Activation of FIX involves the cleavage of two peptide bonds: at Arg191 (R191-A192, the α-cleavage) and at Arg226 (R226-V227, the β-cleavage), releasing an activation peptide (A192-R226). The activation peptide has no known function. Activated FIXa consists of an N-terminal light chain (47-191) and a C-terminal heavy chain (227-461), which are linked by a disulfide bond between Cys178 and Cys335 (Di Scipio RG et al., 1978; Zögg T & Brandstetter H, 2009).
FIX can be cleaved by tissue factor (TF)-bound factor VIIa (FVIIa) during the initiation phase of coagulation and by FXIa during the amplification phase (Komiyama Y et al., 1990; Gailani D et al., 2014). Additionally, plasma kallikrein (PKa) can cleave FIX during the FXIIa-mediated activation of the kallikrein-kinin system (Visser M et al., 2020; Noubouossie DF et al., 2020; Kearney KJ et al., 2021). FXI and prekallikrein (PK) share high sequence homology due to gene duplication (Ponczek MB et al., 2020). Both proteins circulate bound to high-molecular-weight kininogen (HK) and can be activated by FXIIa (Cheng Q et al., 2010; Li C et al., 2023; Mohammed BM et al., 2024).
Despite their similarities, the mechanism of FIX activation by PKa differs from that of FXIa. Unlike FXIa, PKa activates FIX in a manner independent of calcium and phospholipids (Kearney KJ et al., 2021). Structural studies have revealed differences between PKa and FXIa that may influence their interactions with substrates (Li C et al., 2019). Although FXIa is more catalytically efficient than PKa in activating FIX, the higher plasma concentration of prekallikrein (PK) suggests that PKa could have a significant role under physiological conditions (Kearney KJ et al., 2021).
In vivo, PKa-mediated FIX activation has been observed in FXI-deficient mice following FXII activation by agents such as ellagic acid or polyphosphates (Visser M et al., 2020). Furthermore, PKa-mediated FIX activation may contribute to thrombin generation and clot formation in human pathologies involving FXII activation.
This Reactome event shows the PKa-catalyzed cleavage of FIX at Arg191 and Arg226.
