Abstract
By using the substrates of defined anomeric form, with very brief incubation to limit mutarotation, evidence has been obtained that the condensations require donor substrates of specific configuration. That is, crystalline glucoamylase from Rh, niveus has been 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, has been found to catalyze the rapid synthesis of maltotetraose specifically from β-maltose, and crystalline hog pancreatic α-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 (Keq=ca. 0.13) was reached by these homologous hemiacetalto-secondary carbinol condensations. The configurational inversion accompanying both condensations, finally, reveals their mechanism as one of glycosyl transfer. Crystalline a-amylases from six different biological sources, as well as crude salivary amylase, were examined and found to catalyze the synthesis of maltose and maltosaccharides from α-D-glucopyranosyl fluoride, a stereoanalog of α-D-glucopyranose. The entire group of a-amylases had the capacity to promote α-D-glucosyl transfer from α-D-glucosyl fluoride to C4-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 synthesis of maltosaccharides from α-maltosyl fluoride by a-amylases was also discussed.
