Cyclic Peptides with RNase A Inhibitor Activity Based on Amino Acid Sequences Screened from the Fv-Antibody Library

Ribonuclease A (RNase A) is a pancreatic enzyme that rapidly degrades single-stranded RNA and is widely used as a representative enzyme in studies of RNase-mediated RNA degradation. Therefore, effective RNase A inhibitors are needed to regulate RNase activity and protect RNA from enzymatic degradation. Fv-antibodies represent the antigen-binding region of immunoglobulin G (IgG) and are composed of complementarity-determining regions (CDRs) and framework regions (FRs). Previously, CDR3 sequences with RNase A inhibitory activity were screened using RNase A as a probe from an autodisplayed Fv-antibody library constructed by site-directed mutagenesis of the 11-residue CDR3 region and expressed on the outer membrane of Escherichia coli. In this study, these screened CDR3 sequences were synthesized as cyclic peptides by introducing cysteine residues at both the N- and C-termini, enabling intramolecular disulfide bond formation. The metabolic stability of the peptides was enhanced through cyclization, as demonstrated by a significant increase in half-life (t_1/2) during liver microsome assays. The RNase A inhibitory activities of Fv-antibodies, linear peptides, and cyclic peptides were analyzed using cyclic cytidine monophosphate (cCMP) hydrolysis, fluorogenic RNA probe-based fluorescence, and RNA cleavage assays. Among these formats, cyclic peptides exhibited enhanced RNase A inhibitory activity compared with both Fv-antibodies and linear peptides. The influence of peptide conformation on RNase inhibitory activity was further analyzed using docking simulation. These results demonstrated that cyclization of RNase A-inhibitory CDR3 sequences improved inhibitory activity and supported the development of cyclic peptide-based RNase A inhibitors.