S'-SUBSITE MAPPING OF POLYETHYLENE GLYCOL-MODIFIED ALPHA-CHYMOTRYPSINAND ALPHA-CHYMOTRYPSIN - A COMPARATIVE-STUDY

Nucleophile specificities of polyethylene glycol-modified a-chymotryps in and the native enzyme were investigated via acyl transfer reactions using Ac-Tyr-OEt as acyl donor and a large series of peptides and ami no-acid amides as nucleophiles. In acyl transfer reactions with amino- acid amides both enzymes prefer basic and bulky amino-acid residues. H owever, peptides with bulky aliphatic or aromatic residues in P'(1) po sition were very poor nucleophiles for both enzymes. Surprisingly, pep tides having bulky aliphatic or aromatic residues in P'(2) were prefer red by the modified enzyme and were apparently more efficient nucleoph iles for both enzymes than those with such residues in P'(1). Generall y, peptides with a longer chain were weaker nucleophiles in the reacti ons catalyzed by polyethylene glycol-modified enzyme. In the series of peptides containing a positively charged amino-acid residue in variou s locations, the order of nucleophilic efficiency is with this locatio n being: P'(1) > P'(3) > P'(2);this is valid for both enzymes.