The role of human factor X activation peptide in activation of factor X by factor IXa

Identification of a modified coagulation factor X with enhanced activation properties as potential hemostatic agent

Hemophilia A and hemophilia B are X-linked inherited bleeding disorders caused by a deficiency of coagulation factor VIII and IX, respectively. Standard of care is prophylactic factor replacement therapy; however, the development of neutralizing antibodies against these factors represents serious complications underlining the need for alternative treatment approaches. Human coagulation factor X has a central role within the blood coagulation system making it an attractive target for the development of alternative treatment strategies for patients with hemophilia. This study focuses on a modified variant of the human coagulation factor X with enhanced hemostatic bypass activity due to insertion of a factor IX derived activation sequence. This molecule design leads to the direct activation of the modified factor X protein by factor XIa allowing it to bypass the need for coagulation factor VIIIa/factor IXa. The modified variant was able to correct in-vitro activated partial prothrombin time of human and murine factor VIII/factor IX deficient plasma. Furthermore, reduced blood loss in factor VIII knock-out mice was observed after intravenous application of the modified factor X variant. In conclusion, these data suggest that the factor X variant described here could potentially serve as a bypassing agent independent of the inhibitor status of hemophilia patients. However, more research is needed to further investigate the potential of this molecule.

Evolutionary Adaptations in Pseudonaja Textilis Venom Factor X Induce Zymogen Activity and Resistance to the Intrinsic Tenase Complex

The venom of the Australian snake Pseudonaja textilis comprises powerful prothrombin activators consisting of factor X (v-ptFX)- and factor V-like proteins. While all vertebrate liver-expressed factor X (FX) homologs, including that of P. textilis, comprise an activation peptide of approximately 45 to 65 residues, the activation peptide of v-ptFX is significantly shortened to 27 residues. In this study, we demonstrate that exchanging the human FX activation peptide for the snake venom ortholog impedes proteolytic cleavage by the intrinsic factor VIIIa-factor IXa tenase complex. Furthermore, our findings indicate that the human FX activation peptide comprises an essential binding site for the intrinsic tenase complex. Conversely, incorporation of FX into the extrinsic tissue factor-factor VIIa tenase complex is completely dependent on exosite-mediated interactions. Remarkably, the shortened activation peptide allows for factor V-dependent prothrombin conversion while in the zymogen state. This indicates that the active site of FX molecules comprising the v-ptFX activation peptide partially matures upon assembly into a premature prothrombinase complex. Taken together, the shortened activation peptide is one of the remarkable characteristics of v-ptFX that has been modified from its original form, thereby transforming FX into a powerful procoagulant protein. Moreover, these results shed new light on the structural requirements for serine protease activation and indicate that catalytic activity can be obtained without formation of the characteristic Ile¹⁶-Asp¹⁹⁴ salt bridge via modification of the activation peptide.

Anti-beta2-glycoprotein I (beta2GPI) monoclonal antibodies with lupus anticoagulant-like activity enhance the beta2GPI binding to phospholipids

