Factor IXa

PA S1 S01.214 Coagulation factor IXa Possible target for gene therapy of haemophilia B... 

Activated by thrombin factor Villa is a cofactor in the activation of factor X by factor IXa. 

Factor XIa in the presence of activates factor IX (55 kDa, a zymogen containing vitamin K-dependent y-carboxyglutamate [Gla] residues see Chapter 45), to the serine protease, factor IXa. This in turn cleaves an Arg-Ile bond in factor X (56 kDa) to produce the two-chain serine protease, factor Xa. This latter reaction requires the assembly of components, called the tenase... 

Four naturally occurring thrombin inhibitors exist in normal plasma. The most important is antithrombin III (often called simply antithrombin), which contributes approximately 75% of the antithrombin activity. Antithrombin III can also inhibit the activities of factors IXa, Xa, XIa, Xlla, and Vila complexed with tissue factor. a2-Macroglobulin contributes most of the remainder of the antithrombin activity, with heparin cofactor II and aj-antitrypsin acting as minor inhibitors under physiologic conditions. 

Rusconi CP, Scardino E, Layzer J, Pitoc GA, Ortel TL, Monroe D, Sullenger BA (2002) RNA aptamers as reversible antagonists of coagulation factor IXa. Nature 419 90-94... 

Factor XIa, in turn, activates factor IX. Factor IXa then promotes the activation of factor X, but only when it (i.e. IXa) is associated with factor Villa. Factor Villa is formed by the direct action of thrombin on factor VIII. The thrombin will be present at this stage because of prior activation of the intrinsic pathway. 

Antithrombin, already mentioned in the context of heparin, is the most abundantly occurring natural inhibitor of coagulation. It is a single-chain 432 amino acid glycoprotein displaying four oligosaccharide side chains and an approximate molecular mass of 58 kDa. It is present in plasma at concentrations of 150 pig ml 1 and is a potent inhibitor of thrombin (factor Ha), as well as of factors IXa and Xa. It inhibits thrombin by binding directly to it in a 1 1 stoichiometric complex. 

Enzymes are used for a variety of therapeutic purposes, the most significant of which are listed in Table 12.8. A number of specific examples have already been discussed in detail within this chapter, including tPA, urokinase, and factor IXa. The additional therapeutic enzymes now become the focus of the remainder of the chapter. Although a limited number of polymer-degrading enzymes (used as digestive aids) are given orally, most enzymes are administered intravenously. 

Factor VIII process, 12 143 Factor IX, 4 86-87 Factor IXa, 4 86-87, 89 Factor IXa inhibitors, 4 103 Factor IX concentrates, properties of, 72 152t... 

CIEF was also used to follow the production of recombinant antithrombin III (r-AT Iff) in cultures of hamster kidney cells.111 r-AT III inhibits serine proteases such as blood factors (IXa, Xa, and XIa) and thrombin. Interference by the media from which the samples were collected posed some difficulties because some of the media components have similar characteristics to those of the compounds of interest. CIEF was used to determine the pis of the separated components after sample purification by HPLC. Three major peaks showed pis of 4.7, 4.75, and 4.85, and three minor peaks had pis of 5.0, 5.1, and 5.3. These data closely resembled the data already published for serum AT III based on conventional IEF. 

In the extravascular pathway (right), tissue thromboplastin (factor 111), a membrane protein in the deeper layers of the vascular wall, activates coagulation factor Vll. The activated form of this (Vila) autocatalytically promotes its own synthesis and also generates the active factors IXa and Xa from their precursors. With the aid of factor Villa, PL, and Ca factor IXa produces additional Xa, which finally— with the support of Va, PL, and Ca ""—releases active thrombin. 

Other members of the coagulation cascade such as factor Ila (also referred to as thrombin), factor IXa, and factor XIa require a longer heparin unit of 13 or... 

III which rapidly inhibits activated coagulation factors IXa, Xa, XIa and Xlla, plasmin, kallikrein and thrombin, thus inhibiting conversion of fibrinogen to fibrin. 

Hemostasis begins with the formation of the platelet plug, followed by activation of the clotting cascade, and propagation of the clot. One of the major multicomponent complexes in the coagulation cascade consists of activated factor IX (factor IXa) as the protease, activated factor VIII (factor Villa), calcium, and phospholipids as the cofactors, and factor X as the substrate. Factor IXa can be generated by either factor Xa activation of the intrinsic pathway or by the tissue factor/factor Vila complex. 

Human coagulation factor IXa in complex with p-amino benzamidine PDB ID IRFN... 

Factor IXa causes a rapid activation of factor X only if Ca2+, phospholipid,553 554 and the accessory factor Villa555 are present. The IXa Villa complex acts on X about 2 x 105 times faster than does IXa alone. This complex cleaves the same bonds in X as does the VIIa Va complex formed in the tissue factor pathway.514 The 2332-residue factor VIII and factor V have similar structures that include three repeats of a domain homologous to the blue copper-containing plasma protein ceruloplasmin (Chapter 16).556-559 Tyrosine 1680 of VIII apparently must be converted to a sulfate ester for full activity.560... 

Factor IX is activated by factor XIa by cleavage of the Arg-146 and Arg-147 link to give factor IXa, and by further cleavage between Arg-181 and Val-182 in a metal-dependent reaction to give factor IXa,/3. All forms of factor IX have two classes of calcium-binding site. A protease from the venom of Vipera russelli is a coagulant, and activates both factors X and IX. This process requires divalent cations but is relatively non-selective. 

Other aptamers under clinical trials are for coagulation factors including thrombin, Factor VIIA, Factor IXA and Factor XII. Additional aptamers are being tested for kidney and lung cancers. A distinct advantage of aptamers is the availability of antidotes for them, which are short complementary sequences to the aptamers that serve as their antagonists and could be used to avert aptamer toxicity. 

A model of blood coagulation. With tissue factor (TF), factor VII forms an activated complex (VIIa-TF) that catalyzes the activation of factor IX to factor IXa. Activated factor XIa also... 

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