Monosaccharide synthesis using aldolases

It is clear, then, that perhaps no other group of compounds is as well suited to enzyme-based synthesis as is the carbohydrates. During the past several years, we have developed a range of enzyme-based syntheses for mono-, oligo-, and polysaccharides. This paper reviews some of our efforts on the use of aldolases and transketolase for monosaccharide synthesis, as well as the use of Leloir and non-Leloir pathway enzymes for glycosidic bond formation. 

This chapter describes recent developments in carbohydrate synthesis using microbial aldolases. Recombinant type II fructose-1,6> diphosphate aldolase and type I 2-deoxyribose-5-phosphate aldolase have been exploited for the synthesis of monosaccharides and analogs. A new procedure for the synthesis of dihydroxy acetone phosphate has been developed. Sialic acid aldolase has been used in conjunction with other enzyme-catalyzed modifications of monosaccharides for the synthesis of sialic acid-related sugars. 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

The dephosphorylation of 5-chloro and 5-bromo-D-xylulose-l-phosphate was carried out by the addition of acid phosphatase. After purification, 5-chloro-D-xylu-lose and 5-bromo-D-xylulose were recovered as pure compounds in 47 and 12% yields, respectively, from DHAP. In this study, we have shown that DHAP generated from glycidol 7 can be used in situ as a donor substrate of FruA in the presence of 2-halo-acetaldehydes 20 as acceptor substrates for the synthesis of 5-halo-D-xylulose 19. Given that DHAP aldolases display a broad specificity towards acceptor substrates, this strategy can be applied generally to the synthesis of various analogs of monosaccharides. 

Dihydroxyacetone phosphate-dependent aldolases (DHAP-aldolases) have been used widely for preparative synthesis of monosaccharides and sugar analogs (Fessner and Walter 1997 Wymer and Toone 2000 Silvestri et al. 2003). Among them, RAMA RhuA and FucA from E. coli are the most available aldolases, especially the former which was one of the first to be commercialized (Fessner and Walter 1997 Takayama et al. 1997). In many of the chemo-enzymatic strategies they are involved, the biocatalytic aldol addition to the configuration of the newly stereogenic centers is fixed by the enzyme. However, pertinent examples have been reported in which... 

A comprehensive review (260 refs.) on the synthesis of carbohydrates from noncarbohydrate sources covers the use of benzene-derived diols and products of Sharpless asymmetric oxidation as starting materials, Dodoni s thiazole and Vogel s naked sugar approaches, as well as the application of enzyme-catalysed aldol condensations. The preparation of monosaccharides by enzyme-catalysed aldol condensations is also discussed in a review on recent advances in the chemoenzymic synthesis of carbohydrates and carbohydrate mimetics, in parts of reviews on the formation of carbon-carbon bonds by enzymic asymmetric synthesis and on carbohydrate-mediated biochemical recognition processes as potential targets for drug development, as well as in connection with the introduction of three Aldol Reaction Kits that provide dihydroxyacetone phosphate-dependent aldolases (27 refs.). A further review deals with the synthesis of carbohydrates by application of the nitrile oxide 1,3-dipolar cycloaddition (13 refs.). ... 

A potential limitation on the use of FDP aldolases for the synthesis of monosaccharides is that the products are always ketoses with fixed stereochemistry at the newly generated chiral centers on C-3 and C-4. There are, however, methods for establishing this stereochemistiy at other centers and for obtaining aldoses instead of ketoses. This technique makes use of a monoprotected dialdehyde as the acceptor... 

The monosaccharide sialic acid remains relatively costly for preparative-scale synthesis. Although it has been demonstrated that NeuAc can be generated in situ by reaction of ManNAc with pyruvate, catalyzed by NeuAc aldolase [27], ManNAc is also relatively expensive and difficult to prepare. Therefore, a method for the generation of NeuAc from the inexpensive monosaccharide GlcNAc in a two-enzyme system might also prove useful in regeneration schemes. It has been shown that GlcNAc can be converted to ManNAc chemically [28], or enzymatically by the... 

---