1. Glycosidases as a tool for the synthesis of glycosidic bond

The use of natural catalysts relies on their ability to promote very selective modifications difficult to obtain with chemical based methodologies Ref. 1-3. The need for well characterized carbohydrate-containing compounds for chemical, biological and technological Ref.2-7 studies has made the enzymatic synthesis of these molecules an alternative and efficient choice.

Different classes of enzymes have been used in the field of carbohydrate synthesis:

  • (i) lipases or proteases for the direct functionalization (protection/deprotection) of hydroxyl groups Ref. 8;
  • (ii) aldolases and similar enzymes for the synthesis of molecules with a new skeleton Ref. 2-3;
  • (iii) transferases and glycosidases for direct synthesis of the glycosidic bond Ref.1-3.

One great disadvantage of transferases is their limited availability in comparison to lower cost, wider availability and ease of use (i.e. without the need for expensive cofactors) of glycosidases; these latter remain however a class of non specific and low-yielding biocatalysts. Using mesophilic glycosidases the yields of reactions range from 10 to 15% for the equilibrium controlled synthesis and from 20 to 40% for kinetically controlled approach. Very recently a much higher yield of the reaction product has been reported using the acceptor, when it is liquid, as solvent at an elevated temperature Ref.9.The poor stability of the mesophilic enzymes in the presence of denaturing acceptor in the reaction mixture is, however, one of the major drawbacks of these enzymes Ref.6.Studies concerning substrate specificity of the enzyme(s) and selectivity of the reaction are of great interest in that the primary goal in these reactions is the selectivity of functionalization while keeping wide substrate accessibility into the active site of the enzyme. Different examples of functionalization of various hydroxyl groups by the use of mesophilic glycosidases are reported in literature Ref. 2,3. Some useful applications of these mesophilic enzymes in the field of modified monomers are however also recently reported Ref.6. Potential commercial importance of these biocatalysts remains in the field of production of different chemicals not easily synthesized by chemical routes: long-chain alkyl glycosides, synthetic flavor precursors, glucosides with a spacer arm on the anomeric carbon, precursors of glycolipids, glycosylated oligopeptides, natural products, etc., are examples reported in literature Ref.1-3.Among the others, the catalytic properties of thermophilic enzymes Ref.10 are of a particular interest in the field of biocatalysis where solutions to specific problems are requested.

Higher temperatures increase the solubility of many compounds as well as the diffusion rate, reduce the viscosity of the medium and allow removal of volatile products. Enzymes from thermophilic microorganisms are generally thermostable and thermophilic, also in presence of denaturating agents and organic solvents; moreover they could offer very often new types of activity and substrate specificity. Only few examples of application of thermophilic organisms are reported in literature despite the fact that a high number of thermophilic enzymes have been isolated and characterized Ref.10-12. A thermophilic and thermostable -glycosidase has been purified and characterized from Sulfolobus solfataricus, a thermophilic archaeon, and its gene has been cloned and expressed in E. coli and yeast. On the basis of substrate specificity studies it has been possible to define this enzyme as a true glycosyl hydrolase ref13.htm. Other thermophilic organisms, such as Thermus thermophilus, Pyrococcus furiosus and Caldocellum saccharolyticum, possess glycosidase activities of interest in the field of biocatalysis Ref. 10.

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