Combined inhibitor free-energy landscape and structural analysis reports on the mannosidase conformational coordinate

Article abstract of DOI:10.1002/anie.201308334

Mannosidases catalyze the hydrolysis of a diverse range of polysaccharides and glycoconjugates, and the various sequence-based mannosidase families have evolved ingenious strategies to overcome the stereoelectronic challenges of mannoside chemistry. Using a combination of computational chemistry, inhibitor design and synthesis, and X-ray crystallography of inhibitor/enzyme complexes, it is demonstrated that mannoimidazole-type inhibitors are energetically poised to report faithfully on mannosidase transition-state conformation, and provide direct evidence for the conformational itinerary used by diverse mannosidases, including β-mannanases from families GH26 and GH113. Isofagomine-type inhibitors are poor mimics of transition-state conformation, owing to the high energy barriers that must be crossed to attain mechanistically relevant conformations, however, these sugar-shaped heterocycles allow the acquisition of ternary complexes that span the active site, thus providing valuable insight into active-site residues involved in substrate recognition. Shipshape inhibitors: Quantum mechanical calculations of the free-energy landscape (see figure) of the glycosidase transition-state mimics isofagomine and mannoimidazole reveals that only the latter is energetically poised to report upon the mannosidase transition-state conformation. X-ray structures of β-mannanases from different families reveal they both adopt a boat conformation, thus allowing unification of the enzymatic conformational itinerary of a range of diverse α- and β-mannosidases.