Abstract
The action pattern and mechanism of the Taka-amylase A-catalyzed reaction were studied quantitatively and kinetically by product analysis, using a series of maltooligosaccharides from maltotriose (G3) to maltoheptaose (G7) labeled at the reducing end with 14C-glucose. A marked concentration dependency of the product distribution from the end-labeled oligosaccharides was found, especially with G3 and G4 as substrates. The relative cleavage frequency at the first glycosidic bond counting from the nonreducing end of the substrate increases with increasing substrate concentration.
Further product analyses with unlabeled and end-labeled G3 as substrates yielded the following findings: 1) Maltose is produced in much greater yield than glucose from unlabeled G3 at high concentration (73mM). 2) Maltooligosaccharides higher than the starting substrate were found in the hydrolysate of labeled G3. 3) Nonreducing end-labeled maltose (G*-G), which is a specific product of condensation, was found to amount to only about 4% of the total labeled maltose. Based on these findings, it was concluded that transglycosylation plays a significant role in the reaction at high concentrations of G3, although the contribution of condensation cannot be ignored.
A new method for evaluating subsite affinities is proposed; it is based on the combination of the kinetic parameter (k0/Km) and the bond-cleavage distribution at a sufficiently low substrate concentration, where transglycosylation and condensation can be ignored. This method was applied to evaluate the subsite affinities of Taka-amylase A.
Based on a reaction scheme which involves hydrolysis, transglycosylation and condensation, the time courses of the formation of various products were simulated, using the Runge-Kutta-Gill method. Good agreement with the experimental results was obtained.
