Native coagulation factor XIII from human plasma. Plasma Factor XIII is a tetramer (α2β2) composed of two identical α-subunits and two identical β-subunits. After thrombin cleavage converts the α2β2 tetramer to a modified α2′β2 tetramer, Ca2+ promotes both the dissociation of β2 dimer and the activation of the α2′ dimer to its catalytically active form. This activated Factor XIII (Factor XIIIa) strengthens the fibrin clot by catalyzing the formation of glutamyl-lysine bonds between fibrin monomers.
