-X could strongly bind protein S, in addition to factors X and IX. This might inactivate protein S and potentiate the disseminated intravascular coagulation syndrome elicited by Russell's viper envenomation. The N-glycans released from each subunit were profiled and sequenced by MALDI-MS and MS/MS analyses of the permethyl derivatives. All the glycans, one on each light chain and four on the heavy chain, showed a heterogeneous pattern, with a combination of variable terminal fucosylation and sialylation on multiantennary complex-type sugars. Amongst the notable features were the presence of terminal Lewis and sialyl-Lewis epitopes, as confirmed by western blotting analyses. As these glyco-epitopes have specific receptors in the vascular system, they possibly contribute to the rapid homing of RVV-X to the vascular system, as supported by the observation that slower and fewer fibrinogen degradation products are released by desialylated RVV-X than by native RVV-X.

Isolation and characterization of "Reprotoxin", a novel protein complex from Daboia russelii snake venom

In snake venoms, non-covalent protein-protein interaction leads to protein complexes with synergistic and, at times, distinct pharmacological activities. Here we describe a new protein complex containing phospholipaseA(2) (PLA(2)), protease, and a trypsin inhibitor. It is isolated from the venom of Daboia russelii by gel permeation chromatography, on a Sephadex G-75 column. This 44.6 kDa complex exhibits only phospholipase A(2) activity. In the presence of 8M urea it is well resolved into protease (29.1 kDa), PLA(2) (13 kDa), and trypsin inhibitor (6.5 kDa) peaks. The complex showed an LD(50) of 5.06 mg/kg body weight in mice. It inhibited the frequency of spontaneous release of neurotransmitter in hippocampal neurons. It also caused peritoneal bleeding, and edema in the mouse foot pads. Interestingly, the complex caused degeneration of both the germ cells and the mouse Leydig cells of mouse testis. A significant reduction in both the diameter of the seminiferous tubules and height of the seminiferous epithelia were observed following intraperitoneal injection of the sub-lethal dose (3 mg/kg body weight). This effect of the toxin is supported by the increase in the activities of acid and alkaline phosphatases and the nitric oxide content in the testes, and a decrease in the ATPase activity. Because of its potent organ atrophic effects on the reproductive organs, the toxin is named "Reprotoxin". This is the first report demonstrating toxicity to the reproductive system by a toxin isolated from snake venom.

Factor XI homodimer structure is essential for normal proteolytic activation by factor XIIa, thrombin, and factor XIa

Coagulation factor XI (FXI) is a covalent homodimer consisting of two identical subunits of 80 kDa linked by a disulfide bond formed by Cys-321 within the Apple 4 domain of each subunit. Because FXI(C321S) is a noncovalent dimer, residues within the interface between the two subunits must mediate its homodimeric structure. The crystal structure of FXI demonstrates formation of salt bridges between Lys-331 of one subunit and Glu-287 of the other subunit and hydrophobic interactions at the interface of the Apple 4 domains involving Ile-290, Leu-284, and Tyr-329. FXI(C321S), FXI(C321S,K331A), FXI(C321S,E287A), FXI(C321S,I290A), FXI(C321S,Y329A), FXI(C321S,L284A), FXI(C321S,K331R), and FXI(C321S,H343A) were expressed in HEK293 cells and characterized using size exclusion chromatography, analytical ultracentrifugation, electron microscopy, and functional assays. Whereas FXI(C321S) and FXI(C321S,H343A) existed in monomer/dimer equilibrium (K(d) approximately 40 nm), all other mutants were predominantly monomers with impaired dimer formation by analytical ultracentrifugation (K(d)=3-38 microm). When converted to the active enzyme, FXIa, all the monomeric mutants activated FIX similarly to wild-type dimeric FXIa. In contrast, these monomeric mutants could not be activated efficiently by FXIIa, thrombin, or autoactivation in the presence of dextran sulfate. We conclude that salt bridges formed between Lys-331 of one subunit and Glu-287 of the other together with hydrophobic interactions at the interface, involving residues Ile-290, Leu-284, and Tyr-329, are essential for homodimer formation. The dimeric structure of FXI is essential for normal proteolytic activation of FXI by FXIIa, thrombin, or FXIa either in solution or on an anionic surface but not for FIX activation by FXIa in solution.

Characterization of Novel Forms of Coagulation Factor XIa: independence of factor XIa subunits in factor IX activation

Factor XI is the zymogen of a dimeric plasma protease, factor XIa, with two active sites. In solution, and during contact activation in plasma, conversion of factor XI to factor XIa proceeds through an intermediate with one active site (1/2-FXIa). Factor XIa and 1/2-FXIa activate the substrate factor IX, with similar kinetic parameters in purified and plasma systems. During hemostasis, factor IX is activated by factors XIa or VIIa, by cleavage of the peptide bonds after Arg145 and Arg180. Factor VIIa cleaves these bonds sequentially, with accumulation of factor IX alpha, an intermediate cleaved after Arg145. Factor XIa also cleaves factor IX preferentially after Arg145, but little intermediate is detected. It has been postulated that the two factor XIa active sites cleave both factor IX peptide bonds prior to releasing factor IX abeta. To test this, we examined cleavage of factor IX by four single active site factor XIa proteases. Little intermediate formation was detected with 1/2-FXIa, factor XIa with one inhibited active site, or a recombinant factor XIa monomer. However, factor IX alpha accumulated during activation by the factor XIa catalytic domain, demonstrating the importance of the factor XIa heavy chain. Fluorescence titration of active site-labeled factor XIa revealed a binding stoichiometry of 1.9 +/- 0.4 mol of factor IX/mol of factor XIa (Kd = 70 +/- 40 nm). The results indicate that two forms of activated factor XI are generated during coagulation, and that each half of a factor XIa dimer behaves as an independent enzyme with respect to factor IX.

Macromolecular substrate-binding exosites on both the heavy and light chains of factor XIa mediate the formation of the Michaelis complex required for factor IX-activation

Binding of factor IX (FIX) to an exosite on the heavy chain of factor XIa (FXIa) is essential for the optimal activation of FIX (Sinha, D., Seaman, F. S., and Walsh, P. N. (1987) Biochemistry 26, 3768-3775). To gain further insight into the mechanisms of activation of FIX by FXIa, we have investigated the kinetic properties of FXIa-light chain (FXIa-LC) with its active site occupied by either a reversible inhibitor of serine proteases (p-aminobenzamidine, PAB) or a small peptidyl substrate (S-2366) and have examined FIX cleavage products resulting from activation by FXIa or FXIa-LC. PAB inhibited the hydrolysis of S-2366 by FXIa-LC in a classically competitive fashion. In contrast, PAB was found to be a noncompetitive inhibitor of the activation of the macromolecular substrate FIX. Occupancy of the active site of the FXIa-LC by S-2366 also resulted in noncompetitive inhibition of FIX activation. These results demonstrate the presence of an exosite for FIX binding on the FXIa-LC remote from its active site. Furthermore, examination of the cleavage products of FIX indicated that in the absence of either Ca2+ or the heavy chain of FXIa there was substantial accumulation of the inactive intermediate FIXalpha, indicating a slower rate of cleavage of the scissile bond Arg180-Val181. We conclude that binding to two substrate-binding exosites one on the heavy chain and the other on the light chain of FXIa is required to mediate the formation of the Michaelis complex and efficient cleavages of the two spatially separated scissile bonds of FIX.

