Revisiting the mechanism of coagulation factor XIII activation and regulation from a structure/functional perspective

Abstract

The activation and regulation of coagulation Factor XIII (FXIII) protein has been the subject of active research for the past three decades. Although discrete evidence exists on various aspects of FXIII activation and regulation a combinatorial structure/functional view in this regard is lacking. In this study, we present results of a structure/function study of the functional chain of events for FXIII. Our study shows how subtle chronological submolecular changes within calcium binding sites can bring about the detailed transformation of the zymogenic FXIII to its activated form especially in the context of FXIIIA and FXIIIB subunit interactions. We demonstrate what aspects of FXIII are important for the stabilization (first calcium binding site) of its zymogenic form and the possible modes of deactivation (thrombin mediated secondary cleavage) of the activated form. Our study for the first time provides a structural outlook of the FXIIIA2B2 heterotetramer assembly, its association and dissociation. The FXIIIB subunits regulatory role in the overall process has also been elaborated upon. In summary, this study provides detailed structural insight into the mechanisms of FXIII activation and regulation that can be used as a template for the development of future highly specific therapeutic inhibitors targeting FXIII in pathological conditions like thrombosis.

Activation of factor XIII is accompanied by a change in oligomerization state

“…Thus, nPAGE results in the current study indicate that presumably monomeric, activated FXIIIA might reassociate into zymogen-like dimers or higher order aggregates upon loss of Ca 2+ ions. Such reassociations have been proposed by Gupta et al [22]. These phenomena observed in our nPAGE studies occurred specifically in the presence of Ca 2+ but not Mg 2+ , adding to the evidence that Ca 2+ is critical for maintaining the conformation of active FXIIIA.…”

“…As expected, increasing Ca 2+ concentrations resulted in an increase in the number of monomers. The sigmoidal shape of the 'titration curve' indicated cooperative Ca 2+ binding to FXIIIA, consistent with previous computational predictions [22]. Transglutaminase activity was also measured for each Ca 2+ concentration ( Fig.…”

“…Such reassociations have been proposed by Gupta et al . . These phenomena observed in our nPAGE studies occurred specifically in the presence of Ca 2+ but not Mg 2+ , adding to the evidence that Ca 2+ is critical for maintaining the conformation of active FXIIIA.…”

“…Consistent with those conclusions, nonproteolytically activated FXIIIA that had been covalently inhibited was later crystallized with an exposed active site and proposed to be monomeric [21]. In addition, recent in silico steered molecular dynamics simulations have suggested a weakening of intersubunit interactions in both nonproteolytically and proteolytically activated FXIIIA [22].…”

“…In addition, a portion of the intracellular FXIIIA population may exist as nondissociated species, or weak dimers as computationally predicted by Gupta et al . .…”