Glycogen core a glycogenin protein. Glycogen in muscle, liver, and fat cells is stored in a hydrated form, composed of three or four parts of water per part of glycogen associated with 0.45 millimoles (18 mg) of potassium per gram of glycogen. As a meal containing carbohydrates or protein is eaten and digested, blood glucose levels rise, and the pancreas secretes insulin. Blood glucose from the portal vein enters liver cells (hepatocytes). Insulin acts on the hepatocytes to stimulate the action of several enzymes, including glycogen synthase. Glucose molecules are added to the chains of glycogen as long as both insulin
Glycogen synthase glycogen synthase is highly conserved among species, glycogen synthase likely forms a tetramer in humans as well. Glycogen synthase can be classified in two general protein families. The first family (GT3), which is from mammals and yeast, is approximately 80 kDa, uses UDP-glucose as a sugar donor, and is regulated by phosphorylation and ligand binding. The second family (GT5), which is from bacteria and plants, is approximately 50 kDA, uses ADP-glucose as a sugar donor, and is unregulated. Although the catalytic mechanisms used by glycogen synthase are not well known, structural similarities to glycogen phosphorylase at the catalytic and substrate