Functional Analysis of the Factor IX Epidermal Growth Factor-Like Domain Mutation Ile66Thr Associated with Mild Hemophilia B

The present study focused on the functional role of the mutation Ile66Thr located in the N-terminal epidermal growth factor-like domain of coagulation factor IX (FIX). This mutation causes mild hemophilia B with approximately 25% FIX coagulant activity and FIX antigen levels of around 90% of normal. In the 3-dimensional structure of porcine FIXa and in the subsequent 3-dimensional model of human FIXa that we have previously developed, residue 66 is exposed to the solvent and can be replaced by many amino acids, including Thr, without affecting the major folding/stability of the molecule. This is consistent with the basically normal antigen levels observed. We found that the FIX Ile66Thr mutant was activated to a normal extent by FVIIa/TF and FXIa. However, the ability of FIX Ile66Thr to activate FX was impaired in both the presence and absence of FVIIIa, indicating that Ile66 is not directly involved in the binding of FIX to FVIIIa.

Expression, purification and characterization of factor IX derivatives using a novel vector system

Recent studies have indicated that the loop harboring the S1 specificity site (residues 185-189 in chymotrypsin numbering) of coagulation proteases has several charged residues with important structural and functional roles for the catalytic activity of these proteases. This loop is allosterically linked to the Na(+)-binding site in both factor Xa and thrombin. There are three candidate residues (His-185, Glu-186, and Arg-188) on this loop of factor IXa (fIXa) whose side chains can influence the Na(+) binding and the catalytic function of the protease in the intrinsic Xase complex. In this study, we developed a novel expression/purification vector system, substituted all three residues of factor IX individually with Ala, and expressed the mutant zymogens in mammalian cells. Following activation, all three fIXa mutants exhibited normal activity towards a fIXa-specific chromogenic substrate in the presence of Ca(2+) with no obvious requirement for Na(+) in the reaction. Furthermore, all three mutants interacted with factor VIIIa with near normal affinity and catalyzed the activation of factor X in the intrinsic Xase complex with a normal catalytic efficiency. These results suggest that, unlike thrombin and factor Xa, the charged residues of this loop do not play a functional role in modulating the catalytic function of fIXa in the intrinsic Xase complex.

The Interface between the EGF2 Domain and the Protease Domain in Blood Coagulation Factor IX Contributes to Factor VIII Binding and Factor X Activation

The light chain of activated factor IX (FIXa) is involved in a number of functional properties, including FIXa enzymatic activity. This suggests the existence of a functional link between the FIXa light chain and the catalytic domain. The FIXa structure includes a few putative interactions between EGF2 and the protease domain. The role thereof has been addressed in this study. Recombinant FIX variants FIX-N92A, FIX-N92H, FIX-Y295A, and FIX-F299A were produced in 293 cells. After activation, the purified mutants were analyzed for a variety of functional parameters. None of these substitutions had a major effect on the interaction with antithrombin or the cleavage of the chromogenic substrate CH(3)SO(2)-d-CHG-Gly-Arg-p-nitroanilide. All FIXa mutants, however, exhibited a reduced level of factor X (FX) activation. Defective proteolytic activity occurred both in the absence and in the presence of activated factor VIII (FVIIIa). All mutants also exhibited a reduced level of FX activation in the absence of phospholipids. This suggests that putative interdomain contacts involving residues Asn(92), Tyr(295), and Phe(299) affect reactivity toward FX. Detailed kinetic studies in the presence of phospholipids and FVIIIa revealed substrate inhibition, particularly for mutants FIXa-N92A and FIXa-N92H. Surface plasmon resonance demonstrated that the same replacements weaken the association with the isolated factor VIII (FVIII) A2 domain and the FVIII light chain. This implies a defect in the formation of the FX-activating complex that is membrane-independent. We conclude that contacts between EGF2 and the protease domain of FIXa are crucial for FIXa enzymatic activity and for the assembly of the FX-activating complex.

Antivenom treatment and renal dysfunction in Russell's viper snakebite in Taiwan: a case series

Formosan Russell's viper (Daboia russelli siamensis) is the sixth most frequent cause of snakebites in Taiwan. Its bite induces greater kidney injury than other Russell's vipers in Southeast Asia. Poor availability of antivenom might be the major reason. To enhance treatment, we supplied the antivenom to the teaching hospitals that are near the areas where D. r. siamensis is found. We also used an ELISA in diagnosis. From June 1999 to December 2001, a total of 13 cases of D. r. siamensis snakebite were diagnosed with serum venom level of 10-98 ng/ml, 1-6 hours after being envenomed. Abnormal coagulation function and acute renal failure occurred early and were the two most important clinical features. Early specific antivenom treatment, 3-6 hours after systemic envenoming, restored the coagulation function in 1-2 days and seemed to be statistically effective in reducing the severity of renal damage compared with the historical and delayed group by the Wilcoxon rank-sum test. Two to four vials of antivenom were needed to block the systemic toxicity and produced few side effects. The antivenom should be administered as early as possible to prevent systemic dysfunction.

Characterization of a monoclonal antibody B1 that recognizes phosphorylated Ser-158 in the activation peptide region of human coagulation factor IX

Blood coagulation factor IX (FIX) undergoes various post-translational modifications such as gamma-carboxylation and glycosylation. Non-phosphorylated recombinant FIX has been reported to rapidly disappear from plasma, indicating that phosphorylation of FIX plays an important role in the physiological activity of this coagulation factor. In this study, we characterized the human FIX activation peptide (AP) using a monoclonal antibody that recognizes phosphorylated Ser-158 in the AP region. Murine monoclonal antibody B1 against human FIX recognized FIX with an apparent K(d) value of 5 nm in the presence of Ca(2+) (EC(50) = 0.58 mm). B1 bound to the isolated AP of FIX and retained the Ca(2+) dependence of binding to the isolated AP. The deglycosylation of AP did not affect the binding of B1 to AP, while B1 failed to bind to recombinant AP expressed in Escherichia coli. MALDI-TOF mass spectrometry showed that the m/z of plasma-derived deglycosylated AP is 82.54 Da greater than that of recombinant AP. The binding ability of B1 to AP was lost by the dephosphorylation of plasma-derived AP. B1 bound to synthetic peptide AP-(5-19), including phosphoserine-13, but not to the non-phosphorylated AP-(5-19) in the presence of Ca(2+). These data provide direct evidence that Ser-13 of the plasma-derived FIX AP region (Ser-158 of FIX) is phosphorylated and that B1 recognizes the epitope, which includes Ca(2+)-bound phosphoserine-158. B1 should be useful in the quality control of biologically active recombinant FIX containing phosphoserine-158.

