Comparative studies on the structure of active sites. Behavior of "inverse substrates" toward trypsin and related enzymes.

The kinetics of hydrolysis of "inverse substrates," p-amidinophenyl alkanoates, catalyzed by urokinase, plasmin, kallikrein, and trypsins from various sources were studied. Dissociation constants of acyl enzyme-ligand complexes, which are a characteristic parameter of the reaction with "inverse substrates," were analyzed with a view to comparing the spatial requirements of active sites. It was concluded that the spatial restraint of the active site as regards coexistence of the acyl residue and specific ligand is strictest for hog pancreatic kallikrein and this restraint decreases in the following order: human urokinase; bovine plasmin, bovine and hog trypsins; Streptomyces fradiae trypsin; and Streptomyces griseus trypsin.