KINETIC MECHANISM OF MALTODEXTRIN PHOSPHORYLASE

The kinetic mechanism of Escherichia coli maltodextrin phosphorylase has been investigated by (1) initial velocity experiments, (2) studies with inhibitors, and (3) isotopic exchange measurements at equilibrium. Rate equations have been derived for various mechanisms. These equations are different from the common two substrate systems because one of the substrates, polysaccharide, serves as a reactant in both the forward and reverse reactions. Double-reciprocal plots of initial velocity measurements were linear and showed converging line patterns. Inhibition by maltotetraose was competitive with respect to maltoheptaose and mixed with respect to Pi; uridine diphosphate glucose was competitive with respect to Pi and mixed with respect to maltoheptaose. The equilibrium exchange rates for 32Pi ⇄ glucose 1-phosphate and [14C]glucose 1-phosphate ⇄ maltoheptaose increased when the concentrations of Pi and glucose 1-phosphate were increased (in constant ratio) at a fixed concentration of maltoheptaose. The exchanges also increased as a function of maltoheptaose concentration at a constant glucose 1-phosphate and Pi. The data are consistent with a rapid equilibrium random Bi-Bi kinetic mechanism for E. coli phosphorylase. © 1969, American Chemical Society. All rights reserved.