Papain-Catalyzed Peptide Synthesis and Oligomerization of Amino Acid Amides in Organic Solvents

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Abstract
Papain, either modified with polyethylene glycol or adsorbed on porous glass beads, was used to catalyze peptide bond formation between N-acyl-L-amino acid esters and L-phenylalanine amide in organic solvents. Optimal reaction parameters of the modified enzyme were derived from the dependence of its activity upon water and mercaptoethanol concentrations in the reaction mixture. Under the same experimental conditions, immobilized papain was found to be considerably less efficient than modified papain in catalyzing the synthesis of N-α-benzoyl-L-lysine-L-phenylalanine amide in 1,1,1,-trichloroethane. Moreover, the pH at which the enzyme has been lyophilized prior to its adsorption on glass beads also had an important influence on the activity. Peptide synthesis yields higher than 90% were obtained with both papain forms when either basic or acidic amino acids were used as acyl-donor residues, while neutral and aromatic residues were rather poor substrates but initiated a polymerization reaction.