ORGANIC-REACTIONS CATALYZED BY MODIFIED ENZYMES .2. APPLICATION OF QSAR METHODOLOGY TO THE STUDY OF THE CHEMOSELECTIVITY OF MODIFIED ALPHA-CHYMOTRYPSIN IN THE SYNTHESIS OF PEPTIDES

Quantitative Structure Activity Relationship and Molecular Mechanics m ethodologies have been applied to the study of the kinetically control led synthesis of peptides catalyzed by chemically modified alpha-chymo trypsin (alpha-CT-PEG), in order to investigate: I) the structural pro perties of the substrate and their effect on the enzymatic activity. I I) the dimensions of the active site. The chemical modification of alp ha-CT does not alter the relative position of ''ar'', ''am'' and ''h'' subsites of alpha-CT (Figure 1) because only L-aminoacids are accepte d as acyl donors. Using the modified enzyme the active conformation of the acyl donor that interacts with the ''am'', ''ar'' and ''h'' subsi tes of the modified enzyme is described. The relative position of Ser- 195 and the residues that interact with the substrate in the ''am'' su bsite have also been determined. The GO-NH group (interaction with ''a m'' subsite) and the COOCH3 group (interaction with Ser-195) are oppos ite to each other (-177 degrees to 170 degrees) when bound in the acti ve site, The ''ar'' and ''am'' subsites are located with a dihedric an gle of -104 degrees as can be deduced from the relative position of th e two aromatic rings of the analogues of N-benzoyl-phenylalanine methy l ester in the active conformer. The dimensions of the ''ar'' subsite of the modified enzyme are described for the first time. It presents t wo cavities in front of the ortho and meta positions, with dimensions equivalent to those of a CH3 or Br group.