Fig. 1. Structure and activation of the A-subunit of FXIII.

(A) Zymogen FXIIIA₂ (PDB 1F13) is formed by two identical subunits oriented in a head-to-tail fashion (shown as space-filling models, dashed line denotes homodimer interface), with the activation peptide of each subunit blocking access to the active site of the neighboring subunit. Structural domains of the left subunit are highlighted in a ribbon representation. (B) Both plasma and cellular FXIII can be activated by thrombin in the presence of low mM Ca²⁺ or non-proteolytically by high (>50 mM) concentration of Ca²⁺. Aʹ - FXIIIA after cleavage of the activation peptide; A* - FXIIIA activated by thrombin in the presence of Ca²⁺; A° - non-proteolytically activated FXIIIA; question marks denote unknown oligomerization state. A more detailed description can be found in the text.