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PubMed1985

Metal and phospholipid binding properties of partially carboxylated human prothrombin variants.

To study the specific role of gamma-carboxyglutamic acid (Gla) residues in prothrombin, we have isolated a series of partially carboxylated prothrombin variants from a patient with a hereditary defect in vitamin K-dependent carboxylation (Goldsmith, G. H., Pence, R. E., Ratnoff, O. D., Adelstein, D. A., and Furie, B. (1982) J. Clin. Invest. 69, 1253-1260). The three variant prothrombins, purified by DEAE-Sephacel, immunoaffinity chromatography, and preparative gel electrophoresis, were indistinguishable from prothrombin in molecular weight, amino acid composition, and NH2-terminal amino acid sequence, with the exception of Gla residues. Variant prothrombin 1, with 8 Gla residues, had 66% of the coagulant activity of prothrombin, one high affinity metal-binding site (Kd = 15 nM), and three lower affinity sites (Kd = 2.7 microM); prothrombin contained two high affinity (36 nM) and four lower affinity sites (Kd = 1 microM). Ca(II) induced a 23% decrease in the intrinsic fluorescence of variant prothrombin 1 fragment 1, compared to a 35% decrease in that of prothrombin fragment 1. The phospholipid binding activity of variant prothrombin 1 was 44% that of prothrombin. Variant prothrombin 2 and variant prothrombin 3, with 4 and 6 Gla residues, respectively, had about 5% of prothrombin coagulant activity and a single high affinity and two lower affinity metal-binding sites and exhibited no phospholipid binding activity. Variant prothrombin 3 fragment 1 and variant prothrombin 2 fragment 1 demonstrated 18 and 13% of Ca(II)-induced fluorescence quenching, respectively. Abnormal prothrombin, with 1 Gla residue, had 8% of prothrombin coagulant activity, a single lower affinity (1 microM) metal-binding site, and 13% Ca(II)-induced fluorescence quenching of the fragment 1 species and did not bind to phospholipid. These results indicate that Gla residues define the metal binding properties of prothrombin. Most, if not all, of the Gla residues are required for complete prothrombin function, and the prothrombin coagulant activity correlates to the phospholipid binding activity of the prothrombin species.

Amino Acid Sequence↗
PubMed1976

Characteristics of the association between prothrombin fragment 2 and alpha-thrombin.

The esterolytic activity of bovine alpha-thrombin on the synthetic substrate N-alpha-p-tosyl-L-arninine methyl ester (TosArgOMe) is stimulated when the prothrombin activation fragment, prothrombin fragment 2, is added as previously reported by this laboratory (Heldebrant, C. M., and Mann, K. G. (1973), J. Biol. Chem. 248, 3642). A similar stimulation of beta-thrombin is observed upon addition of prothrombin fragment 2. The binding constant of prothrombin fragment 2 to alpha-thrombin has been determined by the method of Gutfreund ((1972), Enzymes, Physical Principles, Wiley, New York, N.Y., pp 67-71). The dissociation constant is 7.7 X 10(-10)M, and there is one molecule of prothrombin fragment 2 bound per molecule of alpha-thrombin. Prethrombin-2 competes for prothrombin fragment 2, so the enhancement of the esterolytic activity of alpha-thrombin by prothrombin fragment 2 was used as a probe to determine the dissociation constant for the binding of prothrombin fragment 2 to prethrombin 2. The dissociation constant for this association is 1.3 X 10(-10)M. The kinetic parameters for the reaction of alpha-thrombin on TosArgOMe were determined in the absence and presence of prothrombin fragment 2 and are as follows: (a) in the absence of prothrombin fragment 2, Km(app) = 1.92 X 10(-4)M, and k3(app) = 35.8 mol of TosArgOMe/mol of alpha-thrombin s(-1); (b) in the presence of prothrombin fragment 2,Km(app = 1.76 X 10(-4)M, and k3(app) = 60.5 mol of TosArgOMe/mol of alpha-thrombin s(-1). Thus, the stimulatory effect of bovine prothrombin fragment 2 on bovine alpha-thrombin is reflected in k3(app) and not in Km(app). In contrast to the stimulatory effect of prothrombin fragment 2 on the thrombin-catalyzed hydrolysis of TosArgOMe, it inhibits the activity of alpha-thrombin toward N-alpha-benzoyl-L-arginine ethyl ester and N-alpha-benzoyl-L-arginine p-nitroanilide. The inhibition of activity toward these substrates by prothrombin fragment 2 is also reflected in k3(app). Activity toward the nonspecific substrate p-nitrophenyl butyrate was completely inhibited by the addition of prothrombin fragment 2. Prothrombin fragment 2 has no effect on the inhibition of alpha-thrombin activity by the active-site serine inhibitors diisopropyl phosphofluoridate, phenylmethanesulfonyl fluoride, or p-nitrophenyl guanidinobenzoate. Inhibition by the active-site-histidine-modifying inhibitor, N-alpha-p-tosyl-L-arginine chloromethyl ketone, was enhanced by the addition of prothrombin fragment 2. Soybean trypsin inhibitor reduces the stimulation by prothrombin fragment 2, but only at high molar ratios. Prothrombin fragment 2 has no effect on the clotting activity of alpha-thrombin, nor inhibition of this activity by heparin, hirudin, or diisopropyl phosphafluoridate. Bovine prothrombin fragment 2 enhances the esterolytic activity of both human and bovine alpha-thrombin, but human prothrombin fragment 2 does not enhance the esterolytic activity of either human or bovine alpha-thrombin.

