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HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
From the Departments of Pathology and Medicine,
Vanderbilt University, Nashville, TN; and the Departments of Medicine,
Biochemistry, and Thrombosis Research, Temple University, Philadelphia,
PA.
Human coagulation factor XI (FXI) is a plasma serine protease
composed of 2 identical 80-kd polypeptides connected by a disulfide bond. This dimeric structure is unique among blood coagulation enzymes.
The hypothesis was tested that dimeric conformation is required for
normal FXI function by generating a monomeric version of FXI (FXI/PKA4)
and comparing it to wild-type FXI in assays requiring factor IX
activation by activated FXI (FXIa). FXI/PKA4 was made by replacing the
FXI A4 domain with the A4 domain from prekallikrein (PK). A dimeric
version of FXI/PKA4 (FXI/PKA4-Gly326) was prepared as a control.
Activated FXI/PKA4 and FXI/PKA4-Gly326 activate factor IX with kinetic
parameters similar to those of FXIa. In kaolin-triggered plasma
clotting assays containing purified phospholipid, FXI/PKA4 and
FXI/PKA4-Gly326 have coagulant activity similar to FXI. The surface of
activated platelets is likely to be a physiologic site for reactions
involving FXI/FXIa. In competition binding assays FXI/PKA4,
FXI/PKA4-Gly326, and FXI have similar affinities for activated
platelets (Ki = 12-16 nM). In clotting assays in which
phospholipid is replaced by activated platelets, the dimeric proteins
FXI and FXI/PKA4-Gly326 promote coagulation similarly; however,
monomeric FXI/PKA4 has greatly reduced activity. Western immunoblot
analysis confirmed that activated monomeric FXI/PKA4 activates
factor IX poorly in the presence of activated platelets. These
findings demonstrate the importance of the dimeric state to FXI
activity and suggest a novel model for factor IX activation in which
FXIa binds to activated platelets by one chain of the dimer, while
binding to factor IX through the other.
(Blood. 2001;97:3117-3122) A paradigm in the field of blood coagulation is
that the zymogen of a plasma protease is activated by limited
proteolysis on a phospholipid surface, in the presence of a protein
cofactor and divalent cations.1-5 In vivo, appropriate
phospholipid surfaces are provided by activated platelets and cell
membranes of damaged tissues. Formation of surface-bound
protease-substrate complexes increases the rate of zymogen activation,
concentrates procoagulant reactions to sites of vessel injury, and
minimizes spread of thrombogenic proteases beyond wound sites. An
apparent exception to this model is the activation of factor IX by
activated factor XI (FXIa). In in vitro coagulation systems, such as
the activated partial thromboplastin time (aPTT) assay, activation of
factor IX by FXIa requires calcium ions.6-8 However,
phospholipids known to promote activation of factor X and prothrombin,
such as brain cephalin, have little effect on the
reaction.9,10 Furthermore, a candidate protein cofactor to
promote surface assembly of a FXIa-based factor IX-activating complex
has not been identified. These observations suggest,
counterintuitively, that factor IX activation by FXIa proceeds to a
significant extent in the fluid phase of blood.
Zymogen factor XI (FXI) and FXIa do bind to activated platelets in a
process that is saturable and reversible and that requires the protein
cofactor high molecular weight kininogen (HK) and zinc
ions.11,12 Evidence strongly suggests that the platelet surface is a physiologic environment for reactions involving FXI. When
bound to activated platelets, FXI activation by the proteases thrombin,
factor XIIa, and FXIa is greatly accelerated.13,14 Furthermore, prothrombin may substitute for HK as a cofactor for FXI/FXIa binding to platelets,13,15 providing an
explanation for the lack of excessive bleeding in patients congenitally
deficient in HK.16 Given these data and the observation
that factor IX binds to activated platelets,17 it is
likely that the surface of activated platelets is a physiologic
environment for activation of factor IX by FXIa.
