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Nature Chemical Biology 4, 306–312 (1 May 2008) | doi:10.1038/nchembio.81

Structural and biochemical evidence for a boat-like transition state in |[beta]|-mannosidases

Louise E Tailford , Wendy A Offen , Nicola L Smith , Claire Dumon , Carl Morland , Julie Gratien , Marie-Pierre Heck , Robert V Stick , Yves Bl|[eacute]|riot , Andrea Vasella , Harry J Gilbert & Gideon J Davies

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 B2,5 (boat) transition states. Here we present the analysis of 25 putative β-mannosidase inhibitors, whose Ki values range from nanomolar to millimolar, on the Bacteroides thetaiotaomicron β-mannosidase BtMan2A. B2,5 or closely related conformations were observed for all tightly binding compounds. Subsequent linear free energy relationships that correlate log Ki with log Km/kcat 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 B2,5 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.