Factor IX participates in the middle phase of the intrinsic pathway of blood coagulation. The reactions leading to the activation of factor IX involve prekallikrein, high molecular weight kininogen, and factor XII. These proteins interact in the presence of a surface such as kaolin and give rise to the activation of factor XI. Factor XIa then converts factor IX to factor IXa in the presence of calcium ions. In this reaction, factor IX (a single-chain glycoprotein of mol. wt.-~55,000) is converted to factor IXa in a two-step reaction. In the first step, an internal peptide bond is cleaved leading to the formation of an intermediate lacking enzymatic activity. This intermediate contains two polypeptide chains held together by a disulfide bond(s). In the second step, an activation peptide is split from the heavy chain of the intermediate giving rise to factor IXa (mol. wt. ~45,000). Factor IXa is composed of a heavy chain (mol. wt.~27,000) and a light chain (mol. wt. ~16,000) held together by a disulfide bond(s). The activation mechanism is essentially identical for human and bovine factor IX. Factor IXa is a serine protease with esterase activity and is sensitive to protease inhibitors such as antithrombin III. Factor IX is also activated by the protease from Russell’s viper venom, but this reaction involves only a single cleavage in the precursor molecule. The critical step in the activation of factor IX by factor XIa or the protease from Russell’s viper venom is the cleavage of the same internal Arg-Val peptide bond and the formation of a new amino-terminal sequence of Val-Val-Gly-Gly- in the heavy chain of the enzyme.
Sequence analysis of cloned cDNA encoding part of an immunoglobulin heavy chain
