5. Glycogenolysis and Glycogenesis Flashcards
How much glycogen is stored as
a) muscle glycogen
b) liver glycogen
a) 400g
b) 100g
When glucose moves across GLUT-4 into the cell it is quickly trapped in the cell by being phosphorylated to glucose-6-phosphate. Only where is the enzyme that will catalyse it back into glucose? what is this enzyme?
only in the liver is there the enzyme glucose-6-phosphatase that’ll catalyse it back into glucose.
How is glycogen assembled?
Chains with branches at 8-10 glucose molecules. The bonds are alpha-1,4-glyosidic bonds for normal bonds. Branch bonds are alpha-1,6-glycosidic bonds.
Explain the formation of glycogen
- Glucose-1-phosphate is joined with uridine tri-phosphate (UTP) under the enzyme UDP-glucose pyrophosphorylase. This forms UDP-glucose and pyrophosphate.
- Once UDP-glucose is formed glycogen synthase will attach the glucose to the glycogen chain already there. This frees up the UDP.
The glycogen chain is now 1 glucose longer.
How are the branches formed in glycogen?
A branching enzyme is used to attach the glucose onto the 6th carbon instead of the 4th
How is glycogen broken down?
A free phosphate joins and breaks off a glucose molecule via the enzyme glycogen Phosphorylase to form glucose-1-phosphate
Insulin is ultimately important in the regulation of glucose homeostasis.
Insulin acts upon the hepatocytes, myocytes and adipocytes to alter plasma glucose.
How does insulin favour the forming of glycogen?
Increase in blood glucose. Trigger of insulin. This stimulates glycogen synthase in the liver to uptake blood glucose.
it can also stimulate the muscles to uptake free blood glucose as well.
Explain the regulation of Glycogen Synthase and the formation of glycogen
Insulin activates protein phosphatase which causes glycogen synthase to become an active form. Glucose-6-phosphate also stimulates protein phosphatase. Adrenaline inhibits it. When this happens glycogen is formed.
Protein kinase A causes glycogen synthase to become and inactive form. Adrenaline activates protein kinase. Ca2+ also has the same effect, and the same with glucagon.
Explain the regulation of Glycogen Phosphorylase and the breaking of glycogen
Insulin stimulates protein phosphatase I to cause glycogen Phosphorylase to become inactive. This doesn’t favour the breaking of glycogen.
Glucagon stimulates the enzyme Phosphorylase kinase to cause glycogen Phosphorylase to become its active form to break glucose molecules off a glycogen chain. Adrenaline and Ca2+ also do the same. AMP can also activate glycogen phosphorylase. ATP and glucose-6-P have inhibitory actions.
