CONVERTING TISSUE-PLASMINOGEN ACTIVATOR TO A ZYMOGEN - A REGULATORY TRIAD OF ASP-HIS-SER

Unlike most serine proteases of the chymotrypsin family, tissue-type p lasminogen activator (tPA) is secreted from cells as an active, single -chain enzyme with a catalytic efficiency only slightly lower than tha t of the proteolytically cleaved form. A zymogenic mutant of tPA has b een engineered that displays a reduction in catalytic efficiency by a factor of 141 in the single-chain form while retaining full activity i n the cleaved form. The residues introduced in the mutant, serine 292 and histidine 305, are proposed to form a hydrogen-bonded network with aspartate 477, similar to the aspartate 194-histidine 40-serine 32 ne twork found to stabilize the zymogen chymotrypsinogen.