The FXI polypeptide is composed of an N-terminal noncatalytic heavy
chain and a C-terminal trypsin-like catalytic light
chain.18,19 The heavy chain consists of 4 homologous
subunits called apple domains (designated A1 to A4, from the
N-terminus),19,20 a feature FXI shares with the plasma
protease prekallikrein (PK).21,22 Mapping studies
identified key amino acids in the A2 and A3 domains that are required
for normal binding to factor IX.23,24 Amino acids involved
in FXI binding to activated platelets have also been localized to the
A3 domain.15,25 The putative factor IX and
platelet-binding sites partially overlap, raising a question as to the
mechanism by which FXIa would bind simultaneously to its substrate and
a platelet surface.23,26 A unique feature of FXI structure
may offer a solution to this dilemma. The protein is a disulfide
bond-linked dimer comprised of 2 of the polypeptides described
above.18-20 In this study we describe the preparation and
characterization of a monomeric version of FXIa and demonstrate that
FXIa must be a dimer to properly promote coagulation in the presence of
activated platelets. The findings suggest a novel model for a factor IX
activation complex on platelets in which one heavy chain of FXIa binds
to the platelet, and the other binds to factor IX.
Preparation and activation of recombinant proteins
Plasma proteins
Gel filtration chromatography Protein (10-20 µg) in 100-200 µL of TBS underwent size-fractionation on a Superose-12 gel filtration column (Amersham Pharmacia Biotech, Piscataway, NJ) fitted to a BioLogic FPLC workstation (BioRad, Richmond, CA). The column was equilibrated with 50 mM sodium phosphate pH 7.3 and 150 mM NaCl. Fractions of eluate (500 µL) were collected. Retention times of proteins (determined by OD 280 nm) were compared to a series of protein standards. The identity of the eluted protein was confirmed by performing Western immunoblot analysis on column fractions (data not shown).Chromogenic substrate assays Chromogenic substrates S-2366 (L-pyroglutamyl-L-prolyl-L-arginine-P-nitroanaline) and S-2765 (N- -benzyoxycarbonyl-D-arginyl-glycyl-L-arginine-P-nitroanaline) were from DiaPharma (Westchester, OH).
Cleavage of S-2366 by FXIa. Activated proteins were diluted to 0.5 µg/mL in TBS with 0.1% bovine serum albumin (TBSA) containing 50-1000 µM S-2366, and change in absorbance at 405 nm was followed on a microtiter plate reader. Michaelis-Menten constants (Km and Vmax) were determined by standard methods, using the average of 2 separate experiments. Values for Vmax were converted to nM pNA/sec, using an extinction coefficient of 9800 optical density units (405 nm)/mole of pNA. Turnover number (kcat) was calculated from the ratio of Vmax to enzyme concentration. Activation of factor IX by FXIa. Activation of factor IX by activated proteases was evaluated as described.23 Briefly, enzyme (0.2 µg/mL) was incubated at 37°C for 1 minute with factor IX (0.05-1.0 µM) in TBSA containing 5 mM CaCl2. Activation was stopped by adding EDTA to 25 mM. The reaction was diluted 1:100 in TBSA, and 10 µL was added to 50 µL of a mixture of factor VIII (8 U/mL; Recombinate Baxter/Hyland, Glendale, CA), CaCl2 (10 mM), and rabbit brain cephalin, and to 30 µL factor X (450 nM). Incubations were at 37° C for 2.5 minutes, and then EDTA was added as above. Fifty microliters of each reaction was mixed with 50 µL 1.0 mM S-2765, and change in absorbance at 405 nm was followed on the microtiter plate reader. Results were compared to a control curve constructed with purified factor IXa. Michaelis-Menten constants were determined, using averages from 3 separate experiments. Rabbit brain cephalin was made from rabbit brain acetone extract (Sigma, St Louis, MO) by the method of Bell and Alton.32 Platelet-binding experiments Gel-filtered human platelets were prepared from fresh blood as previously described.33 Thrombin receptor agonist SFLLRN-amide was prepared at the Protein Chemistry Facility of the University of Pennsylvania.25 Binding experiments were performed by a modification of published methods.12 Briefly, platelets (108/mL) were activated by 5 µM SFFLRN-amide for 5 minutes at 37°C and then supplemented with ZnCl2 (25 µM), CaCl2 (2 mM), HK (50 nM), and 125I-labeled plasma-derived FXI (22 nM) either in the presence or absence of recombinant proteins. Incubation was continued for 30 minutes at 37°C. Aliquots (100 µL) were layered on Dow Corning methyl silicon oil (3 parts 550 density oil:2 parts 200 density oil) and platelets were separated from unbound protein by centrifugation in a microfuge. 