The Rational Design of Therapeutic Peptides for Aminopeptidase N using a Substrate-Based Approach

Joshi, Shilvi; Chen, Lang; Winter, Michael B.; Lin, Yi-Lun; Yang, Yang; Shapovalova, Mariya; Smith, Paige M.; Liu, Chang; Li, Fang; LeBeau, Aaron M.

Understanding substrate specificity is essential to the design of molecules that inhibit the enzymatic activity of APN. Although the preference of APN for neutral amino acid residues at the P1 position has been generally established, little is known about the physical basis for this preference and even less is known about the downstream prime-side specificity of APN. In this study, we performed a comprehensive evaluation of APN substrate specificity and identified key structural features that dictate the specificity of the protease. Using an unbiased mass spectrometry-based peptide library assay, we determined the P1-P4′ substrate preferences of APN and prioritized candidate peptide substrates in the library for rational inhibitor design. Six crystal structures of APN complexed with different amino acids in the P1 position were solved and provided a structural basis for the P1 substrate specificity. From these crystal structures, a peptide was modelled into the specificity pocket to highlight key interactions responsible for dictating the extended prime-side substrate specificity. Using a substrate derived from the peptide library, we developed a novel substrate-based cyclic peptide inhibitor that was specific for APN. Our inhibitor specifically bound to APN-expressing prostate cancer cell lines in vitro, decreasing their clonogenic survival, and was an effective therapeutic, leading to decreased tumor growth in vivo in xenograft models of prostate cancer.