Starch is a major storage product of several economically important crops and the most common carbohydrate in human diets. A variety of enzymes are capable of starch hydrolysis and a large-scale starch processing industry has emerged in the last century. Enzymatic production of dextrose/glucose, maltose and high fructose syrups is increasing day by day as we have seen a shift from the use of traditional cane sugar to these sweeteners all over the world. The best known starch-processing enzymes are α-amylase, β-amylase and glucoamylase. Among starch-processing enzymes, a group whose functions are comparatively less well understood, are 4-α-glucanotransferases. In this review, we report on the classification of starch-processing enzymes based on the amino acid sequence and structural similarity as well as substrate specificity and reaction mechanism with emphasis on 4-α-glucanotransferases. Furthermore, applications of thermostable starch-processing enzymes are discussed.
Genome sequence of hyperthermophilic archaeon Pyrobaculum calidifontis revealed the presence of an open reading frame, Pcal_0768, corresponding to a putative 4-α-glucanotranferase belonging to glycoside hydrolases (GH) family 77. We have produced, in Escherichia coli, and purified recombinant Pcal_0768 which exhibited high disproportionation (690 U mg(-1)) activity. To the best of our knowledge, this is the highest ever reported activity for any member of family GH77. Maltooligosaccharides, when used as sole substrates, were disproportionated into linear maltooligohomologues. The analysis of the reaction end products revealed no evidence for the production of cycloamyloses. Catalytic activity of the enzyme remained unchanged in the presence or the absence of ionic and nonionic detergents. γ-cyclodextrin, an inhibitor of 4-α-glucanotransferases, did not show any inhibitory effect on Pcal_0768 activity. These properties make Pcal_0768 a potential candidate for starch processing industry.
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