Enzymatic synthesis carbohydrates

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... 

Cells grown in the presence of azide analogs of certain amino acids or sugars will incorporate these derivatives into proteins or carbohydrates through enzymatic synthesis using... 

The fhioethyl group present in the anchor group of (40) was activated by treatment with N-iodosuccinimide (NIS) followed by displacement with a variety of alcohols (44-46). To prove the possible application of this linker in solid phase carbohydrate synthesis, protected glycosides (47) and (48) were coupled to linker (40) and released enzymatically. Flitsch et al. also described the immobilization and enzymatic cleavage on a variety of amines [41]. Nevertheless, the application of this enzyme-labile hnker group in multi-step syntheses on the solid phase and subsequent enzyme-initiated release from the polymeric support has not been described yet. 

Several enzymatic procedures have been developed for the synthesis of carbohydrates from acyclic precursors. Aldolases appear to be useful catalysts for the construction of sugars through asymmeteric C-C bond formation. 2-deoxy-KDO, 2-deoxy-2-fluoro-KDO, 9-0-acetyl sialic acid and several unusual sugars were prepared by a combined chemical and enzymatic approach. Alcohol dehydrogenases and lipases have been used in the preparation of chiral furans, hydroxyaldehydes, and glycerol acetonide which are useful as building blocks in carbohydrate synthesis. 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

The most widely adopted methods for carbohydrate synthesis utilize either enzymatic or chemical approaches. Enzymatic carbohydrate synthesis is a vibrant area of research that has been reviewed thoroughly and will... 

U. Kragl, D. Gygax, O. Ghrsalba, and C. Wandrey, Enzymatic two-step synthesis of V-acetyl-neuraminic acid in the enzyme membrane reactor, Angew. Chem. Int. Ed. Engl. 30 827 (1991). M.-J. Kim, W. J. Hennen, H. M. Sweers, and C.-H. Wong, Enzymes in carbohydrate synthesis W-Acetylneuraminic acid aldolase catalyzed reactions and preparation of 7V-acetyl-2-deoxy-D-neuraminic acid derivatives, J. Am. Chem. Soc. 770 6481 (1988). 

Advances in molecular biology have unquestionably diminished most of the restrictions in enzymatic s)mthesis. nourishing cloning and isolating GTs from different sources, particularly bacteria [18,19,20], constructing mutated and fusion enzymes [21] with more favorable properties for s)mthesis than their parent proteins, as well as, bacterial engineering [22], have continuously broadened the GTs utilization in carbohydrate synthesis. 

In research, incremental increases in the capability of research tools have made the localization and functional characterization of glyconjugates possible. Hints at how to solve the many carbohydrate synthesis problems can be gleaned from the developments made in solid and solution phase as well enzymatic... 

Carbohydrate synthesis usually involves selective protection of some groups, reaction of the others, then deprotection. The enzymatic aldol reaction can produce some sugars without the need for any protecting groups.251 An example is the preparation of 6-deoxy-L-sorbose (9.39).252... 

As is evident from the work that is reviewed here, the application of biocatalysts in preparative carbohydrate synthesis has continued to expand and diversify in recent years. Of particular note is the reengineering of biocatalyst properties (e.g., substrate specificity, stability) by the process of directed evolution. Significant progress in the study of the enzymatic biosynthesis of complex carbohydrate has been made with the development in protein purification, molecular genetics, and new methods of enzymological analysis. Bioinformatics provides a large number of putative candidates for carbohydrate-active enzymes. The combined... 

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