Conformational Analysis and Structural Characterization of Furin Inhibitors and Their Enzyme Complexes: Structural...

The three-dimensional structures of two novel and highly potent inhibitors of furin were characterized using NMR spectroscopy. Both compounds are cyclic peptides belonging to the Bowman-Birk family of inhibitors (BBI). In the monocyclic disulfide-bridged inhibitor analysis revealed two conformers (major cis and minor trans) differing in the configuration of the Ile8-Pro9 (P3'-P4') peptide bond. Cis/trans isomerization at this position is well documented feature of BBIs. In contrast, the second disulfide-bridged inhibitor, which has an additional head-to-tail cyclization, exclusively adopted the cis conformation, indicating that the structural constraints imposed by head-to-tail cyclization may effectively restricts cis/trans isomerization. Remarkably, molecular dynamics simulations rank the minor trans conformer of the monocyclic inhibitor as the most favorable binder to furin, challenging previous assumptions that a cis-Pro configuration at the P3' position is essential for BBI activity. Furin appears to be less selective with respect to isomeric configuration at this position. These findings provide new insights into the structural determinants of furin inhibition and highlight the potential of cyclization strategies to fine-tune inhibitor conformations and activity.