Stereochemical aspects of peptide hydrolysis catalyzed by serine proteases of the chymotrypsin type

The steric course of peptide hydrolysis catalyzed by serine proteases has been studied on the basis of the available, extensive structural data and taking into account the stereoelectronic theory of Deslongchamps (Heterocycles, 7, 1271 (1977)). These studies allowed elucidation of the structure of intermediates, in particular of the tetrahedral intermediate, and of the main structural events taking place during catalysis. They reveal a difficulty inherent in the generally accepted mechanism of peptide hydrolysis: protonation of the leaving nitrogen in the configuration arising from nucleophilic attack of Ser-195 on the carbonyl carbon cannot take place internally from His-57. Two alternative mechanisms are discussed which are compatible with all implications of the stereoelectronic theory. The main features of the more probable mechanism are: (i) a conformational change allowing the imidazole ring of His-57 to occupy two distinct positions; in one position a proton is abstracted from O^γ of Ser-195, and in the other this proton is donated to the leaving nitrogen; (ii) a configurational change (inversion) of the pyramidal leaving nitrogen reorienting the lone-pair orbital developed during nucleophilic attack; in one orientation CO bond breaking, and in the other CN bond breaking, is allowed. This inversion process confers on the nitrogen the property of a switch controlling the breakdown of the tetrahedral intermediate.