Discovery of different types of inhibition between the human and Thermotoga maritima α-fucosidases by fuconojirimycin-based derivatives

An efficient method for examining the selectivity of inhibitors on two alpha-fucosidases, one from Thermotoga maritima and the other from human, was established. The X-ray crystal structure of the former enzyme makes possible the homology modeling of the human alpha-fucosidase, indicating the major difference between both enzymes in the periphery of the catalytic site. To investigate the difference at the molecular level, a variety of fuconojirimycin (FNJ) derivatives with substitution at Cl, C2, C6, or N were rapidly prepared in microplates and screened without purification for the inhibition activities of the two alpha-fucosidases. Among the molecules that were tested, only the substitution at Cl can significantly enhance the inhibitory potency, in contrast to the control (no substitution) and compounds with substitution at other positions. The majority of Cl-substituted FNJs were found to be slow tight-binding inhibitors of the Thermotoga enzyme, while acting as the reversible inhibitors of the human fucosidase. The best inhibitor exhibited 13700-fold difference in affinity between the two enzymes, which was attributed to the dissimilar aglycon binding site. Further investigations were carried out, including site-directed mutagenesis, the comparison of K-i values among the wild type and mutants, and the intrinsic fluorescence change upon inhibitor titration, all supporting the idea that Tyr64 and Tyr267 of the Thermotoga alpha-fucosidase are critically involved in closely interacting with the aglycon of inhibitors. The increased level of contact thus induced conformational change, leading to the observed slow tight-binding inhibition.