Animals↗
PubMed1997

Different anticoagulant and immunological properties of anti-prothrombin antibodies in patients with antiphospholipid antibodies.

Lupus anticoagulant (LA) antibodies are acquired inhibitors of coagulation belonging-together with anticardiolipid (aCL) antibodies-to the family of antiphospholipid antibodies. Since LA antibodies affect coagulation reactions via recognition of the complex of lipid-bound prothrombin, they may be better named anti-prothrombin antibodies. We studied their immunological properties in the plasma of 59 patients with antiphospholipid antibodies by means of specific ELISA systems that allowed the characterization of the interaction of these antibodies with human prothrombin and anionic phospholipids. The mode of presentation of prothrombin was found to greatly influence the reactivity of anti-prothrombin antibodies. In fact, when plain polystyrene plates were used to immobilize prothrombin, virtually no binding was observed. Conversely, when prothrombin was coated on high-activated PVC ELISA plates, 34 samples (58%) contained antibodies that recognize human prothrombin in solid phase. In particular, IgG antibodies were found in 21 plasmas and IgM in 22; both IgG and IgM isotypes were present in 9 of these cases. A higher prevalence was observed in the ELISA for the detection of the antibodies directed at the calcium-mediated complex of phosphatidylserine (PS)-bound prothrombin: 53 samples (90%), preadsorbed with cardiolipin liposomes to remove aCL antibodies, showed the presence of IgG and/or IgM anti-prothrombin antibodies. When the results were analyzed according to the immunoglobulin isotypes, 44 (75%) and 39 (66%) samples were found to contain IgG and IgM anti-prothrombin antibodies, respectively. Both IgG and IgM were present in the plasma of 30 patients. Only half of these samples reacted also with PVC-bound prothrombin. Apparently, the higher rate of positivity of the ELISA for the detection of antibodies to the complex of PS-bound prothrombin was not due to differences in the amount of antigen available in the 2 systems, as judged by binding experiments performed with a rabbit polyclonal anti-human prothrombin antiserum. Finally, the anticoagulant properties of 14 total IgG preparations (12 of them contained anti-prothrombin antibodies positive in both ELISA systems, whereas the other 2 cases reacted either with PVC-bound prothrombin only or with PS-bound prothrombin only) were evaluated by diluted Russell's Viper Venom Time and by diluted activated Partial Thromboplastin Time. To rule out the beta 2-glycoprotein I (beta 2-GPI)-dependent anticoagulant effect of the aCL antibodies contained in the preparations, the coagulation tests were performed in beta 2-GPI deficient plasma. Six preparations failed to show anticoagulant activity in both assay systems, suggesting that 2 types of IgG anti-prothrombin antibodies exist, that differ with respect to their anticoagulant properties. These findings suggest that anti-prothrombin antibodies resemble aCL antibodies with respect to the behaviour in "in vitro" coagulation reactions and underline the wide heterogeneity of antiphospholipid antibodies.

Adult↗
PubMed1983

Immunoassays of human prothrombin species which correlate with functional coagulant activities.

