Examining Thrombin Hydrolysis of the Factor XIII Activation Peptide Segment Leads to a Proposal for Explaining the Cardioprotective Effects Observed with the Factor XIII V34L Mutation

In the blood coagulation cascade, thrombin cleaves fibrinopeptides A and B from fibrinogen revealing sites for fibrin polymerization that lead to insoluble clot formation. Factor XIII stabilizes this clot by catalyzing the formation of intermolecular cross-links in the fibrin network. Thrombin activates the Factor XIII a2 dimer by cleaving the Factor XIII activation peptide segment at the Arg37-Gly38 peptide bond. Using a high performance liquid chromatography assay, the kinetic constants Km, kcat, andkcat/Km were determined for thrombin hydrolysis of fibrinogen Aα-(7–20), Factor XIII activation peptide-(28–41), and Factor XIII activation peptide-(28–41) with a Val34 to Leu substitution. This Val to Leu mutation has been correlated with protection from myocardial infarction. In the absence of fibrin, the Factor XIII activation peptide-(28–41) exhibits a 10-fold lower kcat/Kmvalue than fibrinogen Aα-(7–20). With the Factor XIII V34L mutation, decreases in Km and increases inkcat produce a 6-fold increase inkcat/Km relative to the wild-type Factor XIII sequence. A review of the x-ray crystal structures of known substrates and inhibitors of thrombin leads to a hypothesis that the new Leu generates a peptide with more extensive interactions with the surface of thrombin. As a result, the Factor XIII V34L is proposed to be susceptible to wasteful conversion of zymogen to activated enzyme. Premature depletion may provide cardioprotective effects.