Biosynthesis of glycogen in Neurospora crassa. Kinetic mechanism of UDP-glucose: glycogen 4-alpha-glucosyltransferase

Abstract

The kinetic mechanism of glycogen synthase [UDP-glucose: glycogen 4-alpha-glucosyltransferase, EC 2.4.1.11], glucose-6-P-dependent form, from Neurospora crassa has been investigated by initial velocity experiments and studies with inhibitors in the presence of sufficient levels of glucose-6-P. The rate equation was different from those of common two-substrate systems because one of the substrates, glycogen, is also a product. The reaction rates were determined by varying the concentration of one of the substrates while keeping that of the other constant. Double-reciprocal plots of initial velocity measurements were linear and showed converging line patterns. UDP was found to act competitively when the substrate UDP-glucose was varied, but noncompetitively when glycogen was varied. On the basis of these results, it is concluded that glycogen synthase, glucose-6-P-dependent form, from N. crassa has a rapid equilibrium random Bi-Bi mechanism. Rate constant and dissociation constants for each step of this mechanism were estimated.