Shedaoenase, a novel fibrinogenase from the venom of Agkistrodon shedaoenthesis Zhao

Shedaoenase, a serine protease, was isolated from the venom of Agkistrodon shedaoenthesis Zhao with an apparent molecular mass of 36 kDa. It was purified by affinity chromatography on arginine Sepharose 4B column and anion exchange on Mono Q fast protein liquid chromatography. Shedaoenase preferentially cleaved the Aalpha-chain of human fibrinogen and slowly digested the Bbeta-chain. It also showed arginyl esterase activity using Nalpha-benzoyl-L-arginine ethyl ester as a substrate, and some synthetic chromogentic substrates, such as Chromozym PL, S-2266, and S-2160, could also be hydrolyzed. The enzyme activity of shedaoenase could be completely inhibited by phenylmethylsulphonylfluoride and could be little inhibited by the chelating reagent EDTA. The N-terminal sequence of shedaoenase was determined, and its full-length cDNA encoding a protein of 238 amino acid residues was cloned by reverse transcription-polymerase chain reaction from the total mRNA extracted from the snake venom gland. The deduced primary sequence of shedaoenase shares significant homology with other snake venom serine proteases.

Snake venoms and hemostasis

Snake venoms are complex mixtures of biologically active proteins and peptides. Many of them affect hemostasis by activating or inhibiting coagulant factors or platelets, or by disrupting endothelium. Based on sequence, these snake venom components have been classified into various families, such as serine proteases, metalloproteinases, C-type lectins, disintegrins and phospholipases. The various members of a particular family act selectively on different blood coagulation factors, blood cells or tissues. For almost every factor involved in coagulation or fibrinolysis there is a venom protein that can activate or inactivate it. Venom proteins affect platelet function by binding or degrading vWF or platelet receptors, activating protease-activated receptors or modulating ADP release and thromboxane A2 formation. Some venom enzymes cleave key basement membrane components and directly affect capillary blood vessels to cause hemorrhaging. L-Amino acid oxidases activate platelets via H2O2 production.

Exosite-mediated substrate recognition of factor IX by factor XIa. The factor XIa heavy chain is required for initial recognition of factor IX

Studies of the mechanisms of blood coagulation zymogen activation demonstrate that exosites (sites on the activating complex distinct from the protease active site) play key roles in macromolecular substrate recognition. We investigated the importance of exosite interactions in recognition of factor IX by the protease factor XIa. Factor XIa cleavage of the tripeptide substrate S2366 was inhibited by the active site inhibitors p-aminobenzamidine (Ki 28 +/- 2 microM) and aprotinin (Ki 1.13 +/- 0.07 microM) in a classical competitive manner, indicating that substrate and inhibitor binding to the active site was mutually exclusive. In contrast, inhibition of factor XIa cleavage of S2366 by factor IX (Ki 224 +/- 32 nM) was characterized by hyperbolic mixed-type inhibition, indicating that factor IX binds to free and S2366-bound factor XIa at exosites. Consistent with this premise, inhibition of factor XIa activation of factor IX by aprotinin (Ki 0.89 +/- 0.52 microM) was non-competitive, whereas inhibition by active site-inhibited factor IXa beta was competitive (Ki 0.33 +/- 0.05 microM). S2366 cleavage by isolated factor XIa catalytic domain was competitively inhibited by p-aminobenzamidine (Ki 38 +/- 14 microM) but was not inhibited by factor IX, consistent with loss of factor IX-binding exosites on the non-catalytic factor XI heavy chain. The results support a model in which factor IX binds initially to exosites on the factor XIa heavy chain, followed by interaction at the active site with subsequent bond cleavage, and support a growing body of evidence that exosite interactions are critical determinants of substrate affinity and specificity in blood coagulation reactions.

Purification, identification, and characterization of elastase on erythrocyte membrane as factor IX-activating enzyme

In our previous papers, we reported that factor IX (F-IX), when activated by erythrocyte membranes, causes coagulation. We report on purification, identification, and characterization of F-IX-activating enzyme extracted from human erythrocyte membranes. The enzyme whose amino acid sequence is almost in accord with neutrophil elastase was found in normal erythrocyte membrane. The molecular mass was slightly smaller than that of neutrophil elastase. The content of the enzyme in erythrocyte membranes was estimated to be 3.0-3.7 ng per 10(6)erythrocytes. The F-IX sites cleaved by the enzyme were slightly different from those by the ordinary coagulation reaction. The ability of F-IX cleaved by the enzyme to cause coagulation was estimated to be approximately 1/10 as high as that of the F-IX cleaved by activated F-XI. These findings provide evidence that F-IX is activated by erythrocyte membrane, which may serve as a triggering mechanism for blood coagulation.

Contribution of basic residues of the autolysis loop to the substrate and inhibitor specificity of factor IXa

The autolysis loop (residues 143-154 in chymotrypsinogen numbering) plays a pivotal role in determining the macromolecular substrate and inhibitor specificity of coagulation proteases. This loop in factor IXa (FIXa) has 3 basic residues (Arg143, Lys147, and Arg150) whose contribution to the protease specificity of factor IXa has not been studied. Here, we substituted these residues individually with Ala in Gla-domainless forms of recombinant factor IX expressed in mammalian cells. All mutants exhibited normal amidolytic activities toward a FIXa-specific chromogenic substrate. However, Arg143 and Lys147 mutants showed a approximately 3- to 6-fold impairment in FX activation, whereas the Arg150 mutant activated factor X normally both in the absence and presence of factor VIIIa. By contrast, Arg143 and Lys147 mutants reacted normally with antithrombin (AT) in both the absence and presence of the cofactor, heparin. However, the reactivity of the Arg150 mutant with AT was impaired 6.6-fold in the absence of heparin and 33- to 70-fold in the presence of pentasaccharide and full-length heparins. These results suggest that Arg143 and Lys147 of the autolysis loop are recognition sites for FX independent of factor VIIIa, and Arg150 is a specific recognition site for AT that can effectively interact with AT only if the serpin is in the heparin-activated conformation.

