Glycosidase Catalytic Flexibility Catalysis of Substrates with Very Small Aglycon

Glycosidase Catalytic Flexibility Catalysis of Substrates with Very Small Aglycon 

Evidence of stereochemically unique results first emerged from analysis of jS- 

The proposed mechanism of a- and /3-glycosyl fluoride hydrolysis (55) is based on the assumption that catalysis requires a general acid and anionic base on opposite faces of the substrate-reactive center. The assumption is fully consistent with the operative mechanism of lysozyme, modified to catalyze a reaction with anomeric inversion. To continue the example of /3-xyIosidase, hydrolysis of the usual anomeric substrate (/3-D-xylosyl fluoride) involves water attack at the reactive carbon from the si face or below C-1. The general acid, situated above C-1 on the re face, protonates the fluoride (or glycoside oxygen) as it departs from the /3-position (57, 92) [Eq. (1)]. 

Hydrolysis of the stereochemically incompatible substrate, a-D-xylosyl fluoride, generates the anomeric retained a-D-xylose by a mechanism that is somewhat more complex. The reaction is a two-step process that involves free xylose, and requires that the general acid and base switch roles. Following formation of the a-D-xylosyl fluoride Michaelis complex, free xylose (rather than water) attacks from the re face with the aid of what is now the general base a-linked fluoride is displaced with assistance from proton donation at the si face. 

The transfer generates 4-0-)8-D-xylopyranosyl-D-xylopyranose. The disaccha-ride is a reactive substrate and rapidly hydrolyzes to a-D-xylose [Eq. (2)] (87). 

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IV. Glycosidase Catalytic Flexibility Catalysis of Substrates with Very Small Aglycon. .. 198... 

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