The mechanism underlying activation of factor IX by factor XIa

Abstract

Factor XI (fXI) is the zymogen of a plasma protease, factor XIa (fXIa), that contributes to thrombin generation during blood coagulation by proteolytic conversion of factor IX (fIX) to factor IXaβ (fIXaβ). There is considerable interest in fXIa as a therapeutic target because it contributes to thrombosis, while serving a relatively minor role in hemostasis. FXI/XIa has a distinctly different structure than other plasma coagulation proteases. Specifically, the protein lacks a phospholipid-binding Gla-domain, and is a homodimer. Each subunit of a fXIa dimer contains four apple domains (A1 to A4) and one trypsin-like protease domain. The A3 domain contains a binding site (exosite) that largely determines affinity and specificity for the substrate fIX. After binding to fXIa, fIX undergoes a single cleavage to form the intermediate fIXα. FIXα then rebinds to the A3 domain to undergo a second cleavage, generating fIXaβ. The catalytic efficiency for the second cleavage is ~7-fold greater than that of the first cleavage, limiting fIXα accumulation. Residues at the N-terminus and C-terminus of the fXIa A3 domain likely form the fIX binding site. The dimeric conformation of fXIa is not required for normal fIX activation in solution. However, monomeric forms of fXI do not reconstitute fXI-deficient mice in arterial thrombosis models, indicating the dimer is required for normal function in vivo. FXI must be a dimer to be activated normal by the protease fXIIa. It is also possible that the dimeric structure is an adaptation that allows fXI/XIa to bind to a surface through one subunit, while binding to its substrate fIX through the other.

Hydrogen‐deuterium exchange mass spectrometry highlights conformational changes induced by factor XI activation and binding of factor IX to factor XIa

“…This difference could explain ligand selectivity, with FIX binding to FXIa but not to kallikrein. Furthermore, this supports the general hypothesis that the loop between the apple 3 and apple 4 domains in FXI mediates substrate binding …”

“…Additionally, FXI activation induces structural changes that enable FIX binding, since FIX can bind FXIa but not FXI . FXI has been shown to accommodate the FIX Gla domain at an exosite in the apple 3 domain, including amino acids Ile 183 , Arg 184 , and Asp 185 .…”

“…FXI has been shown to accommodate the FIX Gla domain at an exosite in the apple 3 domain, including amino acids Ile 183 , Arg 184 , and Asp 185 . Since it is buried in the FXI zymogen, the structural changes that allow FIX binding are thought to reveal the exosite . However, the full crystal structure of FXIa is yet to become available to confirm this.…”

“…10,12 Additionally, FXI activation induces structural changes that enable FIX binding, since FIX can bind FXIa but not FXI. 13,14 FXI has been shown to accommodate the FIX Gla domain at an exosite in the apple 3 domain, including amino acids Ile 183 , Arg 184 , and Asp 185 . 15 Since it is buried in the FXI zymogen, the structural changes that allow FIX binding are thought to reveal the exosite.…”

Emicizumab, the bispecific antibody to factors IX/IXa and X/Xa, potentiates coagulation function in factor XI‐deficient plasma in vitro

“…In this model, initial tissue injury leads to activation of FIX by the FVIIa/TF complex and mediates tenase activity, which generates thrombin. This reaction is rapidly terminated by tissue factor pathway inhibitor (TFPI) , but sufficient FXIa is produced to maintain activation of FIX in the presence of Ca 2+ and consolidate the formation of tenase with FVIIIa as a cofactor in the propagation phase of coagulation. In the patients with severe FXI deficiency, therefore, tenase formation is markedly decreased and thrombin generation is significantly impaired.…”