125I-FXI bound to the platelet pellet was measured with a Wallace 1470 Wizard gamma counter. The concentration of cold protein that displaced 50% of bound 125I-FXI (IC50) was determined by plotting 125I-FXI bound to platelets against the concentration of competing ligand. Ki was calculated, using the equation IC50 = (1 + [S]/Kd) Ki, where S is the concentration of 125I-FXI (22 nM) and Kd is the binding constant for FXI determined by direct binding experiments (10 nM).Activity of recombinant proteins in plasma clotting assays Coagulant activities for zymogen FXI/PKA4 and FXI/PKA4-Gly326 were determined by aPTT assay. HEPES-Tyrode buffer pH 7.4 (100 µL) containing 1 nM FXI/PKA4 or FXI/PKA4-Gly326 was mixed with 50 µL FXI-deficient plasma. To this mixture was added 50 µL kaolin (5 mg/mL) in HEPES-Tyrode buffer pH 7.4 containing either phospholipid (inosithin 0.04%; Accurate Chemicals, Westbury, NY) or activated platelets (108/mL). Incubation was for 5 minutes at 37°C, followed by addition of 50 µL 50 mM CaCl2. Time to fibrin clot formation was determined on a fibrometer. All proteins were tested in triplicate and were compared to a standard curve prepared with wild-type FXI. One nanomolar wild-type FXI was assigned an activity of 1.00 (100%). FXIa and activated chimeric enzymes were tested in a similar manner, except that phospholipid or platelet suspensions did not contain kaolin, and incubation at 37°C was for 60 seconds prior to addition of CaCl2. A standard curve was prepared with wild-type FXIa (1 nM FXIa was assigned an activity of 1.00 or 100%).Western immunoblot analysis of factor IX activation by FXIa Human factor IX (150 nM) was incubated at 37°C with 1 nM wild-type FXIa or activated FXI/PKA4 in TBSA that contained 2 mM CaCl2. At various time points, 10 µL samples were removed into 5 µL SDS-sample buffer (500 mM Tris-HCl pH 6.8, 40% glycerol, 10% SDS). A second set of experiments was carried out under similar conditions, except that reactions included HK (50 nM), ZnCl2 (25 µM), and activated platelets (0.5 × 108/mL). Samples were size-fractionated on 12% polyacrylamide gels, followed by transfer to nitrocellulose membranes. Blots were developed with a goat antihuman factor IX polyclonal immunoglobulin G (Affinity Biologicals, Hamilton, Ontario, Canada), using an enhanced chemiluminescence Western blotting detection kit (Amersham Pharmacia Biotech).
Recombinant proteins Human FXI is a 160-kd disulfide bond-linked dimer comprised of 2 identical 80-kd polypeptides (Figure 1A, lanes 1 and 4).18,20 The A4 domain mediates dimer formation, with Cys321 in A4 forming the inter-chain disulfide bond.34 FXI in which Cys321 is replaced by alanine (FXI-Ala321, Figure 1A, lane 3) is an 80-kd protein on nonreducing SDS-PAGE; however, gel filtration experiments performed under conditions of physiologic salt concentration and pH demonstrate that it is the same size as plasma factor XI (Figure 1B). This suggests that the protein is a noncovalently associated dimer. These data confirm earlier work, demonstrating that an inter-chain disulfide bond is not required for dimer formation.34 In contrast, PK (Figure 1A, lane 5), which is structurally homologous to FXI,20,22 has a higher retention time on gel filtration (molecular mass ~90 kd; Figure 1), demonstrating it is a monomer.34
To prepare monomeric FXI, the A4 domain was replaced with PKA4. The resulting protein, FXI/PKA4, as expected, is an 80-kd protein on nonreducing SDS-PAGE (Figure 1C, lane 2). In gel filtration experiments, FXI/PKA4 has a similar retention time to PK, indicating it is a monomer (Figure 1D). FXI/PKA4 is expressed poorly by 293 fibroblasts (< 100 ng/mL conditioned media).27 This is consistent with data showing that mutations interfering with FXI intracellular dimerization result in poor protein expression.35,36 Dimeric FXI/PKA4 (FXI/PKA4-Gly326) was made by replacing Cys326 in FXI/PKA4 with glycine. Cys326 in PKA4 is normally paired with Cys321 to form an intra-chain disulfide bond. Its removal leaves Cys321 free to form an inter-chain disulfide-link with Cys321 on another polypeptide. FXI/PKA4-Gly326 is expressed by 293 cells at similar levels to wild-type FXI (data not shown), and the expressed protein is entirely dimeric (Figure 1C, lane 4, and 1D). Parenthetically, this suggests that elements in FXI distinct from the A4 domain are involved in promoting dimer formation. FXIa activity in purified protein assays Activated proteases were studied in 2 purified protein systems. In the first system, the capacities of the proteases to cleave the chromogenic substrate S-2366 were tested. Kinetic parameters for S-2366 cleavage are similar for all proteins tested (Table 1), indicating that the catalytic domains are intact. Kinetic parameters for activation of factor IX by recombinant proteases were determined by a 3-stage assay.23,27 The results demonstrate that activated FXI/PKA4 and FXI/PKA4-Gly326 activate factor IX similarly to FXIa (Table 1). This indicates that both chimeric molecules bind normally to, and have normal catalytic activity toward, factor IX when activation takes place in solution.
