Biosynthesis of glycogen in Neurospora crassa. Purification and properties of the UDPglucose:Glycogen 4-α-glucosyltransferase

Abstract

The Neurospora crassa glycogen synthase (UDPglucose:glycogen 4-α-glucosyltransferase, EC 2.4.1.11) was purified to electrophoretic homogeneity by a procedure involving ultracetrifugation, DEAE-cellulose column chromatography, (NH 4) 2SO 4 fractionation and 3-aminopropyl-Sepharose column chromatography. The final purified enzyme preparation was almost entirely dependent on glucose-6-P and had a specific activity of 6.9 units per mg of protein. The subunit molecular weight of the glycogen synthase was determined by electrophoresis in sodium dodecyl sulfate-polyacrylamide gel to be 88 000–90 000. The native enzyme was shown to have a molecular weight of 270 000 as determined by sucrose density gradient centrifugation. Thus, the glucose-6-P- independent form of the N. crassa glycogen synthase can exist as trimer of the subunit. Limited proteolysis with trypsin or chymotrypsin converted the glucose-6-P- dependent form of the enzyme into an apparent glucose-6-P- dependent form. The enzyme was shown to catalyze transfer of glucose from UDPglucose to glycogen as well as to its phosphorylase limit dextrin, but not to its β-amylase limit dextrin. Moreover, glucose, maltose and maltotriose were not active as acceptors.