The effect of thrombomodulin on the cleavage of fibrinogen and fibrinogen fragments by thrombin

Abstract

Thrombomodulin acts as a linear competitive inhibitor of thrombin with respect to the substrate fibrinogen. In the present study the effect of thrombomodulm on the activity of thrombin with fragments of the Aα and Bβ chain of fibrinogen has been examined. The cleavage of fibrinopeptide A from the N‐terminal disulphide knot, fragment 1–44 and fragment 1–51 of the Aα chain was inhibited by thrombomodulin. The average value for the inhibition constant obtained with these substrates was 0.83 ± 0.09 nM, which was in good agreement with the values obtained previously for the inhibition of thrombin by thrombomodulin with native fibrinogen as the substrate [Hofsteenge, J., Taguchi, H. & Stone, S. R. (1986) Biochem. J. 237, 243–251]. In contrast, the cleavage of fibrinopeptide A from fragment 1–23 and fragment 1–29 of the Aα chain was not affected by thrombomodulin. Although the cleavage of the Bβ chain in the intact fibrinogen molecule was inhibited by thrombomodulin [Hofsteenge, J., Taguchi, H. & Stone, S. R. (1986) Biochem. J. 237, 243–251], the release of fibrinopeptide B from the N‐terminal disulphide knot and the N‐terminal 118‐residue fragment of the Bβ chain was not inhibited by thrombomodulin. In addition, we determined the second‐order rate constants of cleavage of these substrates using human thrombin. Fragments of the Aα chain whose cleavage was inhibited by thrombomodulin were found to have values for k_cat/K_m that were within one order of magnitude of that for the native fibrinogen, whereas those for Aα chain fragments whose cleavage was not inhibited by thrombomodulin were found to be more than two orders of magnitudes lower. From these results we conclude that only a relatively small portion of the Aα chain of the fibrinogen molecule is responsible for the specific binding to thrombin that is affected by thrombomodulin. Moreover, residues 30–44 of the Aα chain play an important role in this thrombin‐fibrinogen interaction.