Thrombin active-site regions.

Knowledge is being gained about topographic features of alpha-thrombin and how they may define thrombin specificity and biologic functions. Thrombin is not just an enzyme with moderately restricted proteolytic capabilities, yet with extraordinarily high specificities for certain bonds (such as the A alpha-cleavage site in fibrinogen), but also is a protein with hormonelike activities involving cell receptor interactions. Such activities do not require the catalytically active enzyme, but are blocked by hirudin (also antithrombin III). These appear to involve an unique insertion and subsequent peptide segment at an exon junction. On the other hand, the enzymic functions of thrombin depend on the catalytic site, per se, and derive specificity from the adjacent apolar-binding site within the fibrinopeptide side and the independent anionic-binding site within the fibrin side of the active groove.