The conformational agenda harnessed by different glycosidases along the reaction pathway has been mapped by X-ray crystallography. The transition state(s) formed during the enzymic hydrolysis of glycosides features strong oxocarbenium-ion-like character involving delocalization across the C-1–O-5 bond. This demands planarity of C-5, O-5, C-1 and C-2 at or near the transition state. It is widely, but incorrectly, assumed that the transition state must be 4H3 (half-chair). The transition-state geometry is equally well supported, for pyranosides, by both the 4H3 and 3H4 half-chair and 2,5B and B2,5 boat conformations. A number of retaining β-glycosidases acting on gluco-configured substrates have been trapped in Michaelis and covalent intermediate complexes in 1S3 (skew-boat) and 4C1 (chair) conformations, respectively, pointing to a 4H3-conformed transition state. Such a 4H3 conformation is consistent with the tight binding of 4E- (envelope) and 4H3-conformed transition-state mimics to these enzymes and with the solution structures of compounds bearing an sp2 hybridized anomeric centre. Recent work reveals a 1S5 Michaelis complex for β-mannanases which, together with the 0S2 covalent intermediate, strongly implicates a B2,5 transition state for β-mannanases, again consistent with the solution structures of manno-configured compounds bearing an sp2 anomeric centre. Other enzymes may use different strategies. Xylanases in family GH-11 reveal a covalent intermediate structure in a 2,5B conformation which would also suggest a similarly shaped transition state, while 2S0-conformed substrate mimics spanning the active centre of inverting cellulases from family GH-6 may also be indicative of a 2,5B transition-state conformation. Work in other laboratories on both retaining and inverting α-mannosidases also suggests non-4H3 transition states for these medically important enzymes. Three-dimensional structures of enzyme complexes should now be able to drive the design of transition-state mimics that are specific for given enzymes, as opposed to being generic or merely fortuitous.
Analysis of transition state mimicry by tight binding aminothiazoline inhibitors provides insight into catalysis by human O-GlcNAcase
