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-Ser-Val-OH at the carboxyl end of ribonuclease A were synthesized in which phenylalanine-120 was replaced by leucine, isoleucine, or tryptophan: [Leu 120 ]-RNase 111–124, [Ile 120 ]-RNase 111–124, [Trp 120 ]-RNase 111–124, [Ile 120 ]-RNase 113–124, [Leu 120 ]-RNase 115–124, [Ile 120 ]-RNase 115–124, [Trp 120 ]-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 Phe 120 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.