Functional analysis of the EGF-like domain mutations Pro55Ser and Pro55Leu, which cause mild hemophilia B

We studied the functional role of two mutations, Pro55Ser and Pro55Leu, located in the N-terminal Epidermal Growth Factor-like domain (EGF1) of coagulation factor (F) IX. Both mutations cause mild hemophilia B with habitual FIX coagulant activities of 10-12% and FIX antigen levels of 50%. We found that activation by FVIIa/TF and FXIa was normal for FIXPro55Ser, but resulted in proteolysis of FIXPro55Leu at Arg318-Ser319 with a concomitant loss of amidolytic activity, suggesting intramolecular communication between EGF1 and the serine protease domain in FIX. This was further supported by experiments using an anti-EGF1 monoclonal antibody. Activation of FX by FIXaPro55Ser was impaired in both the presence and the absence of phospholipid or FVIIIa, indicating that Pro55 is not directly involved in binding to FVIIIa. We also studied the effect of the two Pro55 mutations on Ca2+ affinity and found only small changes. Thus, the Pro55Ser mutation causes hemophilia primarily through to an impaired ability to activate FX whereas at least in vitro the Pro55Leu defect interferes with the activation of FIX.

The factor IX gamma-carboxyglutamic acid (Gla) domain is involved in interactions between factor IX and factor XIa

During hemostasis, factor IX is activated to factor IXabeta by factor VIIa and factor XIa. The glutamic acid-rich gamma-carboxyglutamic acid (Gla) domain of factor IX is involved in phospholipid binding and is required for activation by factor VIIa. In contrast, activation by factor XIa is not phospholipid-dependent, raising questions about the importance of the Gla for this reaction. We examined binding of factors IX and IXabeta to factor XIa by surface plasmon resonance. Plasma factors IX and IXabeta bind to factor XIa with K(d) values of 120 +/- 11 nm and 110 +/- 8 nm, respectively. Recombinant factor IX bound to factor XIa with a K(d) of 107 nm, whereas factor IX with a factor VII Gla domain (rFIX/VII-Gla) and factor IX expressed in the presence of warfarin (rFIX-desgamma) did not bind. An anti-factor IX Gla monoclonal antibody was a potent inhibitor of factor IX binding to factor XIa (K(i) 34 nm) and activation by factor XIa (K(i) 33 nm). In activated partial thromboplastin time clotting assays, the specific activities of plasma and recombinant factor IX were comparable (200 and 150 units/mg), whereas rFIX/VII-Gla activity was low (<2 units/mg). In contrast, recombinant factor IXabeta and activated rFIX/VIIa-Gla had similar activities (80 and 60% of plasma factor IXabeta), indicating that both proteases activate factor X and that the poor activity of zymogen rFIX/VII-Gla was caused by a specific defect in activation by factor XIa. The data demonstrate that factor XIa binds with comparable affinity to factors IX and IXabeta and that the interactions are dependent on the factor IX Gla domain.

Physiological fIXa activation involves a cooperative conformational rearrangement of the 99-loop

Coagulation factor IXa (fIXa) plays a central role in the coagulation cascade. Enzymatically, fIXa is characterized by its very low amidolytic activity that is not improved in the presence of cofactor, factor VIIIa (fVIIIa), distinguishing fIXa from all other coagulation factors. Activation of the fIXa-fVIIIa complex requires its macromolecular substrate, factor X (fX). The 99-loop positioned near the active site partly accounts for the poor activity of fIXa because it adopts a conformation that interferes with canonical substrate binding in S2-S4. Here we show that residues Lys-98 and Tyr-99 are critically linked to the amidolytic properties of fIXa. Exchange of Tyr-99 with smaller residues resulted not only in an overall decreased activity but also in impaired binding in S1. Replacement of Lys-98 with smaller and uncharged residues increased activity. Simultaneous mutagenesis of Lys-98, Tyr-177, and Tyr-94 produced an enzyme with 7000-fold increased activity and altered specificity. This triple mutant probably mimics the conformational changes that are physiologically induced by cofactor and substrate binding. It therefore provides a cooperative two-step activation model for fIXa. Tyr-177 locks the 99-loop in an inactive conformation which, in the physiologic complex, is released by cofactor fVIIIa. FX is then able to rearrange the unlocked 99-loop and subsequently binds to the active site cleft.

Localization of the heparin binding exosite of factor IXa

Factor IXa (FIXa) is known to have a binding site for heparin that has not been mapped by a mutagenesis study. By homology modeling based on structural data, we identified eight basic residues in the catalytic domain of FIXa that can potentially bind to heparin. These residues, Lys(98), Lys(126), Arg(165), Arg(170), Lys(173), Lys(230), Arg(233), and Lys(239) (chymotrypsin numbering) were substituted with Ala in separate constructs in Gla-domainless forms. Following activation, it was found that all FIXa derivatives cleaved the chromogenic substrate CBS 31.39 with near normal catalytic efficiencies. Similarly, antithrombin inactivated FIXa derivatives with a similar second-order association rate constant (k(2)) in both the absence and presence of pentasaccharide. In the presence of a full-length heparin, however, k(2) values were dramatically impaired with certain mutants. Direct binding studies revealed that the same mutants lost their affinities for binding to heparin-Sepharose. Both kinetic and direct binding data indicated that five basic residues of FIXa in the following order of importance, Arg(233) > Arg(165) > Lys(230) > Lys(126) > Arg(170) are critical for binding to heparin. Consistent with these results, examination of the crystal structure of the catalytic domain of FIXa indicated that all five basic residues are spatially aligned in a manner optimal for interaction with heparin.

The interaction of factor XIa with activated platelets but not endothelial cells promotes the activation of factor IX in the consolidation phase of blood coagulation

We have previously shown that the zymogen factor XI (FXI) binds to activated platelets but not to human umbilical vein endothelial cells (HUVEC), a conclusion that is in conflict with previous reports stating that FXI binds to 2.7-13 x 10(6) high affinity sites per HUVEC (Berrettini, M., Schleef, R. R., Heeb, M. J., Hopmeier, P., and Griffin, J. H. (1992) J. Biol. Chem. 267, 19833-19839; Shariat-Madar, Z., Mahdi, F., and Schmaier, A. H. (2001) Thromb. Haemostasis 85, 544-551). It has also been reported that activated FXI (FXIa) binds to 1.5 x 10(6) sites per HUVEC and promotes the activation of factor IX by cell bound FXIa (Berrettini, M., Schleef, R. R., Heeb, M. J., Hopmeier, P., and Griffin, J. H. (1992) J. Biol. Chem. 267, 19833-19839). Therefore, the binding of FXIa to activated platelets was compared with FXIa binding to HUVEC and HEK293 cells immobilized on microcarrier beads. Specific and saturable zinc-dependent FXIa binding was demonstrated to 250 +/- 48 sites per activated platelet (K(D) = 1.7 +/- 0.78 nm) and 6.5 +/- 0.4 x 10(4) sites per HUVEC (K(D) = 2.4 +/- 0.5 nm), whereas no binding to HEK293 cells was detected. A titration with high molecular weight kininogen had no effect on FXIa binding to platelets, but revealed a concentration-dependent decrease in the amount of FXIa bound to HUVEC. The rate of factor IXa generation catalyzed by FXIa was unaffected by the presence of surfaces; however only the activated platelet surface protected FXIa from inhibition by protease nexin 2. The results presented here confirm the conclusion that activated platelets are procoagulant while unstimulated endothelial cells are not.

