グルコアミラーゼの生澱粉分解作用と基質特異性

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タイトル別名
  • Raw-starch-digesting activity and kinetic properties of glucoamylases.
  • グルコアミラーゼ ノ ナマ デンプン ブンカイ サヨウ ト キシツ トクイセイ

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The raw-starch-digesting activities of five glucoamylases, one each from Aspergillus niger, and Aspergillus sp. K-27, and three forms from Rhizopus delemar, GI, GII and GIII, were discussed as to the kinetic parameters (Km and Vmax) for maltooligosaccharides, amylopectin and glycogen, and their molecular structures. The R. delemar GIII and Aspergillus sp. K-27 enzymes, which showed strong raw starch digesting activity, showed much lower Km values for amylopectin and glycogen than the A. niger and R. delemar GI and GII enzymes, which lack raw-starchdigesting activity. The R. delemar GI and GII enzymes were suggested to be the products of limited proteolysis of the GIII enzyme after biosynthesis from the results of amino acid and carbohydrate analyses of these three forms and the partial proteolysis of the GIII enzyme with chymotrypsin. From these results and other lines of evidence, “starch binding site” other than the active site on the raw-starch-digesting enzyme was postulated. It is considered that this site shows high affinity to high molecular substrates and raw starch, and is involved in the hydrolyses of raw starch granules, amylopectin and glycogen, and the absorption on starch granules. The amino acid composition of this site of the GIII enzyme was suggested to be rich in serine, tyrosine and aspartic acid (including asparagine).<br>All glucoamylases hydrolyze starch from its non-reducing end and their action stops at the C-6-phosphorylated residues and the residue just before the C-3-phosphorylated residue. Therefore, they are unable to completely hydrolyze potato starch, -20%, depending on the amount of the phosphate, remaining as limit dextrin. It is concluded that the cooperative action of α-amylase and phosphatase is essential for the complete degradation of starch.<br>Aspergillus sp. K-27 produces a strong raw starch degrading enzyme, which is a mixture of glucoamylase and α-amylase, with an activity ratio of 7:3. The glucoamylase alone showed strong activity toward various raw starches, including potato starch, but the activity was enhanced synergistically with the addition of a small amount (1%) of the α-amylase. However, this synergistic action was observed also more or less with other α-amylases, Taka-amylase, and Bacillus subtilis liquefying and saccharogenic α-amylases.<br>The relationship between the structure and function of the “starch binding site” and the mechanism of the synergistic action of glucoamylase and α-amylase as to raw starch degradation is an interesting problem for future studies.

収録刊行物

  • 澱粉科学

    澱粉科学 34 (2), 98-105, 1987

    日本応用糖質科学会

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