アミラ-ゼの逆合成反応と転移反応 (アミラ-ゼシンポジウム(1977)特集)
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概要
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By using the pure anomeric forms of substrates separately under conditions to limit mutarotation, it was found that the condensation reactions by amylases require donor substrates of specific configuration. That is, crystalline glucoamylase from Rhizopus niveus was found to catalyze the rapid synthesis of maltose and a slower synthesis of isomaltose specifically from β-D-glucopyranose. Crystalline sweet potato β-amylase, likewise, was found to catalyze the rapid synthesis of maltotetraose specifically from β-maltose, and crystalline hog pancreatic β-amylase the rapid synthesis of maltotetraose specifically from α-maltose. A rapid approach to equilibrium was found both in maltose synthesis from β-D-glucopyranose by glucoamylase, and in maltotetraose synthesis from β-maltose by β-amylase. Moreover, essentially the same equilibrium level (K<SUB>eq</SUB> =ca.0.13) was obtained. The configurational inversion accompanying both condensations reveals their mechanism as one of glycosyl transfer. Crystalline α-amylases from six different sources, as well as crude salivary amylase, were found to catalyze the synthesis of maltose and maltosaccharides from α-D-glucopyranosyl fluoride, a stereoanalog of α-D-glucopyranose. The entire group of α-amylases had the capacity to promote α-D-glucosyl transfer from α-D-glucosyl fluoride to C<SUB>4</SUB>-carbinol sites, demonstrating for the first time that α-amylases possess in common the capacity to catalyze glycosylation(i.e., glycosyl-hydrogen interchange) reaction extending beyond hydrolysis and its reversal. Similar de novo syntheses of maltosaccharides from α-maltosyl fluoride by α-amylases were also discussed.
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