Polysaccharides modification using enzymes

Other methods of stabilization include chemical or carbohydrate modification of enzymes. Modifications of reactive groups on proteins without insolubilization has been used to enhance stability in solution. Grafting of polysaccharides or synthetic polymers, alkalation, acetylation and amino acid modification have all been reported (5)... 

SCHEME 9.1 Biosynthesis of HS. (a) Elongation reactions to prepare the precursor polysaccharide backbone using both EXT1 and EXT2 enzymes, (b) Polysaccharide modifications. 

The modification of polysaccharides using enzymes has been studied to attain more uniformly substituted products under mild reaction conditions. Hydroxyethylcellulose (HEC) particles were suspended in dimethylacetamide and acylated with vinyl stearate using Candida antartica Lipase B (CAL-B) as the catalyst. After 48 hrs, a product with degree of substitution (D.S.) of 0.1 was formed(8). HEC in film or powder form was modified using lipase-catalysis and e-caprolactone (CL) to form low D.S. HEC-g-PCL copolymers(9). Problems that limit the utility of these reactions include the use of polar aprotic solvents that strip the critical water from enzymes lowering their activities(lO) and the use of heterogeneous reaction conditions that restrict the modification of large particles and films to a small fraction of the substrate that resides at die surface. 

Only a few methods for the specific degradation of polysaccharides were known when an earlier article on structural polysaccharide chemistry appeared in this Series.3 The Smith degradation,4 which has become the most frequently used, had only just been introduced. Since then, a number of specific degradation techniques have been developed. In addition, there have also been some modifications and improvements to existing methods. In this article, chemical methods for specific or selective degradation of polysaccharides, and their applications in structural analysis, will be discussed. Enzymic methods, which may be of considerable value, have already been treated in this Series.5... 

In a second class of regulatory enzymes the active and inactive forms are inter-converted by covalent modifications of their structures by enzymes. The classic example of this type of control is the use of glycogen phosphorylase from animal tissues to catalyse the breakdown of the polysaccharide glycogen yielding glucose-1-phosphate, as illustrated in Fig. 5.37. 

Recent trends in glycochemistry are presented in three parts synthesis, principles and applications. The first six chapters provide a comprehensive, up-to-date review on the chemical synthesis of complex carbohydrates for their potential use in biological systems. The following seven chapters reveal some fundamental principles that are used to design and exploit carbohydrates for their effects in biological settings. The remaining five chapters examine the applicability of enzymes towards the chemo-enzymatic synthesis and modification of carbohydrates and polysaccharides. 

Immobilization through inclusion into a gel, similarly to immobilization by adsorption, is a physical method of protein fixation. Advantages of such a method are its simplicity and the absence of chemical modification of the enzyme molecules. Polyacrylamide, polyethylene glycol methacrylate, polysaccharide, polyionite, as well as various inorganic gels are used. 

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