Structural and biochemical evidence for a boat-like transition state in β-mannosidases.

Enzyme inhibition through mimicry of the transition state is a major area for the design of new therapeutic agents. Emerging evidence suggests that many retaining glycosidases that are active on α- or β-mannosides harness unusual B_2,5 (boat) transition states. Here we present the analysis of 25 putative β-mannosidase inhibitors, whose K_i values range from nanomolar to millimolar, on the Bacteroides thetaiotaomicron β-mannosidase BtMan2A. B_2,5 or closely related conformations were observed for all tightly binding compounds. Subsequent linear free energy relationships that correlate log K_i with log K_m/k_cat for a series of active center variants highlight aryl-substituted mannoimidazoles as powerful transition state mimics in which the binding energy of the aryl group enhances both binding and the degree of transition state mimicry. Support for a B_2,5 transition state during enzymatic β-mannosidase hydrolysis should also facilitate the design and exploitation of transition state mimics for the inhibition of retaining α-mannosidases—an area that is emerging for anticancer therapeutics. [ABSTRACT FROM AUTHOR]