FXI binding to platelets FXI/PKA4 and FXI/PKA4-Gly326 were tested for their capacity to compete with 125I-labeled FXI for binding to activated platelets (Figure 2). Results were compared to those for plasma-derived FXI (positive control) and to FXI/PKA3 (negative control). FXI/PKA3, a chimera consisting of FXI with the A3 domain replaced by the PKA3 domain,27 binds poorly to platelets because PKA3 lacks critical amino acids required for platelet binding.15,25 The Ki for FXI/PKA4 (14 nM) and FXI/PKA4-Gly326 (16 nM) are similar to the value for plasma FXI (12 nM), indicating the 3 proteins bind to platelets with similar avidity. In contrast, the Ki for FXI/PKA3 is more than 500 nM, a result similar to reported values for PK binding to platelets.12
Activity of FXI and FXIa in plasma clotting assays In contact activation-initiated clotting assays, such as the aPTT, a negatively charged substance is used to initiate coagulation, and phospholipid is required for several enzymatic steps.3 Activated platelets may serve as a source of phospholipid. Clot formation in this type of assay depends on factor IX activation by FXIa. In aPTT assays using purified phospholipid (inosithin), FXI/PKA4 and FXI/PKA4-Gly326 correct the defect in FXI-deficient plasma similarly to wild-type FXI (Table 2, column 1). In contrast, when the phospholipids are replaced by activated platelets, only the dimeric protein FXI/PKA4-Gly326 shows significant activity (Table 2, column 2). In the presence of activated platelets, the activity of monomeric FXI/PKA4 is below the lower limit of detection of the assay.
These data indicate that activated FXI/PKA4 bound to platelets does not properly activate factor IX. Alternatively, zymogen FXI/PKA4 may not be activated well in this system. To distinguish between these possibilities, a modified clotting assay was performed in which coagulation is initiated by FXIa or activated chimera rather than by kaolin. Poor activation of FXI/PKA4 is not an issue in this case as the protease is added in the active form. Activated FXI/PKA4 and activated FXI/PKA4-Gly326 demonstrate significant activity in the presence of phospholipid (Table 2, column 3). However, consistent with the results obtained using zymogens in the aPTT assay, activated FXI/PKA4 has little activity in the presence of activated platelets compared to its dimeric counterpart FXI/PKA4-Gly326 (Table 2, column 4). Factor IX activation by FXIa in the presence of platelets During activation of factor IX (molecular mass 55 kd), an approximately 11-kd activation peptide is released to generate the active enzyme, factor IXa (45 kd).6-8 Therefore, the
activation of factor IX by FXIa can be directly observed by Western
immunoblot assay under nonreducing conditions. Wild-type FXIa and
activated FXI/PKA4 activate factor IX to factor IXa similarly in the
absence of platelets (Figure 3A). In
contrast, and consistent with the results of the clotting assays,
activation by activated FXI/PKA4 is significantly reduced compared to
wild-type FXIa and activated XI/PKA4-Gly326 when activated platelets
are included in the reaction (Figure 3B). It is not clear if the
relatively small amount of factor IXa generated by FXI/PKA4
represents enzyme activity on the platelet surface, or activation in
solution phase by FXI/PKA4 that has not bound to the platelet.
