Mapping the conformational itinerary of β-glycosidases by X-ray crystallography

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 H3 (half-chair). The transition-state geometry is equally well supported, for pyranosides, by both the H3 and H4 half-chair and 2,5B and B2,5 boat conformations. A number of retaining β-glycosidases acting on gluco-configured substrates have been trapped inMichaelis and covalent intermediate complexes in S3 (skew-boat) and C1 (chair) conformations, respectively, pointing to a H3-conformed transition state. Such a H3 conformation is consistent with the tight binding of 4E(envelope) and H3-conformed transition-state mimics to these enzymes and with the solution structures of compounds bearing an sp2 hybridized anomeric centre. Recent work reveals a S5 Michaelis complex for β-mannanases which, together with the S2 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 S0-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-H3 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