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Functional and structural characterization of factor Xa dimer in solution.

Rima Chattopadhyay1, Roxana Iacob, Shalmali Sen

  • 1Department of Biochemistry and Biophysics and Program in Molecular and Cellular Biophysics, University of North Carolina, Chapel Hill, North Carolina, USA.

Biophysical Journal
|February 3, 2009
PubMed
Summary

Phosphatidylserine (PS) binding causes Factor Xa (FXa) to dimerize, significantly reducing its prothrombin activity. This dimerization alters FXa structure, impacting Factor Va binding sites.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Structural Biology

Background:

  • Factor Xa (FXa) is a key enzyme in the coagulation cascade.
  • Previous studies indicated that water-soluble phosphatidylserine (C6PS) binding induces Ca2+-dependent dimerization of bovine FXa.
  • The structural and activity consequences of this dimerization on human and bovine FXa were not fully understood.

Purpose of the Study:

  • To investigate the effects of Ca2+, C6PS, and dimerization on the activity and structure of human and bovine FXa.
  • To elucidate the structural basis for the reduced activity of FXa dimers.
  • To identify potential changes in Factor Va binding regions upon C6PS-induced dimerization.

Main Methods:

  • Enzyme activity assays (prothrombinase and amidolytic activity).

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  • Circular dichroism spectroscopy for structural analysis.
  • Mass spectrometry to identify lysine residues at the dimer interface.

    • Main Results:

    • FXa dimers (human and bovine) exhibited a 10^6- to 10^7-fold decrease in prothrombinase activity compared to monomers, primarily due to reduced k(cat).
    • Amidolytic activity on a synthetic substrate was largely unaffected, suggesting the active site remains accessible.
    • Circular dichroism showed significant structural changes, including a decrease in alpha-helix content, upon dimerization.
    • Mass spectrometry identified K270 buried at the dimer interface, potentially affecting FVa binding.
    • C6PS binding exposed specific lysine residues (K351, K242, K420) in regions relevant to FVa and substrate binding.

    Conclusions:

    • C6PS-induced dimerization of FXa leads to significant conformational changes or domain rearrangements.
    • The reduced prothrombinase activity of FXa dimers is mainly due to a decrease in catalytic efficiency (k(cat)).
    • Dimerization alters the structure of FXa in ways that may impact its interaction with Factor Va.
    • Further structural studies of PS-activated FXa are necessary to fully understand the FXa dimer and FXa-FVa complex structures.