The N-terminal epidermal growth factor-like domain of coagulation factor IX. Probing its functions in the activation of factor IX and factor X with a monoclonal antibody

The absence or reduced activity of coagulation factor IX (FIX) causes the severe bleeding disorder hemophilia B. FIX contains an N-terminal Gla domain followed by two epidermal growth factor-like (EGF) domains and a serine protease domain. In this study, the epitope of monoclonal antibody AW, which is directed against the C-terminal part of the first EGF domain in human FIX, was defined, and the antibody was used to study interactions between the EGF domain of FIX and other coagulation proteins. Antibody AW completely blocks activation of FIX by activated factor XI, but activation by activated factor FVII-tissue factor is inhibited only slightly. The antibody also causes a marginal reduction in the apparent k(cat) for factor X both in the presence and absence of activated factor VIII. Based on these results, we produced a preliminary model of the structure of the activated factor IX-activated factor VIII-AW complex on the surface of phospholipid. The model suggests that in the Xase complex, EGF1 of activated factor IX is not involved in direct binding to activated factor VIII. Studies of the interaction of antibody AW with a mutated FIX molecule (R94D) also suggest that the Glu(78)-Arg(94) salt bridge is not important for maintaining the structure of FIX.

Russell's viper snakebite in Taiwan: differences from other Asian countries

Formosan Russell's viper (Daboia russelli siamensis) is the sixth most frequent cause of snakebite in Taiwan. Its venom has been thought to have both neurotoxic and hematoxic properties. This viper's snakebite is rare and thus scarcely subjected to systemic studies. In this paper, we retrospectively analyzed and described 18 cases of viper snakebite from 1987 to 1999. Like that of the Russell's viper snakebite in other South East Asian areas, varied degrees of acute renal failure, incoagulable blood with bleeding diathesis and hemolysis were the major symptoms found in the systemic envenoming patients. Systemic thrombosis seems to be the distinguishing feature in Formosan Russell's viper snakebite. Neither symptoms nor signs of neuromuscular junction blocking effects were observed, which is another difference from symptoms observed after bites of some other Russell's viper subspecies, suggesting a significant geographic variation. These findings confirmed the clinical importance of Russell's viper snakebite in Taiwan.

Identification of functionally important residues of the epidermal growth factor-2 domain of factor IX by alanine-scanning mutagenesis. Residues Asn(89)-Gly(93) are critical for binding factor VIIIa

This paper describes the consequences of alanine-scanning mutagenesis on 28 positions of the second epidermal growth factor (EGF-2) domain of factor IX. We identified four positions of Gln(97), Phe(98), Tyr(115), and Leu(117) that are critical for secretion of factor IX. Of the remaining mutations, 4 mutants (V86A, E113A, K122A, and S123A) are as active as wild-type factor IX (IXwt); 16 (D85A, K100A, N101A, D104A, N105A, R116A, E119A, T87A, I90A, K91A, R94A, E96A, S102A, K106A, T112A, and N120A) retain reduced but detectable activity, and 4 (N89A, N92A, G93A, and V107A) are nearly inert in the clotting assay. Both factor XIa and the factor VIIa-tissue factor complex effectively catalyzed the activation of these mutants except N89A. The mutant V107A failed to form the factor tenase complex with factor VIIIa because of a 35-fold increase in K(d). The mutants N89A and N92A did not compete with factor IXwt for factor VIIIa binding, and G93A exhibited a 6-fold increase in K(i) values in the competitive binding assay. It appears that mutations at these positions have significantly affected the interaction between factor IX and factor VIIIa, although other mutations had little effect on the binding of factor IX to factor VIIIa. Mutations in two regions, Thr(87)-Gly(93) and Asn(101)-Val(107), significantly increased the K(m) value of factor IXa (2-10-fold) in cleavage of factor X in the absence of factor VIIIa. In the presence of factor VIIIa, the catalytic efficiency of each mutant toward factor X paralleled its clotting activity. Briefly, we propose two relatively distinctive functions of factor IX for two adjacent regions in the EGF-2 domain; the first loop region (residues 89-94) is involved with the binding of its cofactor, factor VIIIa, and the third loop with connected beta-sheets (residues 102-108) is involved in the proper binding to the substrate, factor X.

The connecting segment between both epidermal growth factor-like domains in blood coagulation factor IX contributes to stimulation by factor VIIIa and its isolated A2 domain

The light chain of activated factor IX comprises multiple interactions between both epidermal growth factor-like domains that contribute to enzymatic activity and binding of factor IXa to its cofactor factor VIIIa. To investigate the association between factor IXa-specific properties and surface-exposed structure elements, chimeras were constructed in which the interconnection between the modules Leu(84)-Thr(87) and the factor IX-specific loop Asn(89)-Lys(91) were exchanged for corresponding regions of factor X and factor VII. In absence of factor VIIIa, all chimeras displayed normal enzymatic activity. In the presence of factor VIIIa, replacement of loop Asn(89)-Lys(91) resulted in a minor reduction in factor IXa activity. However, chimeras with substitutions or insertions in the spacer between the epidermal growth factor-like domains showed a major defect in response to factor VIIIa. Of these chimeras, some displayed a normal response to isolated factor VIII A2 domain as a cofactor in factor X activation. Surprisingly, chimeras containing elongated inter-domain spacers from factor X or VII displayed reduced response to both complete factor VIIIa and the isolated A2 domain. Moreover, these chimeras still displayed effective association with immobilized A2 domain as assessed by surface plasmon resonance. We conclude that both sequence and length of the junction Leu(84)-Thr(87) between both epidermal growth factor-like domains contribute to the enhancement of factor IXa enzymatic activity that occurs upon assembly with factor VIIIa.

