Carbohydrate enzymatic catalysis

Considering oxidations employing air or molecular oxygen as the oxidizing reagent, the reactivity of different chemical groups in carbohydrates can be predicted on the basis of thermodynamic and kinetic models. Under ambient conditions, monomeric molecules can be more easily oxidized than polymeric materials but, in any case, the kinetics are very slow, and most of the processes require catalysis. On the basis of reaction models underpinned by experimental data, the reactivity of carbohydrates is more predictable in the case of chemical catalysis than in the case of enzymatic catalysis. 

Both enzymatic catalysis and chemical catalysis can be applied under proper experimental conditions in order to prepare the desired product, and literature data may assist in making the best choice. According to the pioneering work of Heyns and Paulsen, the chemical oxidation of carbohydrates with finely divided platinum is first directed toward the anomeric carbon. 

Enzymatic mode. Synthesis of some carbohydrates has been also carried out by the use of enzymatic catalysis.Sugar type and name of enzyme used have been collected in Table 4. 

Several improvements of the process, such as the reaction with acyl chlorides or the application of two-phase reaction systems with propylene glycol and an emulsifier in order to build a microemulsion, have been described in the literature and patents [21]. Another approach for the synthesis of sugar esters is the use of enzymes. Enzymatic catalysis in the field of carbohydrate chemistry has been actively explored over years in laboratory... 

Special attention is given to the integration of biocatalysis with chemocatalysis, i.e., the combined use of enzymatic with homogeneous and/or heterogeneous catalysis in cascade conversions. The complementary strength of these forms of catalysis offers novel opportunities for multi-step conversions in concert for the production of speciality chemicals and food ingredients. In particular, multi-catalytic process options for the conversion of renewable feedstock into chemicals will be discussed on the basis of several carbohydrate cascade processes that are beneficial for the environment. 

Catalytic aldol reactions are among the most useful synthetic methods for highly stereo-controlled asymmetric synthesis. In this account we discuss the recent development of a novel synthetic technique which uses tandem enzyme catalysis for the bi-directional chain elongation of simple dialdehydes and related multi-step procedures. The scope and the limitations of multiple one-pot enzymatic C-C bond formations is evaluated for the synthesis of unique and structurally complex carbohydrate-related compounds that may be regarded as metabolically stable mimetics of oligosaccharides and that are thus of interest because of their potential bioactivity. 

About 3000 plant species are known to release HCN from their tissues, a process which is known as cyanogenesis[17, 181. Storage compounds are cyanohydrins where the hydroxy function is glycosylated to a carbohydrate or protected as a fatty acid ester. The plant defence mechanism in the case of sugar compounds is a two-step reaction. Initially a glycosidase liberates the cyanohydrin moiety, which is cleaved either spontaneously by base catalysis or enzymatically by the action of oxynitrilases to release the corresponding carbonyl compound and HCN1191. 

Evidently, enzyme catalysis is thus most attractive for the synthesis and modification of biologically relevant classes of organic compounds that are typically complex, multifunctional, and vater soluble. Typical examples are those structurally related to amino acids [16, 17] or carbohydrates [18-24], vhich are difficult to prepare and handle by conventional methods of chemical synthesis. Because of the multitude of factors that might be critical to the success of an enzymatic conversion, and because of the empirical nature of their development, it is mandatory in the design of new biocatalytic processes to become familiar vith the scope and limitations of synthetically useful enzymes, both as a source of inspiration and for reference. Thus, this overview attempts to outline the current status of development for the most important aldolase biocatalysts and their preparative potential for asymmet-... 

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