Manno-epi-cyclophellitols Enable Activity-Based Protein Profiling of Human α-Mannosidases and Discovery of New Golgi Mannosidase II Inhibitors

Golgi mannosidase II (GMII) catalyzes the sequential hydrolysis of two mannosyl residues from GlcNAcMan 5 GlcNAc 2 o produce GlcNAcMan 3 GlcNAc 2 , the precursor for all complex N -glycans, including the branched N -glycans associated with cancer. Inhibitors of GMII are potential cancer therapeutics, but their usefulness is limited by off-target effects, which produce α-mannosidosis-like symptoms. Despite many structural and mechanistic studies of GMII, we still lack a potent and selective inhibitor of this enzyme. Here, we synthesized manno- epi -cyclophellitol epoxide and aziridines and demonstrate their covalent modification and time-dependent inhibition of GMII. Application of fluorescent manno- epi -cyclophellitol aziridine derivatives enabled activity-based protein profiling of α-mannosidases from both human cell lysate and mouse tissue extracts. Synthesized probes also facilitated a fluorescence polarization-based screen for dGMII inhibitors. We identified seven previously unknown inhibitors of GMII from a library of over 350 iminosugars and investigated their binding modalities through X-ray crystallography. Our results reveal previously unobserved inhibitor binding modes and promising scaffolds for the generation of selective GMII inhibitors.