[Conformational aspects of beta-trypsin interaction with substrates and pancreatic trypsin inhibitor. I. Conformational properties of residues in the enzyme-active center and the structure of a non-valent enzyme-substrate complex].

The theoretical conformational analysis of potential surfaces of Ser-195, His-57, Asp-102 and Gln-192 side chains in the active center of native beta-trypsin has been carried out. The above residues are shown to exist in low-energy conformational states in the free enzyme and in its nonbonded substrate complexes. Interrelations between the flexibility of the residues and their catalytical functions were revealed. Conformational aspects of interaction of trypsin with N-acetyl-L-lysine and N-acetyl-L-alanyl--L-alanyl--L-lysyl--L-alanyl methyl amide and with the Gly-12--Ile-19 BPTI fragment were analysed. The productive binding of the substrate at the nonbonded complex stage is shown to take place exclusively in the lowest energy conformation of the enzyme-substrate complex. Basing on theoretical and experimental evidence, the problems of primary and secondary specificity of trypsin, and potential properties of the native protein to form a productive nonbonded complex and to react at the subsequent stages of the catalytical act are discussed. Conformational changes in the active center and interactions with a substrate are shown to result from stabilizing enzyme-substrate interactions. Trypsin inhibition by BPTI molecule does not take place at the nonbonded complex stage.