Mechanism of action of thrombin on fibrinogen: Reaction of the N-terminal CNBr fragment from the Bβ chain of bovine fibrinogen with bovine thrombin

Abstract

The N-terminal cyanogen bromide fragment from the Bβ chain of bovine fibrinogen was isolated, and its molecular weight was estimated to be approximately 14,000–15,500. The ratio of the Michaelis-Menten constants, k cat K m , for its hydrolysis by bovine thrombin was found to be 3 × 10 −7 [(NIH unit/liter)s] −1, indicating that the Bβ fragment is a poor substrate for thrombin compared to the corresponding Aα chain fragment. This value of k cat K m is too small to account for the rate of release of fibrinopeptide B from fibrinogen by thrombin. It is suggested that, while the Aα chain contains all of the amino acid residues necessary to interact with thrombin, the Bβ chain does not; i.e., some of the binding sites that are used in the hydrolysis of the Bβ chain are assumed to be located on either the α or γ chains of fibrinogen. An alternative hypothesis is that, after the Bβ chain fragment is removed from the fibrinogen molecule, it does not have the proper conformation to be hydrolyzed by thrombin.