Kinetic Mechanism of Glycogen Synthase D from Human Polymorphonuclear Leukocytes

The steady state bisubstrate kinetics of the reaction UDP-glucose + glycogen ⇄ UDP + glycogen catalyzed by glycogen synthase D (UDP-glucose:glycogen α-4-glucosyltransferase, EC 2.4.1.11) was studied, using 14C isotope transfer rates. The reaction rates were determined at varying concentrations of either substrate at constant amounts of the other substrate. The inhibition patterns of UDP as well as the activation patterns of the activator glucose 6-phosphate toward either substrate were determined. UDP was found to act competitively when the substrate UDP-glucose was varied, and noncompetitively when glycogen was the varied substrate, while glucose 6-phosphate showed an intersecting pattern toward glycogen and an equilibrium ordered pattern toward UDP-glucose. The reciprocal plots were found to be linear in all cases. On the basis of the results, it is concluded that glycogen synthase D from human polymorphonuclear leukocytes has a rapid equilibrium random bi-bi mechanism, in which the attachment of the activator glucose 6-phosphate is a necessary prerequisite for the addition of the substrate UDP-glucose (and UDP in the reverse reaction).