After a meal, blood glucose enters cells and is stored as glycogen, particularly in the liver. Which of the following is the donor of new glucose molecules in glycogen?
|D||Glucose -6- phosphate|
a. Upon entering cells, blood glucose is rapidly converted to glucose-6-phosphate by hexokinase or, in the case of the liver, by glucokinase. Glucose-6-phosphate is in equilibrium with glucose-1-phosphate via the action of phosphoglucomutase.
b. Glucose-1- phosphate is activated by UTP to form UDP-glucose, which is added to glycogen by an αl →4 linkage in the presence of glycogen synthase.
c. To increase the solubility of glycogen and to increase the number of terminal residues, glycogen-branching enzyme transfers a block of about 7 residues from a chain at least 11 residues long to a branch point at least 4 residues from the last branch point. The branch is attached by an αl→6 linkage.