Reactivation of des (119-124) ribonuclease A by mixture with synthetic COOH-terminal peptides; the role of phenylalanine-120.

Several peptides contained within the amino acid sequence -Glu-Gly-Asn-Pro-Tyr-Val-Pro-Val-His-Phe-Asp-Ala-SerVal-OH at the carboxyl end of ribonuclease A were synthesized in which phenylalanine-120 was replaced by leucine, isoleucine, or tryptophan: [Leul*O]-RNase 111-124, [Ile120]RNase 111-124, [Trp’*Ol-RNase 111-124, ple*20]-RNase 113124, [Leul*O]-RNase 115-124, ple120]-RNase 115-124, [Trpl*O]RNase 115-124. The peptides were examined for their ability to regenerate enzymatic activity when mixed with RNase 1-118 which had been prepared by enzymatic degradation of RNase A. A study of the dissociation constants of the peptide-protein complexes, of the Michaelis constants of the complexes with cyclic 2’,3’-cytidylic acid, and of the inhibition constants with 2’-cytidylic acid led to the conclusion that Phel*O plays an important role in binding the peptide and protein and in aligning the catalytic site of the complex, but that it does not have a specific effect on binding of substrate.