Role of calcium in the conformational dynamics of factor XIII activation examined by hydrogen–deuterium exchange coupled with MALDI-TOF MS

Factor XIII catalyzes formation of γ-glutamyl-ε-lysyl crosslinks within fibrin clots. FXIII A2 can be activated proteolytically with thrombin and low mM Ca2+ or nonproteolytically with high monovalent/divalent cations along with low mM Ca2+. Physiologically, FXIII A2 is poised to respond to transient influxes of Ca2+ in a Na+ containing environment. A successful strategy to monitor FXIII conformational events is hydrogen–deuterium exchange (HDX) coupled with mass spectrometry. FXIII A2 was examined in the presence of different cations (Ca2+, Mg2+, Ba2+, Cu2+, Na+, TMAC+, and EDA2+) ranging from 1 to 2 mM, physiological Ca2+ concentration, to 50–500 mM for nonproteolytic activation. Increases in FXIII solvent exposure could already be observed at 1 mM Ca2+ for the dimer interface, the catalytic site, and glutamine substrate regions. By contrast, solvent protection was observed at the secondary cleavage site. These events occurred even though 1 mM Ca2+ is insufficient for FXIII activation. The metals 1 mM Mg2+, 1 mM Ba2+, and 1 mM Cu2+ each led to conformational changes, many in the same FXIII regions as Ca2+. FXIII could also be activated nonproteolytically with 500 mM tetramethylammonium chloride (TMAC+) and 500 mM ethylenediamine (EDA2+), both with 2 mM Ca2+. These different HDX studies help reveal the first FXIII segments that respond to physiological Ca2+ levels.

► Conformational changes for FXIII activation initiated at physiological Ca2+.
► Ca2+, Mg2+, Ba2+, and Cu2+ can protect the FXIII secondary cleavage site 513–514.
► Exposures observed at dimer interface, catalytic site, and substrate binding sites.
► FXIII A2 can be activated by organic cations in the presence of low mM Ca2+.
► As divalent cation concentration increases, conformation effects often enhanced.