Residues 88-109 of factor IXa are important for assembly of the factor X activating complex

Activated platelets and phospholipid vesicles promote assembly of the intrinsic factor X (FX) activating complex by presenting high-affinity binding sites for blood coagulation FIXa, FVIIIa, and FX. Previous reports suggest that the second epidermal growth factor (EGF)-like domain of FIXa mediates assembly of the FX activating complex (Ahmad, S. S., Rawala, R., Cheung, W. F., Stafford, D. W., and Walsh, P. N. (1995) Biochem. J. 310, 427-431; Wong, M. Y., Gurr, J. A., and Walsh, P. N. (1999) Biochemistry 38, 8948-8960). To identify important residues, we prepared several chimeric FIXa proteins using homologous sequences from FVII: FIXa(FVIIEGF2) (FIX Delta 88-124,inverted Delta FVII91-127), FIXa(loop1) (FIX Delta 88-99,inverted Delta FVII91-102), FIXa(loop2) (FIX Delta 95-109,inverted Delta FVII98-112), FIXa(loop3) (FIX Delta 111-124,inverted Delta FVII114-127), and point mutants (FIXaR94D and FIXa(loop1)G94R). In the presence and absence of FVIIIa, a 2- to 10-fold reduced V(max) of FX activation (nm FXa min(-1)) was observed for FIXa(FVIIEGF2), FIXa(loop1), FIXa(loop2), and FIXa(loop1)G94R, whereas FIXa(loop3) and FIXaR94D were normal. For all of the FIXa proteins, K(m)((app)) values were normal as were EC(50) values for interactions with FVIIIa. However, K(d)((app)) (in nm) for the FX activating complex assembled on phospholipid vesicles was increased for FIXa(FVIIEGF2) (43.3 +/- 2.70), FIXa(loop1)(10.9 +/- 2.8), FIXa(loop2) (70.5 +/- 1.60), and FIXa(loop1)G94R (17.1 +/- 2.90) relative to FIXa(N) (3.9 +/- 0.11), FIXa(WT) (4.6 +/- 0.17), FIXa(loop3) (4.5 +/- 0.20), and FIXaR94D (2.2 +/- 0.09) suggesting that reduced V(max) is a result of impaired complex assembly. These data indicate that residues 88-109 (but not Arg(94)) are important for normal assembly of the FX activating complex on phospholipid vesicles.

An anti-EGF monoclonal antibody that detects intramolecular communication in factor IX

Coagulation factor IX contains a gamma-carboxyglutamic acid (Gla) module, two epidermal growth factor-like (EGF) modules, and a serine protease region. We have characterized a mouse monoclonal antibody that binds the N-terminal EGF-like module of human factor IX with high affinity. Studies of recombinant factor IX mutants indicated that the epitope is located in the C-terminal end of the EGF-like module, which is consistent with the binding being non-Ca(2+)-dependent. The antibody bound factor IXa (K(D) = 7.6 x 10(-10) M) with about 10-fold higher affinity than factor IX (K(D) = 6.2 x 10(-9) M). Binding of the antibody to factor IXa did not affect the amidolytic activity of the protein, nor was binding affected by active site inhibition of factor IXa. These results are consistent with long-range interactions between the serine protease region and the N-terminal EGF-like module in factor IX.

Activation of factor IX zymogen results in exposure of a binding site for low-density lipoprotein receptor–related protein

The interaction between the endocytic receptor low density lipoprotein receptor–related protein (LRP) and either coagulation factor IX or its active derivative factor IXa was studied. Purified factor IX was unable to associate with LRP when analyzed by surface plasmon resonance. By contrast, factor XIa–mediated conversion of factor IX into factor IXa resulted in reversible dose- and calcium-dependent binding to LRP. Active-site blocking of factor IXa did not affect binding to LRP, whereas LRP binding was efficiently inhibited in the presence of heparin or antibodies against factor IX or LRP. The factor IXa–LRP interaction could be described by a 2-site binding model with equilibrium dissociation constants of 27 nmol/L and 69 nmol/L. Consistent with this model, it was observed that factor IXa binds to 2 different recombinant receptor fragments of LRP (denoted cluster II and cluster IV) with equilibrium dissociation constants of 227 nmol/L and 53 nmol/L, respectively. The amount of factor IXa degraded by LRP-deficient cells was 35% lower than by LRP-expressing cells, demonstrating that LRP contributes to the transport of factor IXa to the intracellular degradation pathway. Because ligand binding to LRP is often preceded by binding to proteoglycans, the contribution of proteoglycans to the catabolism of factor IXa was addressed by employing proteoglycan-deficient cells. Degradation of factor IXa by proteoglycan-deficient cells proceeded at a 83% lower rate than wild-type cells. In conclusion, the data presented here indicate that both LRP and proteoglycans have the potential to contribute to the catabolism of factor IXa.

Activation of factor IX zymogen results in exposure of a binding site for low-density lipoprotein receptor–related protein

The interaction between the endocytic receptor low density lipoprotein receptor–related protein (LRP) and either coagulation factor IX or its active derivative factor IXa was studied. Purified factor IX was unable to associate with LRP when analyzed by surface plasmon resonance. By contrast, factor XIa–mediated conversion of factor IX into factor IXa resulted in reversible dose- and calcium-dependent binding to LRP. Active-site blocking of factor IXa did not affect binding to LRP, whereas LRP binding was efficiently inhibited in the presence of heparin or antibodies against factor IX or LRP. The factor IXa–LRP interaction could be described by a 2-site binding model with equilibrium dissociation constants of 27 nmol/L and 69 nmol/L. Consistent with this model, it was observed that factor IXa binds to 2 different recombinant receptor fragments of LRP (denoted cluster II and cluster IV) with equilibrium dissociation constants of 227 nmol/L and 53 nmol/L, respectively. The amount of factor IXa degraded by LRP-deficient cells was 35% lower than by LRP-expressing cells, demonstrating that LRP contributes to the transport of factor IXa to the intracellular degradation pathway. Because ligand binding to LRP is often preceded by binding to proteoglycans, the contribution of proteoglycans to the catabolism of factor IXa was addressed by employing proteoglycan-deficient cells. Degradation of factor IXa by proteoglycan-deficient cells proceeded at a 83% lower rate than wild-type cells. In conclusion, the data presented here indicate that both LRP and proteoglycans have the potential to contribute to the catabolism of factor IXa.

