Chapter 7 - Enzymatic Cleavage of Glycosides: Mechanism, Inhibition and Synthetic Applications

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This chapter focuses on mechanism, inhibition, and application of glycoside hydrolases as enzymatic cleavage of glycosides. Over the years, many conflicting mechanisms for catalysis by glycoside hydrolases have been proposed. There is now convincing evidence for detailed mechanisms that apply to the major classes of inverting and retaining glycoside hydrolases. Glycoside hydrolases may also be usefully classified into endo- or exo-glycoside hydrolases. This classification is made on the basis of whether a glycoside hydrolase cleaves an internal glycosidic linkage in a chain (endo) or at the end of a chain (exo; most commonly the nonreducing end, but not always). Whether an enzyme is exo- or endo-acting has no relationship to the mechanism used—thus cellulases, for example, can be endo- or exo-acting and retaining or inverting β-glucanases. Inverting glycoside hydrolases are arguably the simplest mechanistic class of enzymes that cleave the glycosidic linkage. These enzymes act through a one-step mechanism that involves strategically positioned active-site base and acid residues. A single molecule of water is positioned such that deprotonation by the base residue occurs simultaneously to formation of a new bond to the anomeric center and cleavage of the bond to the aglycon, which occurs with general acid assistance from the acid residue. The major glycoside hydrolase inhibitors are nitrogen-containing “sugar-shaped” heterocycles. Many of these compounds have been found in nature, and others have been designed based on considerations of enzyme mechanisms. Another group of interesting glycoside hydrolase inhibitors is those that contain a cyclitol ring. The most significant of these compounds is acarbose, which was originally isolated from an Actinoplanes sp. strain.

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