The formation of protease-substrate complexes on activated
platelets and damaged tissue is crucial for normal
hemostasis.1-5 In complexes involving vitamin K-dependent
proteases (prothrombin and factors VII, IX, and X), both protease and
substrate bind to phospholipid in an interaction involving the
N-terminal "Gla-domain" of each protein.2,3
Gla-domains contain 10 to 12 glutamic acid residues that undergo
post-translational modification by addition of a carboxyl group to the
Human FXI is a disulfide-bond linked homodimer,19,20 a unique feature among coagulation proteases. Meijers et al34 demonstrated that the A4 domain is involved in dimer formation,34 with Cys321 involved in the inter-chain disulfide bond.20,34 To test the significance of the dimeric state to FXI function, a FXI monomer is required for comparison. Taking advantage of the homology between FXI and PK (a monomeric protein), we generated the monomeric chimera FXI/PKA4. As in FXI, there is a cysteine at position 321 in PKA4; however, it is involved in an intra-chain bond with Cys326, a residue unique to PK. FXI/PKA4 is expressed poorly in 293 cells,27 consistent with work showing that dimerization is necessary for proper protein secretion.35,36 Indeed, Meijers et al34 postulated that FXI may be dimeric to facilitate intracellular processing and secretion. In our experiments with FXI/PK chimeras, arrangements placing the A3 domain of FXI in a monomeric protein result in poor expression (D.G., unpublished observation, November 1999). This suggests that FXIA3, in contrast to PKA3, prefers to be a component of a dimer. This is supported by the normal expression of FXI/PKA4-Gly326 in 293 cell culture. Despite lacking FXIA4, a free cysteine at Cys321 facilitates dimer formation (possibly driven by the A3 domain) and improves the poor expression seen with FXI/PKA4. We compared FXI/PKA4 to wild-type FXI and FXI/PKA4-Gly326 in a series
of assays requiring factor IX activation by FXIa. In the absence of
activated platelets (either in purified protein or plasma clotting
assays) the 3 molecules performed similarly. In contrast, when
activated platelets are used as a lipid source, monomeric FXI/PKA4
demonstrates a defect in factor IX activation. Several scenarios must
be considered as explanations for this observation. FXI/PKA4 may be
activated poorly on the platelet surface in an aPTT assay. However,
FXI/PKA4 has poor activity even when added to the assay in the
activated state. Similarly, it is difficult to invoke abnormalities in
FXI/PKA4 platelet binding because FXI/PKA4 and wild-type FXI have
similar affinities for platelets. A hypothesis that fits all
experimental data well is that monomeric FXIa is unable to interact
simultaneously with activated platelets and factor IX. The findings are
consistent with a scenario in which FXIa binds to a platelet by one
polypeptide of the dimer, while interacting with factor IX through the
other. A model based on this hypothesis is shown in Figure
4.
Several assumptions were made in preparing the model that require further testing and may, therefore, not be accurate. For example, it is likely that factor IX is bound to the platelet through its Gla-domain during activation, rather than being in solution as shown in Figure 4. In addition, it is not clear if one or both FXIa catalytic domains interact with the substrate. A study by Wolberg et al38 suggested that the 2 proteolytic cleavages made in factor IX during activation by FXIa may require both FXIa catalytic domains. Finally, for most reactions involving vitamin K-dependent coagulation proteases, phospholipid or activated platelets are both suitable surfaces. In contrast, our results strongly indicate that FXIa behaves differently in the presence of activated platelets compared to purified phospholipid. This suggests that FXIa may be interacting with a platelet membrane protein rather than the phospholipid component of the platelet membrane as shown in Figure 4. In this regard, preliminary data indicating that FXI binds to glycocalycin, the extramembrane portion of glycoprotein Ib, are of interest (P.N.W., unpublished observations, June 1999). Additional work will be required to validate this model; however, it offers a reasonable explanation as to why FXI, alone among coagulation proteases, is dimeric.
The authors are grateful to Dr George J. Broze Jr for his thoughtful reading of the manuscript and to Jean McClure for graphics work.
Submitted December 8, 2000; accepted January 22, 2001.
Supported by grants HL58837 and HL02917 (D.G.) and by grants HL46213, HL56153, and HL56914 (P.N.W.) from the National Heart, Lung, and Blood Institute. D.G. is an Established Investigator of the American Heart Association.
The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked "advertisement" in accordance with 18 U.S.C. section 1734.
Reprints: David Gailani, Division of Hematology/Oncology, Vanderbilt University, 538 MRB II, 2220 Pierce Ave, Nashville, TN 37232-6305; e-mail: dave.gailani{at}mcmail.vanderbilt.edu.
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© 2001 by The American Society of Hematology.
