Organic Reactions Catalyzed by Modified Enzymes Part II. Application of QSAR methodology to the study of the chemoselectivity of modified α-chymotrypsin in the synthesis of peptides

Abstract

Quantitative Structure Activity Relationship and Molecular Mechanics methodologies have been applied to the study of the kinetically controlled synthesis of peptides catalyzed by chemically modified α-chymotrypsin (α-CT-PEG), in order to investigateI) the structural properties of the substrate and their effect on the enzymatic activity.II) the dimensions of the active site.The chemical modification of α-CT does not alter the relative position of “ar”, “am” and “h” subsites of α-CT (Figure 1) because only L-aminoacids are accepted as acyl donors. Using the modified enzyme the active conformation of the acyl donor that interacts with the “am”, “ar” and “h” subsites of the modified enzyme is described. The relative position of Ser-195 and the residues that interact with the substrate in the “am” subsite have also been determined. The CO-NH group (interaction with “am” subsite) and the COOCH3 group (interaction with Ser-195) are opposite to each other (−177° to 170°) when bound in the active site. The “ar” and “am” subsites are located with a dihedric angle of − 104° as can be deduced from the relative position of the two aromatic rings of the analogues of N-benzoyl-phenylalanine methyl ester in the active conformer. The dimensions of the “ar” subsite of the modified enzyme are described for the first time. It presents two cavities in front of the ortho and meta positions, with dimensions equivalent to those of a CH3 or Br group.