Hydrophobic contact between the two epidermal growth factor-like domains of blood coagulation factor IX contributes to enzymatic activity

The three-dimensional structure of activated factor IX comprises multiple contacts between the two epidermal growth factor (EGF)-like domains. One of these is a salt bridge between Glu(78) and Arg(94), which is essential for binding of factor IXa to its cofactor factor VIII and for factor VIII-dependent factor X activation (Christophe, O. D., Lenting, P. J., Kolkman, J. A., Brownlee, G. G., and Mertens, K. (1998) J. Biol. Chem. 273, 222-227). We now addressed the putative hydrophobic contact at the interface between the EGF-like domains. Recombinant factor IX chimeras were constructed in which hydrophobic regions Phe(75)-Phe(77) and Lys(106)-Val(108) were replaced by the corresponding sites of factor X and factor VII. Activated factor IX/factor X chimeras were indistinguishable from normal factor IXa with respect to factor IXa enzymatic activity. In contrast, factor IXa(75-77)/factor VII displayed approximately 2-fold increased factor X activation in the presence of factor VIII, suggesting that residues 75-77 contribute to cofactor-dependent factor X activation. Activation of factor X by factor IX(106-108)/factor VII was strongly decreased, both in the absence and presence of factor VIII. Activity could be restored by simultaneous substitution of the hydrophobic sites in both EGF-like domains for factor VII residues. These data suggest that factor IXa enzymatic activity requires hydrophobic contact between the two EGF-like domains.

Factor XI Messenger RNA in Human Platelets

The bleeding diathesis associated with congenital deficiency of factor XI (FXI) is variable and correlates poorly with standard coagulation assays. Platelets are reported to contain FXI activity that may substitute for the plasma protein. The presence of this platelet activity in some patients deficient in plasma FXI could partly explain the variable bleeding associated with the deficiency state. Polyclonal antibodies to plasma FXI recognize a 220 kD platelet membrane protein distinct in structure from plasma FXI. The messenger RNA (mRNA) coding for this protein has been postulated to be an alternatively spliced FXI message lacking the fifth exon found in the liver (wild type) message. We analyzed RNA from platelets, leukocytes, and bone marrow for FXI mRNA by reverse transcription polymerase chain reaction (RT-PCR) technology. Single FXI mRNA species were identified by RT-PCR in platelet and bone marrow RNA, but not leukocyte RNA, that are the same size as the message from liver RNA. Sequencing of PCR products confirmed that the FXI mRNA species in platelets is identical to the one in liver. Wild-type FXI mRNA was also identified in three leukemia cell lines with megakaryocyte features (MEG-01, HEL 92.1.7, and CHRF-288-11). The data show that platelets contain wild-type FXI mRNA. FXI protein, therefore, may be present in platelets and may be released during platelet activation. The data do not support the premise that the 220 kD platelet protein that cross-reacts with FXI antibodies is a product of an alternatively spliced mRNA from the FXI gene.

Coagulation factor IXa: the relaxed conformation of Tyr99 blocks substrate binding

BACKGROUND

Among the S1 family of serine proteinases, the blood coagulation factor IXa (fIXa) is uniquely inefficient against synthetic peptide substrates. Mutagenesis studies show that a loop of residues at the S2-S4 substrate-binding cleft (the 99-loop) contributes to the low efficiency. The crystal structure of porcine fIXa in complex with the inhibitor D-Phe-Pro-Arg-chloromethylketone (PPACK) was unable to directly clarify the role of the 99-loop, as the doubly covalent inhibitor induced an active conformation of fIXa.

RESULTS

The crystal structure of a recombinant two-domain construct of human fIXa in complex with p-aminobenzamidine shows that the Tyr99 sidechain adopts an atypical conformation in the absence of substrate interactions. In this conformation, the hydroxyl group occupies the volume corresponding to the mainchain of a canonically bound substrate P2 residue. To accommodate substrate binding, Tyr99 must adopt a higher energy conformation that creates the S2 pocket and restricts the S4 pocket, as in fIXa-PPACK. The energy cost may contribute significantly to the poor K(M) values of fIXa for chromogenic substrates. In homologs, such as factor Xa and tissue plasminogen activator, the different conformation of the 99-loop leaves Tyr99 in low-energy conformations in both bound and unbound states.

CONCLUSIONS

Molecular recognition of substrates by fIXa seems to be determined by the action of the 99-loop on Tyr99. This is in contrast to other coagulation enzymes where, in general, the chemical nature of residue 99 determines molecular recognition in S2 and S3-S4. This dominant role on substrate interaction suggests that the 99-loop may be rearranged in the physiological fX activation complex of fIXa, fVIIIa, and fX.

Snake venoms and the hemostatic system

Snake venoms are complex mixtures containing many different biologically active proteins and peptides. A number of these proteins interact with components of the human hemostatic system. This review is focused on those venom constituents which affect the blood coagulation pathway, endothelial cells, and platelets. Only highly purified and well characterized snake venom proteins will be discussed in this review. Hemostatically active components are distributed widely in the venom of many different snake species, particularly from pit viper, viper and elapid venoms. The venom components can be grouped into a number of different categories depending on their hemostatic action. The following groups are discussed in this review: (i) enzymes that clot fibrinogen; (ii) enzymes that degrade fibrin(ogen); (iii) plasminogen activators; (iv) prothrombin activators; (v) factor V activators; (vi) factor X activators; (vii) anticoagulant activities including inhibitors of prothrombinase complex formation, inhibitors of thrombin, phospholipases, and protein C activators; (viii) enzymes with hemorrhagic activity; (ix) enzymes that degrade plasma serine proteinase inhibitors; (x) platelet aggregation inducers including direct acting enzymes, direct acting non-enzymatic components, and agents that require a cofactor; (xi) platelet aggregation inhibitors including: alpha-fibrinogenases, 5'-nucleotidases, phospholipases, and disintegrins. Although many snake venoms contain a number of hemostatically active components, it is safe to say that no single venom contains all the hemostatically active components described here. Several venom enzymes have been used clinically as anticoagulants and other venom components are being used in pre-clinical research to examine their possible therapeutic potential. The disintegrins are an interesting group of peptides that contain a cell adhesion recognition motif, Arg-Gly-Asp (RGD), in the carboxy-terminal half of their amino acid sequence. These agents act as fibrinogen receptor (integrin GPIIb/IIIa) antagonists. Since this integrin is believed to serve as the final common pathway leading to the formation of platelet-platelet bridges and platelet aggregation, blockage of this integrin leads to inhibition of platelet aggregation regardless of the stimulating agent. Clinical trials suggest that platelet GPIIb/IIIa blockade is an effective therapy for the thrombotic events and restenosis frequently accompanying cardiovascular and cerebrovascular disease. Therefore, because of their clinical poten tial, a large number of disintegrins have been isolated and characterized.