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  W. Wu, D. Sinha, S. Shikov, C. K. Yip, T. Walz, P. C. Billings, J. D. Lear, and P. N. Walsh Factor XI Homodimer Structure Is Essential for Normal Proteolytic Activation by Factor XIIa, Thrombin, and Factor XIa J. Biol. Chem., July 4, 2008; 283(27): 18655 - 18664. [Abstract] [Full Text] [PDF]
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S. B. Smith, I. M. Verhamme, M.-f. Sun, P. E. Bock, and D. Gailani Characterization of Novel Forms of Coagulation Factor XIa: INDEPENDENCE OF FACTOR XIa SUBUNITS IN FACTOR IX ACTIVATION J. Biol. Chem., March 14, 2008; 283(11): 6696 - 6705. [Abstract] [Full Text] [PDF]
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 D. Samuel, H. Cheng, P. W. Riley, A. A. Canutescu, C. Nagaswami, J. W. Weisel, Z. Bu, P. N. Walsh, and H. Roder Solution structure of the A4 domain of factor XI sheds light on the mechanism of zymogen activation PNAS, October 2, 2007; 104(40): 15693 - 15698. [Abstract] [Full Text] [PDF]
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T. Shi, B. Giannakopoulos, G. M. Iverson, K. A. Cockerill, M. D. Linnik, and S. A. Krilis Domain V of {beta}2-Glycoprotein I Binds Factor XI/XIa and Is Cleaved at Lys317-Thr318 J. Biol. Chem., January 14, 2005; 280(2): 907 - 912. [Abstract] [Full Text] [PDF]
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F. A. Baglia, D. Gailani, J. A. Lopez, and P. N. Walsh Identification of a Binding Site for Glycoprotein Ib{alpha} in the Apple 3 Domain of Factor XI J. Biol. Chem., October 29, 2004; 279(44): 45470 - 45476. [Abstract] [Full Text] [PDF]
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 U. Seligsohn A new mechanism for inherited factor XI deficiency Blood, July 1, 2004; 104(1): 7 - 8. [Full Text] [PDF]
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T. Shi, G. M. Iverson, J. C. Qi, K. A. Cockerill, M. D. Linnik, P. Konecny, and S. A. Krilis {beta}2-Glycoprotein I binds factor XI and inhibits its activation by thrombin and factor XIIa: Loss of inhibition by clipped {beta}2-glycoprotein I PNAS, March 16, 2004; 101(11): 3939 - 3944. [Abstract] [Full Text] [PDF]
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C. Leon, M. Alex, A. Klocke, E. Morgenstern, C. Moosbauer, A. Eckly, M. Spannagl, C. Gachet, and B. Engelmann Platelet ADP receptors contribute to the initiation of intravascular coagulation Blood, January 15, 2004; 103(2): 594 - 600. [Abstract] [Full Text] [PDF]
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T. H. Yun, F. A. Baglia, T. Myles, D. Navaneetham, J. A. Lopez, P. N. Walsh, and L. L. K. Leung Thrombin Activation of Factor XI on Activated Platelets Requires the Interaction of Factor XI and Platelet Glycoprotein Ib{alpha} with Thrombin Anion-binding Exosites I and II, Respectively J. Biol. Chem., November 28, 2003; 278(48): 48112 - 48119. [Abstract] [Full Text] [PDF]
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A. Aktimur, M. A. Gabriel, D. Gailani, and J. R. Toomey The Factor IX gamma -Carboxyglutamic Acid (Gla) Domain Is Involved in Interactions between Factor IX and Factor XIa J. Biol. Chem., February 28, 2003; 278(10): 7981 - 7987. [Abstract] [Full Text] [PDF]
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T. R. Baird and P. N. Walsh 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 J. Biol. Chem., October 4, 2002; 277(41): 38462 - 38467. [Abstract] [Full Text] [PDF]
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T. Renne, D. Gailani, J. C. M. Meijers, and W. Muller-Esterl Characterization of the H-kininogen-binding Site on Factor XI. A COMPARISON OF FACTOR XI AND PLASMA PREKALLIKREIN J. Biol. Chem., February 8, 2002; 277(7): 4892 - 4899. [Abstract] [Full Text] [PDF]
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Top
Abstract
Introduction
), FXI/PKA3 (
), FXI/PKA4 (
), and
FXI/PKA4-Gly326 (
). Data are mean values (± SEM) of total binding
results from 3 separate experiments, each performed in
triplicate.
-carbon.