beta2-Glycoprotein I (beta2GPI), a plasma glycoprotein with phospholipid-binding property, is known to be the actual target antigen for autoimmune type anticardiolipin antibodies (aCLs). Certain groups of aCLs (anti-beta2GPI antibodies) exert lupus anticoagulant (LA) activity and perturb the function of vascular endothelial cells. This investigation aimed at highlighting some insights into the molecular basis by which aCLs exert their biological effects by using anti-beta2GPI mAbs with well-characterized epitopes from mice and from patients with antiphospholipid syndrome. Anti-beta2GPI mAbs directed against the third domain (Cof-20 and Cof-22) and fourth domain (Cof-21, EY1C8, and EY2C9) of beta2GPI inhibited the thrombin generation induced by Russell's viper venom in diluted plasma and that induced by the prothrombinase complex reconstituted with purified clotting factors. This anticoagulant activity was abrogated in the presence of an excess amount of phospholipids, thus resembling the LA activity. In stark contrast, anti-beta2GPI mAbs directed against the fifth domain and the carboxy-terminal region of the fourth domain showed no LA-like activity. These findings suggest that the LA activity of anti-beta2GPI antibodies depends on their epitope specificity. Experiments carried out to clarify the mechanism of the LA activity showed that anti-beta2GPI mAbs with LA-like activity, but not those without this effect, enhance the beta2GPI binding to phospholipids. In addition, the F(ab')2 fragment, but not the Fab' fragment, of the anti-beta2GPI mAbs was found to enhance the LA activity and the beta2GPI binding to phospholipids, suggesting that anti-beta2GPI antibodies induce formation of multiple complexes of beta2GPI on the surface of phospholipids because of their bivalent property. This clustering of beta2GPI molecules induced by anti-beta2GPI antibodies, probably because of their multivalent property and epitope specificity, might hinder the lateral mobility and activation of clotting factors on the surface of phospholipids and thus exert LA activity. Clustering of beta2GPI molecules may also explain the molecular mechanism by which anti-beta2GPI antibodies alter the function of leukocytes and endothelial cells. The well-documented heterogeneous LA activity of aCLs (anti-beta2GPI antibodies) may also be explained by their epitope specificity.

Role of the activation peptide domain in human factor X activation by the extrinsic Xase complex

The activation of factor X by the extrinsic coagulation system results from the action of an enzyme complex composed of factor VIIa bound to tissue factor on phospholipid membranes in the presence of calcium ions (extrinsic Xase complex). Proteolysis at the Arg52-Ile53 peptide bond in the heavy chain of factor X leads to the formation of the serine protease, factor Xa, and the generation of a heavily glycosylated activation peptide comprising residues 1-52 of the heavy chain. The role of the activation peptide region in mediating substrate recognition and cleavage by the extrinsic Xase complex is unclear. The protease Agkistrodon rhodostoma hydrolase gamma (ARHgamma), from the venom of the Malayan pit viper, was used to selectively cleave human factor X in the activation peptide region. Three cleavage sites were found within this region and gave products designated Xdes1-34, Xdes1-43, and Xdes1-49. The products were purified to yield Xdes 1-49 and a mixture of Xdes 1-34 and Xdes 1-43. Reversed phase high pressure liquid chromatography analysis indicated that the cleaved portion of the activation peptide was likely removed during purification. All cleaved species were inactive and could be completely activated to factor Xa by the extrinsic Xase complex or by a purified activator from Russell's viper venom. Steady state kinetic studies using tissue factor reconstituted into membranes yielded essentially equivalent kinetic constants for the activation of intact factor X and the cleaved derivatives under a wide range of conditions. Since Xdes 1-49 lacks all but three residues of the activation peptide and is devoid of the carbohydrate present in this region, the data suggest that the specific recognition of human factor X by the extrinsic Xase complex is not achieved through specific interactions with residues 1-49 of the activation peptide or with carbohydrate structures attached to these residues.

Molecular mechanism of the dysfunction of protein S(Tokushima) (Lys155-->Glu) for the regulation of the blood coagulation system

The congenital abnormal protein S(Tokushima) has Glu substituted for Lys155 in the second epidermal growth factor domain of the protein S molecule (Hayashi T., Nishioka J., Shigekiyo, T. Saito, S. and Suzuki, K. (1994) Blood 83, 683-690). To elucidate the molecular mechanism of the dysfunction of the protein S(Tokushima), a comparative evaluation between the molecular interaction of the abnormal protein S and that of normal protein S with other clotting factors was carried out using recombinant normal protein S (rPSN) and protein S(Tokushima) (rPST) expressed in baby hamster kidney cells. While rPSN and plasma protein S exhibited cofactor activity for activated protein C (APC), rPST did not show this property. rPSN and rPST bound equally to phospholipids and C4b-binding protein fixed on microplate wells. APC bound to rPSN but not to rPST in an assay using immobilized monoclonal anti-protein S antibody. On the other hand, rPSN and plasma protein S inhibited the activity of prothrombinase complex composed of factor Xa and thrombin-stimulated platelets, whereas rPST lacked this inhibitory effect. Assessment of the mechanism by which rPST lacks inhibitory activity on the platelet-prothrombinase complex was also performed. Factor Xa bound to rPSN but not to rPST. Binding to rPSN to biotinylated factor Va in solution phase did not differ significantly from that of rPST. Binding of prothrombin to factor Va in solution phase was not inhibited either by rPSN or rPST. Binding of 4-amidinophenylmethanesulfonyl-factor Xa to factor Va in solution phase increased in the presence of rPSN but not in that of rPST. These findings suggest that the dysfunction of protein S(Tokushima) occurs because it fails to interact with APC and factor Xa. This molecular interaction is required for the expression of the APC cofactor activity and for the inhibition of the prothrombinase complex activity.