Specific immunoassays have been developed for forms of human prothrombin that vary in their degree of carboxylation. Human abnormal (des-gamma-carboxyl) prothrombin was isolated in 18% yield from the plasma of a patient treated with warfarin. The purified protein migrated as a single band in electrophoresis and contained an average of three gamma-carboxyglutamic acid residues per molecule. A specific antibody subpopulation was isolated from rabbit anti-abnormal prothrombin antiserum by using affinity chromatography. These antibodies, which bound to abnormal prothrombin but which cross-reacted minimally with prothrombin, were used to establish an immunoassay specific for abnormal prothrombin. In parallel, a specific antibody subpopulation, anti-prothrombin: Ca(II), was isolated from rabbit anti-prothrombin antiserum by conformation perturbation affinity chromatography. This antibody, which bound prothrombin but minimally cross-reacted with abnormal prothrombin, was used to establish a specific immunoassay for native prothrombin. An anti-prethrombin 1 subpopulation bound abnormal prothrombin and prothrombin equivalently and was used for an immunoassay that measured total prothrombin. These assays permit the quantitation of abnormal prothrombin and prothrombin in plasma and serum. The level of native prothrombin antigen correlates precisely with the functional prothrombin activity. These assays provide an example of the use of specific antibodies against functionally important antigenic surfaces to monitor properties of coagulation proteins with the precision and reliability of immunoassy.

Electrophoresis, Polyacrylamide Gel↗
PubMed2003

Venous thromboembolism, oral contraceptives and high prothrombin levels.

The G20210A prothrombin mutation, associated with elevated prothrombin levels, is a risk factor for venous thromboembolism (VTE) and displays a strong interaction with oral contraceptives (OC). No data are available on VTE risk of OC use in women with high prothrombin levels, either associated or not with the mutation. The aim of this study was to evaluate the risk of VTE in OC users with high prothrombin levels, either including or excluding carriers of the prothrombin mutation. Prothrombin levels were measured by a chromogenic assay in 152 women who suffered from VTE in reproductive age and in 296 healthy women. Subjects carrying thrombophilic alterations other than the G20210A prothrombin mutation were excluded. Prothrombin levels were stratified into quartiles. The OR of subjects in the upper quartile were 3.10 [95% confidence interval (CI) 1.73-5.55] and 2.07 (95% CI 1.11-3.85) in all women and in those not carrying the prothrombin mutation, respectively. Among the 152 patients, 88 had experienced VTE during OC; in the control group we considered as OC users the women who had used OC for at least 6 months in the 2 years before presentation but had stopped the treatment at least 3 months before the time of blood sampling (n = 127). For the interaction between OC and prothrombin levels only the two extreme strata of prothrombin were considered. Women with the lowest prothrombin levels and who did not use OC were used as reference category. The VTE risk of using OC in subjects with prothrombin levels in the upper quartile was increased 5.4-fold (95% CI 2.38-12.3) and 3.5-fold (95% CI 1.48-8.22) in all women and in those not carrying the prothrombin mutation, respectively. We conclude that elevated prothrombin levels, even in women without the G20210A prothrombin mutation, are associated with an increased risk for venous thromboembolism and that oral contraceptive use potentiates such association.

Adolescent↗
PubMed2006

Prothrombin A19911G polymorphism and the risk of venous thromboembolism.

BACKGROUND: The A > G polymorphism at position 19911 of the prothrombin gene is associated with increased plasma prothrombin levels but its role as a risk factor for venous thromboembolism (VTE) is not established. OBJECTIVE: To investigate the role of prothrombin 19911 A > G polymorphism in the risk of VTE in patients with heterozygous prothrombin 20210GA or factor (F) V Leiden and in those without thrombophilia. PATIENTS AND METHODS: Case-control study of 793 patients with prothrombin 20210 GA (n = 167) or FV Leiden (n = 198), and without thrombophilia (n = 428), and of 795 healthy individuals with the corresponding coagulation profile, investigated for the presence of prothrombin 19911 A > G. Plasma prothrombin levels were measured in 342 individuals. RESULTS: Prothrombin 19911 A > G did not increase the risk of VTE in carriers of prothrombin 20210 GA [odds ratio (OR) 1.2, 95% CI (95% CI) 0.8-1.8] but significantly increased the risk in carriers of FV Leiden (OR 2.1, 95% CI 1.3-3.4) and in patients without thrombophilia (OR 1.5, 95% CI 1.0-2.2). Higher plasma prothrombin levels in carriers of prothrombin 19911 A > G polymorphism than in non-carriers were found among individuals without thrombophilia (P =0.05) and with FV Leiden (P = 0.07), but not in carriers of prothrombin 20210 GA (P = 0.2). CONCLUSIONS: Prothrombin 19911 A > G polymorphism was independently associated with a 1.5-fold increased risk of VTE and increased 2-fold the risk of VTE associated with FV Leiden, both increases statistically significant. No effect was observed in carriers of prothrombin 20210 GA, perhaps because this polymorphism has a stronger influence on plasma prothrombin levels than the prothrombin 19911 polymorphism.

