Comparison between Glucoamylase and α-Glucosidase

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  • CHIBA Seiya
    Department of Agricultural Chemistry , Faculty of Agriculture, Hokkaido University

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  • グルコアミラーゼとα‐グルコシダーゼの比較
  • グルコアミラーゼ ト アルファ グルコシダーゼ ノ ヒカク

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The difference between glucoamylase [EC 3.2.1.3 exo-1, 4-α-D-glucosidase] and α-glucosidase [EC 3.2.1.20 α-glucosidase] was discussed on the basis of the kinetic parameters, Km, ko(-V/eo; eo, enzyme concentration), and ko/Km, for maltooligosaccharides . In glucoamylase, there is a large difference between the ko values of maltose and those of other maltooligosaccharides. In α-glucosidases, however, the ko values are little dependent on the degree of polymerization of glucosyl residues in a series of maltooligosaccharides. The active site of α-glucosidase, like glucoamylase, have been also considered to be made up by the subsite structure. The subsite affinities of three kinds of glucoamylases were compared with those of five kinds of α-glucosidases from various origins. The difference in the substrate specificities between glucoamylase and aglucosidase was reasonably interpreted by their subsite affinities. In the "Enzyme Nomenclature, " the distinction between glucoamylase and a group of α-glucosidases capable of attacking α-glucan is not always clear, that is, lysosomal a-glucosidase and acid maltase, which usually mean mammalian acid α-glucosidase, are classified into the category of EC 3.2.1.3. However, it has been reported that the acid a-glucosidases from pig liver and rabbit muscle also produce α-glucose. Glucoamylase can be definitely distinguished from α-glucosidase in their anomeric forms of the product glucose: the former releases J3-anomer, and the latter releases a-anomer. Therefore, it appears to be not proper that certain kinds of α-glucosidase are included in the group of EC 3.2.1.3, and also that the trivial name recommended for glucoamylase is exo-1, 4-α-D-glucosidase. The term glucoamylase itself may be more proper for the trivial name. The reaction mechanisms of glucoamylase and α-glucosidase were proposed, by which the mechanisms of hydrolysis, transglucosylation and reverse reaction were discussed . Moreover, the reason why the configuration of the carbonyl carbon of the product glucose is retained or inverted was explained.

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