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Factor XIa dimer in the activation of factor IX
Dipali Sinha1, Mariola Marcinkiewicz, James D Lear
1Sol Sherry Thrombosis Research Center, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA. dipali@temple.edu
Biochemistry
|July 27, 2005
Summary
The dimeric nature of coagulation factor XI (FXI) is not essential for its function. Even when FXI exists primarily as a monomer, it effectively supports blood clotting and factor IX activation, particularly with activated platelets.
Area of Science:
- Biochemistry
- Hematology
- Molecular Biology
Background:
- Factor XI (FXI) is a coagulation protein that typically exists as a homodimer.
- The physiological importance of this dimeric structure is not fully understood.
- Other plasma proteins with similar structures exist as monomers.
Purpose of the Study:
- To investigate the functional significance of the dimeric state of factor XI.
- To determine if a preformed dimer is necessary for FXI activity, especially in the presence of activated platelets.
Main Methods:
- Expression of a mutant factor XI (FXI/G326C) designed to favor monomeric form.
- Analysis of protein structure using SDS-PAGE and detection of free thiols.
- Assessment of monomer-dimer equilibrium via gel-filtration chromatography and ultracentrifugation.
- Functional assays including plasma clotting and factor IX activation in the presence and absence of activated platelets.
Main Results:
- The mutant FXI/G326C protein migrated as a monomer under non-reducing conditions and lacked detectable free thiols.
- In physiological buffer, the mutant protein exhibited a monomer-dimer equilibrium with a K(d) of approximately 36 nM.
- FXI/G326C demonstrated functional activity comparable to wild-type FXI in plasma clotting and factor IX activation assays, even at low dimer concentrations.
Conclusions:
- A preformed dimer of factor XI is not required for its optimal function.
- Factor XI can be functionally active as a monomer or in a monomer-dimer equilibrium, particularly in the context of activated platelets.