Adenine↗
PubMed1987

Expression of completely gamma-carboxylated recombinant human prothrombin.

Human prothrombin cDNA has been expressed in mammalian cells to yield biologically active, fully gamma-carboxylated prothrombin. A 2.0-kilobase cDNA encoding full-length prothrombin was isolated from a human fetal liver library using a cDNA fragment recovered from a lambda gt11 human hepatoma expression library. Prothrombin cDNA was cloned into a mammalian expression vector and transfected into Chinese hamster ovary cells. Selection for expression of dihydrofolate reductase yielded cell lines secreting up to 0.55 microgram/ml of prothrombin. Recombinant prothrombin synthesized in the presence of vitamin K was quantitatively recovered from tissue culture medium by affinity chromatography using conformation-specific antibodies directed against the metal-stabilized, gamma-carboxylated conformer. The purified material migrated as a single band on denaturing polyacrylamide gels with an electrophoretic mobility equivalent to that of plasma-derived human prothrombin. Automated Edman degradation of recombinant prothrombin revealed a single amino-terminal sequence identical to that of plasma-derived prothrombin. Recombinant and plasma-derived prothrombin interacted similarly with antibodies specific for total prothrombin, abnormal des-gamma-carboxyprothrombin, and two metal-stabilized conformers of prothrombin. Recombinant prothrombin exhibited a specific coagulant activity equivalent to that of plasma-derived prothrombin. The gamma-carboxyglutamic acid analysis of recombinant prothrombin demonstrated 9.9 +/- 0.4 mol of gamma-carboxyglutamic acid/mol of prothrombin. These results represent the first description of the expression of a recombinant vitamin K-dependent protein in which all of the expressed protein is gamma-carboxylated.

1-Carboxyglutamic Acid↗
PubMed1991

Prenatal and postnatal expression of mRNA coding for rat prothrombin.

The levels of prothrombin mRNA in prenatal and postnatal rat tissues were analyzed in order to determine tissue distribution of prothrombin expression and to determine if increases in liver prothrombin mRNA during development correlated with previously documented developmental increases in plasma prothrombin levels. Maternal tissues were also analyzed in order to determine if prothrombin mRNA levels varied due to gestational or postpartum influences. Northern analysis demonstrated that rat liver prothrombin mRNA levels increased several-fold late in gestation and reached maximal levels by 13 days after birth. Prothrombin mRNA was also expressed in diaphragm, stomach, intestine, kidney, spleen and adrenal tissues during development. In maternal tissues during pregnancy, prothrombin mRNA was expressed in liver, diaphragm, stomach, uterus and placenta. Prothrombin mRNA levels in each of these tissues that were positive by Northern analysis were quantitated by solution hybridization analysis. Between gestational day 18 and postnatal day 13, liver prothrombin mRNA levels increased from approx. 600 to 2100 molecules per cell (a 3.5-fold increase). In maternal liver during pregnancy, between day 18 and day 22, prothrombin mRNA levels increased from approx. 1800 to 2100 molecules per cell. Immediately after delivery, maternal liver prothrombin mRNA levels decreased to approx. 50% of preparturition levels. Prothrombin mRNA levels in placental tissue ranged from approx. 100 to 250 molecules per cell. In other fetal, postnatal and maternal tissues, prothrombin mRNA expression was less than 100 molecules per cell. These results demonstrate that the level and tissue-type expression of prothrombin mRNA varies in response to prenatal and postnatal influences.

Aging↗
PubMed1985

Acquired hypoprothrombinemia due to non-neutralizing antibodies to prothrombin: mechanism and management.