Characterization of the binding of factor Xa to fibrinogen/fibrin derivatives and localization of the factor Xa binding site on fibrinogen

The binding of human factor Xa to fibrinogen and its derivatives was characterized. Factor Xa bound to immobilized fibrin with a concentration at half-maximal binding (C50) of 100 nM. The 4-carboxyglutamic acid (Gla) domain of factor Xa is important in factor Xa binding to fibrin monomer, based on the following observations; the binding requires Ca2+; Gla-domain-lacking factor Xa could not bind to fibrin; factor Xa binding was significantly reduced by prior treatment of factor Xa with factor IX/factor-X-binding protein from the venom of Trimeresurus flavoviridis which specifically binds to the Gla domain of human factors IX and X. Factor Xa also bound to fibrinogen, fibrinogen degradation products (FDP)-D and FDP-E, with a similar affinity (C50 = 75-131 nM). In a solution-phase equilibrated binding assay, approximately 0.76 mol factor Xa bound to 1 mol fibrinogen with a dissociation constant of 180 nM. The binding of 125I-labeled factor Xa to the fibrin monomer was inhibited markedly by unlabeled factor Xa, but only slightly by thrombin, suggesting that the binding site of factor Xa on fibrin monomer differs from that of thrombin. We localized the binding site of factor Xa on fibrinogen: factor Xa bound strongly to the A alpha chain, but weakly to the B beta and gamma chains of fibrinogen. The A alpha chain was then digested with lysyl endopeptidase and separated by reverse-phase HPLC. Among resulting peptides, factor Xa bound specifically to a peptide corresponding to residues Asp82-Lys123 of the A alpha chain. This factor-Xa-binding site is located in the boundary between the central E domain and the terminal D domain of fibrinogen and is apparently distinct from the reported thrombin-binding site.

Identification of the oligosaccharide structures of human coagulation factor X activation peptide at each glycosylation site

Human blood coagulation factor X has two N-linked oligosaccharides at Asn39 and Asn49 residues and two O-linked oligosaccharides at Thr17 and Thr29 residues in the region of the factor X activation peptide (XAP) which is cleaved off during its activation by factor IXa. We determined the structure of oligosaccharides in the XAP region of human factor X. Four glycopeptides each containing a glycosylation site were isolated by digestion of XAP with endoproteinase Asp-N followed by reversed-phase HPLC. N-linked oligosaccharides released from the glycopeptides by glycoamidase A digestion were derivatized with 2-aminopyridine. Pyridylamino(PA)-oligosaccharides were separated by HPLC into neutral and sialyl oligosaccharides using an anion-exchange column. Structures of oligosaccharides and their contents at each glycosylation site were determined by a two-dimensional sugar mapping method. The contents of the neutral oligosaccharides at Asn39 and Asn49 residues were 32.5% and 30.0%, respectively. Six neutral and twelve monosialyl oligosaccharides isolated from both N-linked glycosylation sites showed similar elution profiles composed of bi-, tri- and tetra-antennary complex type oligosaccharides. The predominant component in neutral oligosaccharides was biantennary without a fucose residue. Two major monosialyl oligosaccharides were also biantennary without fucose and with a Neu5Ac alpha 2-->6 residue. In addition, the structures of O-linked oligosaccharides at Thr17 and Thr29 residues were suggested to be disialylated Gal beta 3GalNAc sequences by their component analyses.