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-1-0-5 bond. This demands planarity of C-5, 0-5, C-1 and C-2 at or near the transition state. It is widely, but incorrectly, assumed that the transition state must be H-4(3) (half-chair). The transition-state geometry is equally well supported, for pyranosides, by both the H-4(3) and H-3(4) half-chair and B-2,B-5 and B-2,B-5 boat conformations. A number of retaining,beta-glycosidases acting on gluco-configured substrates have been trapped in Michaelis and covalent intermediate complexes in S-1(3) (skew-boat) and C-4(1) (chair) conformations, respectively, pointing to a H-4(3)-conformed transition state. Such a H-4(3) conformation is consistent with the tight binding of E-4-(envelope) and H-4(3)-conformed transition-state mimics to these enzymes and with the solution structures of compounds bearing an sp(2) hybridized anomeric centre. Recent work reveals a S-1(5) Michaelis complex for beta-mannanases which, together with the S-0(2) covalent intermediate, strongly implicates a B2,s transition state for beta-mannanases, again consistent with the solution structures of manno-configured compounds bearing an sp(2) anomeric centre. Other enzymes may use different strategies. Xylanases in family GH-11 reveal a covalent intermediate structure in a B-2,B-5 conformation which would also suggest a similarly shaped transition state, while S-2(0)-conformed substrate mimics spanning the active centre of inverting cellulases from family GH-6 may also be indicative of a B-2,B-5 transition-state conformation. Work in other laboratories on both retaining and inverting alpha-mannosidases also suggests non-H-4(3) 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.