A patient developed bleeding due to an acute acquired specific prothrombin deficiency. Unlike previously described patients, this patient had no evidence of an associated lupus anticoagulant. Prothrombin activity and antigen were decreased concordantly and the patient's plasma did not neutralize the activity of added prothrombin or interfere with its measurement by electroimmunoassay. Nevertheless, immunoelectrophoresis and experiments using 125I-prothrombin revealed a prothrombin-binding antibody. The residual prothrombin in the patient's plasma was in the form of a prothrombin-antibody complex. Administration of adrenal corticosteroids was associated with a rise in prothrombin activity and cessation of bleeding, but circulating prothrombin was still bound to the antibody. This suggests that non-neutralizing antibodies to prothrombin cause plasma prothrombin deficiency because of a rapid clearance of prothrombin-antibody complexes, which is slowed by adrenal corticosteroids. The antibody had a relatively low affinity for prothrombin (Kd 5 to 8 X 10(-7)) and was transient. It is possible, therefore, that the antibody arose not to prothrombin itself, but to an antigen sharing an epitope with prothrombin.

Aged↗
PubMed1994

Functional and binding studies of the roles of prothrombin and beta 2-glycoprotein I in the expression of lupus anticoagulant activity.

We present functional and binding data relevant to the reported roles for prothrombin and beta 2-glycoprotein I (beta 2GPI) in the expression of lupus anticoagulant activity. In a purified system containing human prothrombin, Xa, Va, and a rate-limiting concentration of phosphatidylserine (PS)/phosphatidylcholine (PC) vesicles, the preliminary incubation of vesicles with protein A separated IgG preparations from 10 lupus anticoagulant plasmas, calcium, and prothrombin enhanced the inhibitory effect of all IgG preparations upon thrombin generation. Experiments in a purified factor X activation system provided supporting data that a similar preliminary incubation with prothrombin enhanced the inhibitory effect of many of the IgG preparations upon factor X activation. However, we could not obtain unequivocal evidence that prothrombin was an obligatory cofactor for lupus anticoagulant IgG to inhibit procoagulant phospholipid function, because lupus anticoagulant IgG separated by protein A chromatography contained traces of prothrombin. The binding of many IgG preparations to immobilized PS was enhanced by prothrombin when calcium ions were present. beta 2GPI enhanced binding of many of the IgG preparations to immobilized PS both in the presence and absence of calcium, yet beta 2GPI failed to enhance the ability of the IgG preparations to inhibit phospholipid function in purified prothrombin and factor X assays. Moreover, the IgG preparations prolonged the dilute Russell's viper venom time (dRVVT) of beta 2GPI-depleted normal plasma. Nine of 10 IgG preparations bound to prothrombin on Western blots in the absence of calcium and phospholipid, whereas no preparation bound to beta 2GPI. Passage of five citrated lupus anticoagulant plasmas through a prothrombin affinity column in the absence of added calcium and phospholipid removed most of the activity prolonging the dRVVT of normal plasma, and IgG in the pass-through plasma no longer bound to PS in the presence of prothrombin and calcium ions. IgG in prothrombin column eluates had strikingly enhanced specific lupus anticoagulant activity and also specific PS binding activity in the presence of prothrombin and calcium ions. Thus, lupus anticoagulant plasmas were shown to contain IgG binding to prothrombin, in the absence of calcium ions and phospholipid, which could also, in the presence of calcium ions and prothrombin, bind to PS and express lupus anticoagulant activity.

Adult↗
PubMed1998

A kinetic assay to determine prothrombin binding to membranes.

Activation of prothrombin by multisquamase, the prothrombin activator from the venom of Echis multisquamatus (Central Asian sand viper), is inhibited by membranes containing negatively charged anionic phospholipids. This inhibition appears to be due to the fact that the venom activator cannot activate membrane-bound prothrombin. Initial steady state rates of prothrombin activation by multisquamase in the presence of phospholipids appeared to depend on the fraction unbound prothrombin only and this phenomenon was used to quantitate binding of prothrombin to membranes of varying phospholipid composition. In this method, the initial rate of prothrombin activation by multisquamase is measured in the absence (total prothrombin) and in the presence of a procoagulant surface (rate depending only on free prothrombin) and from the difference in activation rates the amount of membrane-bound prothrombin is calculated. The validity of the method was established by determination of the binding parameters for prothrombin binding to 100 microM phospholipid vesicles composed of 20 mole% phosphatidylserine and 80 mole% phosphatidylcholine. The binding parameters obtained were Kd=0.84 microM and n=0.021 micromoles prothrombin bound per micromole phospholipid which is in agreement with literature. Due to the nature of the measurement the method is especially suitable to quantitate binding of prothrombin at concentrations as low as 5 nM prothrombin.

