CRITICAL ROLE OF INTERACTION OF THROMBIN ANION-BINDING EXOSITE WITH COMPLEMENTARY RECOGNITION SITE OF FIBRINOGEN A-ALPHA-CHAIN FOR HIGH SPECIFICITY OF THE ENZYME

Bovine fibrinogen N-terminal fragments were hydrolyzed by native alpha -thrombin and its nonclotting gamma-form in which the anion binding ex osite (ABE) is disrupted. The bond of bovine fibrinogen A alpha-chain which is subject to cleavage (between residues 19 and 20) was cleaved by alpha-thrombin more efficiently than by gamma-thrombin. The differe nce between alpha- and gamma-forms disappears if the substrate sequenc e A alpha 37-54 is disrupted and is also absent on hydrolysis of nonsp ecific macromolecular substrates; these are hydrolyzed equally ineffic iently by alpha- and gamma-thrombins. These data demonstrate that the 37-54 sequence of bovine fibrinogen A alpha-chain contains a thrombin recognition site (TRS) which is complementary to the ABE of the enzyme . Thus, a hypothesis suggesting two types of high molecular weight thr ombin substrates differing in the presence or absence of the TRS is co nfirmed. The possible enhancement of the catalytic process by ABE-TRS interaction is discussed.