glycogen needs to be converted back to glucose for energy.
glycogen needs to be converted back to glucose for energy.
when would glycogen need to be converted back to glucose?
for energy
glycogen is a molecular polymer of glucose used to store energy. it is important for providing energy for skeletal and cardiac muscle contraction, and for maintaining glucose hemostasis in the blood. molecules of glucose are linked into linear chains by α-1,4-glycosidic bonds. additionally, branches of glucose are formed off of the chain via α-1,6-glycosidic bonds. 2 molecules of glucose are joined into an α-1,4-glycosidic bonds by an enzyme known as glycogen synthase. this bond may be broken by amylase when the body wishes to break down glycogen into glucose for energy. glycogen branching enzyme is responsible for the required α-1,6-glycosidic bonds needed to start a branch off of these linear chains. these branches are important, as they provide additional "free ends" for linear chains of α-1,4-glycosidic bonds, which can then be broken down by amylase. this allows for glucose to be removed at a faster rate than if all glucose molecules were in a single chain with only two free ends on which amylase could attach.
glycogen-branching enzyme deficiency
when glucose has been converted into g6p by hexokinase or glucokinase, it can either be converted to glucose-1-phosphate (g1p) for conversion to glycogen, or it is alternatively converted by glycolysis to pyruvate, which enters the mitochondrion where it is converted into acetyl-coa and then into citrate. excess citrate is exported from the mitochondrion back into the cytosol, where atp citrate lyase regenerates acetyl-coa and oxaloacetate (oaa). the acetyl-coa is then used for fatty acid synthesis and cholesterol synthesis, two important ways of utilizing excess glucose when its concentration is high in blood. the rate limiting enzymes catalyzing these reactions perform these functions when they have been dephosphorylated through the action of insulin on the liver cells. between meals, during fasting, exercise or hypoglycemia, glucagon and epinephrine are released into the blood. this causes liver glycogen to be converted back to g6p, and then converted to glucose by the liver-specific enzyme glucose 6-phosphatase and released into the blood. glucagon and epinephrine also stimulate gluconeogenesis, which coverts non-carbohydrate substrates into g6p, which joins the g6p derived from glycogen, or substitutes for it when the liver glycogen store have been depleted. this is critical for brain function, since the brain utilizes glucose as an energy source under most conditions. the simultaneously phosphorylation of, particularly, phosphofructokinase, but also, to a certain extent pyruvate kinase, prevents glycolysis occurring at the same time as gluconeogenesis and glycogenolysis.
glycolysis