Fig. 2.

Activation and action of plasma FXIII (From [7], © 2012 International Society on Thrombosis and Haemostasis. Reprinted with permission). FXIII circulates in plasma consisting of two catalytic A subunits and two carrier B subunits. Thrombin (Factor IIa) performs the activation step of FXIII, where thrombin cleaves the activation peptide from the A subunits before Ca²⁺ ions cause dissociation of the subunits. The result is a dimer of two activated FXIIIa A subunits. Presence of fibrin/fibrinogen enhances both FXIII activation steps. Following activation, FXIIIa cross-links lysine (Lys) and glutamine (Gln) residues of fibrin α- and γ-chains leading to a three-dimensional network of insoluble fibrin molecules