Fluorescent Probes for Studying Thioamide Positional Effects on Proteolysis Reveal Insight into Resistance to Cysteine Proteases.

Thioamide substitutions of the peptide backbone have been shown to reduce proteolytic degradation, and this property can be used to generate competitive protease inhibitors and to stabilize peptides toward degradation in vivo. Here, we present a straightforward sensor design that allows a systematic study of the positional effects of thioamide substitution by using real-time fluorescence. Thioamide scanning in peptide substrates of five papain family cysteine proteases demonstrates that a thioamide at or near the scissile bond can slow proteolysis in all cases, but that the magnitude of the effects varies with position and protease in spite of high sequence homology. Mechanistic investigation of papain proteolysis reveals that the thioamide effects derive from reductions in both affinity (KM ) and turnover number (kcat ). Computational modeling allows these effects to be understood based on disruption of key enzyme-substrate hydrogen bonds, providing a model for future rational use of thioamides to confer cysteine protease resistance.