Animals↗
PubMed1988

Calcium-independent activation of prothrombin on membranes with positively charged lipids.

The activation of prothrombin by factor Xa is strongly accelerated by negatively charged phospholipids plus calcium ions. In this paper we report that positively charged membranes can also stimulate prothrombin activation provided that the activation reaction is carried out in the absence of calcium ions. Membranes composed of a mixture of phosphatidylcholine (PC) and positively charged lipids like stearylamine, sphingosine, or hexadecyltrimethylammonium bromide caused a more than 1000-fold increase of the rate of prothrombin activation. Prothrombin activation by the factor Xa-factor Va complex was also considerably stimulated by such membranes. Stimulation of prothrombin activation by positively charged membranes was suppressed at high ionic strength. This suggests that electrostatic attraction of negatively charged proteins by positively charged membranes is the major driving force in the association of prothrombin and factor Xa with the lipid surface. Calcium ions strongly inhibited prothrombin activation on vesicles composed of PC and stearylamine (80/20 M/M), which indicates that the regions of prothrombin and/or factor Xa containing gamma-carboxyglutamic acid (gla) are important for the interaction of these proteins with positively charged membranes. The importance of the gla domain was confirmed by the observation that PC/stearylamine vesicles had much less effect on the reactions between proteins that lack gla residues [gla-domainless (des-1-45) prothrombin, prethrombin 1, prethrombin 2, or gla-domainless (des-1-44) factor Xa]. The efficiency of prothrombin and prothrombin derivatives to act as substrate decreased in the order prothrombin greater than des-1-45-prothrombin = prethrombin 1 greater than prethrombin 2, while prothrombin activation by gla-domainless (des-1-44) factor Xa was hardly stimulated by positively charged membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

1-Carboxyglutamic Acid↗
PubMed2000

A binding site for the kringle II domain of prothrombin in the apple 1 domain of factor XI.

Previously we defined binding sites for high molecular weight kininogen (HK) and thrombin in the Apple 1 (A1) domain of factor XI (FXI). Since prothrombin (and Ca(2+)) can bind FXI and can substitute for HK (and Zn(2+)) as a cofactor for FXI binding to platelets, we have attempted to identify a prothrombin-binding site in FXI. The recombinant A1 domain (rA1, Glu(1)-Ser(90)) inhibited the saturable, specific and reversible binding of prothrombin to FXI, whereas neither the rA2 domain (Ser(90)-Ala(181)), rA3 domain (Ala(181)-Val(271)), nor rA4 domain (Phe(272)-Glu(361)) inhibited prothrombin binding to FXI. Kinetic binding studies using surface plasmon resonance showed binding of FXI (K(d) approximately 71 nm) and the rA1 domain (K(d) approximately 239 nm) but not rA2, rA3, or rA4 to immobilized prothrombin. Reciprocal binding studies revealed that synthetic peptides (encompassing residues Ala(45)-Ser(86)) containing both HK- and thrombin-binding sites, inhibit (125)I-rA1 (Glu(1)-Ser(90)) binding to prothrombin, (125)I-prothrombin binding to FXI, and (125)I-prothrombin fragment 2 (Ser(156)-Arg(271)) binding to FXI. However, homologous prekallikrein-derived peptides (encompassing Pro(45)-Gly(86)) did not inhibit FXI rA1 binding to prothrombin. The peptides Ala(45)-Arg(54), Phe(56)-Val(71), and Asp(72)-Ser(86), derived from sequences of the A1 domain of FXI, acted synergistically to inhibit (125)I-rA1 binding to prothrombin. Mutant rA1 peptides (V64A and I77A), which did not inhibit FXI binding to HK, retained full capacity to inhibit rA1 domain binding to prothrombin, and mutant rA1 peptides Ala(45)-Ala(54) (D51A) and Val(59)-Arg(70) (E66A), which did not inhibit FXI binding to thrombin, retained full capacity to inhibit rA1 domain binding to prothrombin. Thus, these experiments demonstrate that a prothrombin binding site exists in the A1 domain of FXI spanning residues Ala(45)-Ser(86) that is contiguous with but separate and distinct from the HK- and thrombin-binding sites and that this interaction occurs through the kringle II domain of prothrombin.

Binding Sites↗