Proteolytic and nonproteolytic activation mechanisms result in conformationally and functionally different forms of coagulation factor XIII A

Factor XIIIA (FXIIIA) is a transglutaminase that cross‐links intra‐ and extracellular protein substrates. FXIIIA is expressed as an inactive zymogen, and during blood coagulation, it is activated by removal of an activation peptide by the protease thrombin. No such proteolytic FXIIIA activation is known to occur in other tissues or the intracellular form of FXIIIA. For those locations, FXIIIA is assumed instead to undergo activation by Ca2+ ions. Previously, we demonstrated a monomeric state for active FXIIIA. Current analytical ultracentrifugation and kinetic experiments revealed that thrombin‐activated FXIIIA has a higher conformational flexibility and a stronger affinity toward glutamine substrate than does nonproteolytically activated FXIIIA. The proteolytic activation of FXIIIA was further investigated in a context of fibrin clotting. In a series of fibrin cross‐linking assays and scanning electron microscopy studies of plasma clots, the activation rates of FXIIIA V34X variants were correlated with the extent of fibrin cross‐linking and incorporation of nonfibrous protein into the clot. Overall, the results suggest conformational and functional differences between active FXIIIA forms, thus expanding the understanding of FXIIIA function. Those differences may serve as a basis for developing therapeutic strategies to target FXIIIA in different physiological environments. Enzymes Factor XIIIA ( EC 2.3.2.13) The transglutaminase Factor XIIIA (FXIIIA) cross‐links its protein substrates in a Ca2+‐dependent manner and has a key role in processes such as blood coagulation where it is activated proteolytically by thrombin and targets fibrin. Here, Muriel Maurer and colleagues investigate the impact of proteolytic activation with low Ca2+, slow, non‐proteolytic activation with low Ca2+, or nonproteolytic activation at high Ca2+ concentrations on FXIIIA. The resultant transglutaminase enzyme species exhibit conformationally and functionally different enzymatic properties. FXIIIA variants are tested for differences in fibrin clot structure, supporting development of therapeutic strategies to target FXIIIA in different physiological environments.