Characterization of Novel Forms of Coagulation Factor XIa: independence of factor XIa subunits in factor IX activation.

Smith SB, Verhamme IM, Sun MF, Bock PE, Gailani D
J Biol Chem. 2008 283 (11): 6696-705

PMID: 18192270 · PMCID: PMC2633474 · DOI:10.1074/jbc.M707234200

Factor XI is the zymogen of a dimeric plasma protease, factor XIa, with two active sites. In solution, and during contact activation in plasma, conversion of factor XI to factor XIa proceeds through an intermediate with one active site (1/2-FXIa). Factor XIa and 1/2-FXIa activate the substrate factor IX, with similar kinetic parameters in purified and plasma systems. During hemostasis, factor IX is activated by factors XIa or VIIa, by cleavage of the peptide bonds after Arg145 and Arg180. Factor VIIa cleaves these bonds sequentially, with accumulation of factor IX alpha, an intermediate cleaved after Arg145. Factor XIa also cleaves factor IX preferentially after Arg145, but little intermediate is detected. It has been postulated that the two factor XIa active sites cleave both factor IX peptide bonds prior to releasing factor IX abeta. To test this, we examined cleavage of factor IX by four single active site factor XIa proteases. Little intermediate formation was detected with 1/2-FXIa, factor XIa with one inhibited active site, or a recombinant factor XIa monomer. However, factor IX alpha accumulated during activation by the factor XIa catalytic domain, demonstrating the importance of the factor XIa heavy chain. Fluorescence titration of active site-labeled factor XIa revealed a binding stoichiometry of 1.9 +/- 0.4 mol of factor IX/mol of factor XIa (Kd = 70 +/- 40 nm). The results indicate that two forms of activated factor XI are generated during coagulation, and that each half of a factor XIa dimer behaves as an independent enzyme with respect to factor IX.

MeSH Terms (15)

Arginine Binding Sites Biochemistry Catalysis Catalytic Domain Dimerization Factor IX Factor XIa Hemostasis Humans Hydrolysis Kinetics Models, Biological Peptides Recombinant Proteins

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