Sugar isomerases

Many sugar isomerases are activated by Three Mn " " activated sugar isomerases have been... 

Sugar isomerases catalyze the interconversion between aldose and ketose. Tautomerases catalyze a keto-enol tautomerization. A-Isomerases catalyze the shift of a double bond. The reactions catalyzed by these enzymes proceed through intramolecular oxidation and reduction. 

This hydride shift reaction mechanism is quite different from the base-catalyzed enolization mechanism proposed for phospho sugar isomerases such as triosephos-phate isomerase which generally do not require a metal ion for activity12261. 

A more general access to biologically important and structurally more diverse aldose isomers makes use of ketol isomerases for the enzymatic interconversion of ketoses to aldoses. For a full realization of the concept of enzymatic stereodivergent carbohydrate synthesis, the stereochemically complementary i-rhamnose (Rhal EC 5.3.1.14) and i-fucose isomerases (Fuel EC 5.3.1.3) from E. coli have been shown to display a relaxed substrate tolerance [16,99,113,131]. Both enzymes convert sugars and their derivatives that have a common (3 J )-OH configuration, but may deviate in... 

The glycolytic enzyme triosephosphate isomerase (TIM) catalyzes the interconversion of the sugar phosphates dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde 3-phosphate (GAP). Although the reaction is extremely fast, the enzyme is an attractive candidate for mapping... 

The enzyme is also being studied for use in converting of biomass to ethanol for fuel usage. Prospects for the conversion of cellulolytic biomass to ethanol for fuel or as a fuel additive have improved within the last decade because of the development of methods for the fermentation of xylose, which can comprise as much as 50% of the fermentable sugars in these feedstocks. One of these methods uses xylose isomerase to convert xylose, which is difficult to ferment by ethanol-tolerant yeasts, to the fermentable sugar xylulose (12,13). 

L-Rhamnose isomerase catalyzes the interconversion of L-rhamnose and L-rhamnulose. L-Rhamnose, a deoxy sugar, is found in bacteria and plants and it plays an essential role in many pathogenic bacteria. The pathway for the metabolism of this sugar does not exist in humans, and this makes enzymes of this pathway attractive targets for therapeutic intervention. " Rhamnose isomerase from E. coli is a tetramer of (/ /a)8-barrels similar to xylose... 

Figure 1. Synthesis of usual and unusual sugars using FDP aldolase and glucose isomerase as catalysts.

Combined Use of Aldolases and Isomerases for the Synthesis of Natural and Unnatural Sugars... 

However, the applicability of this strategy is limited by the substrate specificity of the isomerases so that only a fraction of the ketoses that can be obtained from the aldose-catalyzed reaction can be enzymatically isomerized to the corresponding aldose. Moreover, the isomerization reaction is reversible and, as a ketone is generally more stable than an isomeric aldehyde, the equilibrium produces substantial aldose isomer only if the aldose sugar can exist in a very stable aldopyra-nose form [38b]. 

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