Fibrin(ogen) residues γ390-396 mediate the acceleratory effect of fibrin(ogen) on FXIII-A₂B₂ activation. FXIII-A₂B₂ (20 µg/mL [60 nM], final) was mixed with recombinant fibrinogens (γA/γA or Fibγ^390-396A, 150 µg/mL [440 nM], final) or buffer (No Fgn). Reactions were triggered by addition of thrombin (2 nM, final) and CaCl₂ (10 mM, final), quenched at the indicated time points, and analyzed by SDS-PAGE with western blotting and densitometry. Activation peptide cleavage was detected using anti-FXIII-A antibody. Fibrin crosslinking was detected using anti-fibrinogen antibody. (A) Representative western blots and (B) quantitation of FXIII-A₂B₂ activation over time from all blots. (C) Maximal rates of FXIII-A₂B₂ activation calculated from panel B. (D) Representative western blots of fibrin crosslinking, and quantitation of (E) γ-γ dimer formation and (F) γ-γ dimer formation rate. Data are means ± SEM; N = 3-6 replicates per time point.