Dramatic enhancement of the catalytic activity of coagulation factor IXa by alcohols

The coagulation factor IXa (FIXa) exhibits a very weak proteolytic activity towards natural or synthetic substrates. Upon complex formation with its cofactor FVIIIa and Ca2+-mediated binding to phospholipid membranes, FIXa becomes a very potent activator of FX. The presence of FVIIIa has no effect on the cleavage of peptide substrates by FIXa, however. We found that several alcohols dramatically enhance the catalytic activity of human FIXa towards synthetic substrates. Substrates with the tripeptidyl moiety R-D-Xxx-Gly-Arg are especially susceptible to the enhanced FIXa catalysis. Maximal increase up to 20-fold has been measured in the presence of ethylene glycol. We suggest that alcohols modify the conformation of FIXa rendering the active-site cleft more easily accessible to tripeptide substrates with a hydrophobic residue in the P3-position.

Factor IX activation by factor XIa proceeds without release of a free intermediate

Factor IX activation by factor XIa is thought to proceed through the singly-cleaved free intermediate, factor IX alpha. However, we observed no intermediate development during factor IX activation by factor XIa when using a low substrate to enzyme ratio (44:1 mol/mol). This result can be explained by one of two mechanisms: (1) factor XIa-catalyzed activation proceeds via a singly-cleaved free intermediate with a much higher efficiency of cleavage than factor IX zymogen, or (2) the reaction occurs without free intermediate generation, whereby factor XIa makes both proteolytic cleavages in a single substrate molecule before releasing the final product (processive mechanism). We compared the factor XIa cleavage rates of free factor IX alpha and factor IXa alpha with that of factor IX zymogen. In contrast to the requirements of mechanism (1), the cleavage rate constants of factor IX zymogen, factor IX alpha, and factor IXa alpha were similar: 0.38 +/- 0.02 s(-1), 0.34 +/- 0.05 s(-1), and 0.27 +/- 0.01 s(-1), respectively. It seems likely that factor XIa-generated intermediates observed under some reaction conditions are produced through the occasional failure of a processive mechanism. Indeed, in reactions using a high substrate to enzyme ratio (1900:1 mol/mol), we observed some factor IX alpha development; however, the pattern of intermediate and product development over time was inconsistent with a mechanism involving an obligate intermediate. Rather, it corresponded to behavior expected from a processive mechanism undergoing a consistent low failure. We conclude that factor XIa-catalyzed activation of factor IX proceeds via a processive mechanism without release of a free intermediate.

Ca2+ binding to the first epidermal growth factor-like domain of human blood coagulation factor IX promotes enzyme activity and factor VIII light chain binding

Ca2+ binding to the first epidermal growth factor (EGF)-like domain of factor IX is known to be required for biological activity, but the mechanism by which Ca2+ contributes to factor IX function has remained unclear. We have studied recombinant factor IX mutants which lack Ca2+ binding to the first EGF-like domain, due to a replacement of Asp64 by Glu, Lys, or Val. The purified mutants (factors IX D64E, D64K, and D64V), were compared to plasma-derived and recombinant wild-type factor IX with regard to a number of metal-ion dependent functional parameters. In the presence of Mg2+, the activated mutants were indistinguishable from normal factor IXa in hydrolyzing the synthetic substrate CH3-SO2-Leu-Gly-Arg-p-nitroanilide. Replacing Mg2+ by Ca2+ further stimulated the activity of normal factor IXa but not of mutant factor IXa. In factor VIII-independent factor X activation, factor IXa D64K and D64E displayed reduced catalytic activity compared to normal factor IXa (apparent kcat/Km approximately 1, 2, and 4 x 10(3) M-1 s-1, respectively). In the presence of factor VIIIa, factor X activation rates by normal and mutant factor IXa were stimulated by factor VIIIa to a different extent ( approximately700- and 200-fold, respectively), indicating that Asp64 replacements affect the interaction with factor VIIIa. This possibility was addressed in inhibition studies employing synthetic peptides comprising the factor IXa-binding motifs of factor VIII heavy or light chains. Whereas the heavy chain peptide (Ser558-Gln565) inhibited factor VIII-dependent factor X activation by normal and mutant factor IXa with similar efficiency, the light chain peptide (Lys1804-Lys1818) inhibited normal factor IXa 2-3-fold more efficiently than did mutant factor IXa. This indicates that the reduced response to factor VIIIa may be due to impaired binding of mutant factor IXa to the factor VIII light chain. This was further explored in direct binding studies. In the presence of Mg2+, normal and mutant factor IXa were similar in binding to the factor VIII light chain. However, in the presence of Ca2+, factor IXa mutants were less efficient than normal factor IXa, which was illustrated by a 4-5-fold lower affinity than normal factor IXa for factor VIII light chain. Collectively, our data demonstrate that a number of factor IXa functions, including enzymatic activity and assembly into the factor IXa-factor VIIIa complex, are dependent on Ca2+ binding to the first EGF-like domain of factor IX.

A binding site expressed on the surface of activated human platelets is shared by factor X and prothrombin

We have demonstrated the presence of a saturable, reversible, and Ca(2+)-dependent binding site for 125I-labeled factor X ([125I]factor X) on human platelets (16000 +/- 2000 sites per platelet, Kd = 320 +/- 40 nM, n = 12) activated with either thrombin or the thrombin receptor agonist peptide, SFLLRN-amide, but not with ADP. Bound [125I]factor X could be completely removed by the addition of a Ca2+ chelator or an excess of unlabeled factor X. Antibodies that inhibit binding of factor X to the MAC-1 integrin receptor of monocytes and those directed against human factor V, failed to disrupt [125I]factor X binding to platelets. Prothrombin, but neither factor VII, factor IX, protein C, nor protein S, was an effective competitor of [125I]factor X binding with a K1 approximately Kd. [125I]Prothrombin also binds to activated (but not unactivated) platelets in a saturable, reversible, and Ca(2+)-dependent manner (20500 +/- 1500 sites, Kd = 470 +/- 110 nM, n = 3). Annexin V potently inhibited the binding of both [125I]factor X and [125I]prothrombin (IC50 approximately 3 nM). Factor X, prothrombin, and prothrombin fragment 1 (residues 1-155) were equipotent inhibitors of [125I]prothrombin and [125I]factor X binding, whereas Gla-domain-less factor X was unable to compete with [125I]factor X for platelet binding sites. Thus, it is the Gla-domains of factor X and prothrombin that appear to contain the regions necessary for platelet binding. The results of studies utilizing artificial phospholipid surfaces have led to the hypothesis that the substrates (FX and prothrombin) for the intrinsic pathway FXase and prothrombinase complexes are bound to the phospholipid surface. The factor X/prothrombin binding site we have described on the surface of activated platelets permits the utilization of surface-bound substrates by these complexes when they are assembled on a physiologic surface.