Conformationally restricted glycoside derivatives as mechanistic probes and/or inhibitors of sugar processing enzymes and receptors - Carbohydrate Chemistry: Chemical and Biological Approaches - page 424

competitive inhibitor of cellobiohydrolase Cel6A from Humicola insolens
that cleaves b1,4 linkage of polymers of glucose with inversion of
anomeric configuration, displays a distorted conformation close to 2,5 B
upon binding. 42 A 0.8 Å-resolution structure of an unrelated inverting
b-glucosidase, the endoglucanase Cel(8)A from Clostridium thermocellum
in complex with cellopentaose also reveals a 2,5 B conformation. 43 This is
also the case for Cel5A from Bacillus agaradhaerens. 7b The 2,5 B con-
formation is attained more easily for xylosides than for glucosides owing
to the lack of the hydroxymethyl substituent at C5. Indeed, a 2,5 B con-
formation has been trapped for the xylosyl unit of a covalently linked
a-2-fluoro-xylobioside-enzyme intermediate located in the 1 subsite of
the GH-11 b-xylanases family active site, and has been proposed for the
corresponding transition state. 44 This hypothesis has been further sup-
ported by MM-PBSA free energy analysis 45 of Xyl11-substrate complexes
and by molecular dynamic simulations using hybrid QM/MM method-
ology applied to non-covalent complexes of phenyl b-xyloside with BCX
xylanase and a Tyr69Phe mutant. 43 In addition, Bl´riot and co-workers
hypothesized that a xyloside analogue locked in a 2,5 B conformation
should be cleaved at a high rate as the energy penalty required to go from
the chair to the TS conformation is already paid. They synthesized xylo-
side analogues 26 and 27 based on a 2-oxabicyclo[2.2.2]octane framework
that proved to hydrolyze 100-1200 times and 2000 times faster than the
parent sugar respectively. 46 Early on, Hosie and Sinnott reported that yeast
a-glucosidase also proceeds via a 2,5 B TS conformation. 47 To further probe
this conformation, Bennet and co-workers synthesized a bicyclo[2.2.2]
analogue of the glycosidase inhibitor 1-deoxynojirimycin (DNJ), the
2,6-anhydro-1-deoxymannojirimycin 28 that weakly inhibited several gly-
cosidases despite lacking the 2-OH and 6-OH present in DNJ (Fig. 9). 48

B 2,5 CONFORMATION. Half-chair conformation is not compatible with
b-mannopyranoside hydrolysis as it results in an unfavourable syn-diaxial
orientation for the 2-OH group and the anomeric leaving group, an issue
that can be solved by adopting a B 2,5 TS conformation that places the
2-OH group pseudoequatorial. Such conformation has been demonstrated
in the case of GH26 49 and BtMan26A 50 b-mannosidases but also in the case
of Golgi 51 and GH38 a-mannosidases. 51 Partial support also came from the
fact that some mannosidase inhibitors, sp 2 hybridized at the anomeric
position such as mannono-d-lactone 29 14 and mannono-d-lactame 30 52
and nanomolar inhibitor mannoamidine 31 53 also adopt this unusual boat
conformation in solution (Fig. 10).
In the case of BtMan26A, screening of 25 putative inhibitors of this
enzyme showed that the tight binding compounds displayed a B 2,5 or

NH

O O O

HO HO HO

OMe

HO HO HO

HO OMe HO

26 27 28

Fig. 9 Structures of bicycles 26-28.