Abstract
Kinetic studies of two glucosylation reactions catalyzed by an amyloglucosidase from Rhizopus sp. leading to the synthesis of vanillin-α/β-D-glucoside from D-glucose and vanillin and curcumin-bis-α-D-glucoside from D-glucose and curcumin were investigated in detail. Initial reaction rates were determined from kinetic runs involving different concentrations of D-glucose and vanillin (5 mM to 0.1 M) or D-glucose and curcumin (5 mM to 0.1 M). Graphical double reciprocal plots showed that the kinetics of the two enzyme catalyzed reactions exhibited Ping-Pong Bi-Bi mechanism where competitive substrate inhibition by vanillin/curcumin led to dead-end amyloglucosidase–vanillin/curcumin complexes at higher concentrations of vanillin/curcumin. An attempt to obtain the best fit of this kinetic model through computer simulation yielded in good approximation, the values of four important kinetic parameters, vanillin-α/β-D-glucoside: kcat=35.0±3.2 10−5M h−1·mg, Ki=10.5±1.1 mM, KmD-glucose=60.0±6.2 mM, Kmvanillin=50.0±4.8 mM; curcumin-bis-α-D-glucoside: kcat=6.07±0.58 10−5M h−1·mg, Ki=3.0±0.28 mM, KmD-glucose=10.0±0.9 mM, Kmcurcumin=4.6±0.5 mM.