Structure-function of anticoagulant TIX-5, the inhibitor of factor Xa-mediated FV activation.
Research article published in Journal of thrombosis and haemostasis : JTH (2021)
Abstract
BACKGROUND: The prothrombinase complex consists of factors Xa (FXa) and Va (FVa) on an anionic phospholipid surface and converts prothrombin into thrombin. Both coagulation factors require activation before complex assembly. We recently identified TIX-5, a unique anticoagulant tick protein that specifically inhibits FXa-mediated activation of FV. Because TIX-5 inhibited thrombin generation in blood plasma, it was concluded that FV activation by FXa contributes importantly to coagulation. OBJECTIVE: We aimed to unravel the structure-function relationships of TIX-5. METHOD: We used a structure model generated based on homology with the allergen Der F7. RESULTS: Tick inhibitor of factor Xa toward FV was predicted to consist of a single rod formed by several beta sheets wrapped around a central C-terminal alpha helix. By mutagenesis we could show that two hydrophobic loops at one end of the rod mediate the phospholipid binding of TIX-5. On the other end of the rod an FV interaction region was identified on one side, whereas on the other side an EGK sequence was identified that could potentially form a pseudosubstrate of FXa. All three interaction sites were important for the anticoagulant properties of TIX-5 in a tissue factor-initiated thrombin generation assay as well as in the inhibition of FV activation by FXa in a purified system. CONCLUSION: The structure-function properties of TIX-5 are in perfect agreement with a protein that inhibits the FXa-mediated activation on a phospholipid surface. The present elucidation of the mechanism of action of TIX-5 will aid in deciphering the processes involved in the initiation phase of blood coagulation.
Abstract sourced from PubMed (NCBI) for the cited record. See the original publication for the authoritative version.
Summary
The prothrombinase complex consists of factors Xa (FXa) and Va (FVa) on an anionic phospholipid surface and converts prothrombin into thrombin. Both coagulation factors require activation before complex assembly.
Why This Matters for Hirudotherapy
This study examined the structure-function relationships of TIX-5, a tick-derived anticoagulant protein that specifically inhibits FXa-mediated activation of factor V, a key step in prothrombinase assembly and thrombin generation. Using homology modeling and mutagenesis, researchers identified three functional interaction sites—phospholipid-binding hydrophobic loops, an FV interaction region, and a potential FXa pseudosubstrate EGK sequence—all critical for anticoagulant activity. While TIX-5 is an anticoagulant from a blood-feeding arthropod, it is derived from ticks, not leeches, and the abstract makes no mention of leeches, hirudotherapy, or leech-derived molecules. The relevance to ASH's domain is limited to the general thematic context of hematophagous invertebrate anticoagulants, with no direct leech connection.
Citation
Structure-function of anticoagulant TIX-5, the inhibitor of factor Xa-mediated FV activation.
Maag et al. · Journal of thrombosis and haemostasis : JTH, 2021
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