The 380-residue FIXa contains a typical serine protease catalytic triad of S365, H221, and D269, all located in the heavy chain of FIXa (Figure 11.3)1[23,24]. FIXa (EC 22.214.171.124), a 385-residue polypeptide, interacts with a complex (the tenase complex) composed of FVIII, phoshoplipids, FX, and calcium that are located on the surface of platelet membranes. FIXa binds the light chain of factor VIIIa through its Gla domain . FIXa functions as a protease to cleave the peptide bond between R52 and I53 (on the C terminus of the FX heavy chain) converting it into activated factor X (FXa). FXa then interacts with factors V (FV) and II (FII; prothrombin) to convert FII into FIIa (thrombin). FIIa proteolyzes fibrinogen into fibrin monomers, which undergo polymerization in the final stage of the blood-clotting cascade. These fibrin polymers are stabilized by crosslinking activity of factor XIII (FXIII) in association with thrombin and calcium (Figure 11.1).
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