LYSINE-156 PROMOTES THE ANOMALOUS PROENZYME ACTIVITY OF TPA - X-RAY CRYSTAL-STRUCTURE OF SINGLE-CHAIN HUMAN TPA

Tissue type plasminogen activator (tPA) is the physiological initiator of fibrinolysis, activating plasminogen via highly specific proteolys is; plasmin then degrades fibrin with relatively broad specificity, Un like other chymotrypsin family serine proteinases, tPA is proteolytica lly active in a single-chain form, This form is also preferred for the rapeutic administration of tPA in cases of acute myocardial infarction , The proteolytic cleavage which activates most other chymotrypsin fam ily serine proteinases increases the catalytic efficiency of tPA only 5- to 10-fold, The X-ray crystal structure of the catalytic domain of recombinant human single-chain tPA shows that Lys156 forms a salt brid ge with Asp194, promoting an active conformation in the single-chain f orm, Comparisons with the structures of other serine proteinases that also possess Lys156, such as trypsin, factor Xa and human urokinase pl asminogen activator (uPA), identify a set of secondary interactions wh ich are required for Lys156 to fulfil this activating role. These find ings help explain the anomalous single-chain activity of tPA and mag s uggest strategies for design of new therapeutic plasminogen activators .