Abstract
Trypsinogen is converted to trypsin by the removal of a peptide from the N terminus, which permits formation of a salt bridge between the new N-terminal Ile (residue 16) and Asp194. Formation of this salt bridge triggers a conformational change in the “activation domain” of trypsin, creating the S1 binding site and oxyanion hole. Thus, the activation of trypsinogen appears to represent an example of protein folding driven by electrostatic interactions. The following trypsin mutants have been constructed to explore this problem: Asp194Asn, Ile16Val, Ile16Ala, and Ile16Gly. The bovine pancreatic trypsin inhibitor (BPTI), benzamidine, and leupeptin affinities and activity and pH−rate profiles of these mutants have been measured. The changes in BPTI and benzamidine affinity measure destabilization of the activation domain. These experiments indicate that hydrophobic interactions of the Ile16 side chain provide 5 kcal/mol of stabilization energy to the activation domain while the salt bridge accounts for 3 kcal/mol. Thus, hydrophobic interactions provide the majority of stabilization energy for the trypsinogen to trypsin conversion. The pH−rate profiles of I16A and I16G are significantly different than the pH−rate profile of trypsin, further confirming that the activation domain has been destabilized. Moreover, these mutations decrease k cat/K m and leupeptin affinity in parallel with the decrease in stability of the activation domain. Acylation is selectively decreased, while substrate binding and deacylation are not affected. Together these observations indicate that the stability of protein structure is an important component of transition state stabilization in enzyme catalysis. These results also suggest that active zymogens can be created without providing a counterion for Asp194, and thus have important implications for the elucidation of the structural features which account for the zymogen activity of tissue plasminogen activator and urokinase.