Glycogen Synthesis & Regulation Flashcards
(29 cards)
Where is glycogen stored?
- Liver for fuel during fasting states
- Muscles for local metabolism
Which enzyme acts to synthesise glycogen?
Glycogen synthase!
Which enzyme acts to break down glycogen into glucose?
What does this enzyme release glucose as from glycogen?
Glycogen phosphorylase.
Glucose released as: glucose-1-phosphate.
Which enzyme converts glucose-1-phosphate (from glycogen breakdown) into glucose-6-phosphate?
Phosphoglucomutase.
NOT hexokinase or glucokinase as with free glucose in the blood, as the glucose is already phosphorylated.
What is the chain of events when we eat carbohydrates or proteins that results in glycogen formation?
1) We ingest the food
2) blood glucose levels rise which causes insulin to be released
3) the insulin causes the blood glucose traveling to the liver (via the hepatic portal vein) from the intestines to enter the hepatocytes
4) the insulin results in activated glycogen synthase, which synthesised glycogen
What is the process of synthesis of glycogen called?
Glycogenesis.
What is the name of the process of breaking down glycogen?
Glycogenolysis.
Why is it the case that glucose-6-phosphate from glycogen breakdown in the liver can enter the blood for use by all tissues in the body, yet glucose-6-phosphate in the muscles can not?
Muscle cells lack glucose-6-PHOSPHATASE = the enzyme which hydrolyses glucose-6-phosphate into glucose + Pi. For this reason it can not leave the myocyte and must be used locally only.
What do the following enzymes do:
1) kinases
2) phosphorylases
3) phosphotases
1) phosphorylase using ATP
2) hydrolyse using phosphate
3) remove a phosphate
Which molecule is needed first in the synthesis of glycogen via glycogenesis?
Glucose-1-phosphate.
What does glucose-1-phosphate link with to form the glucose molecule units which are involved in glycogen synthesis?
What is the complex/molecule called?
Which enzyme is involved in this reaction?
Glucose-1-phosphate bonds with UTP to make UDP glucose and PPi (pyrophospate, X1 comes from UTP and X1 from the glucose-1-phosphate).
The enzyme = UDP-glucose pyrophosphotase.
What is the name of the primer used for glycogen synthesis?
What is its role?
What does the UDP-glucose attach to on this molecule?
Glycogenin.
Provides an attachment (an OH group) for the first UDP-glucose molecule.
How does glycogen synthase work with UDP-glucose?
It binds new UDP-glucose molecules to an existing glycogen chain, removing UDP in the process.
Which enzyme is involved in the branching of glycogen chains?
How does this work?
Glycogen branching enzyme.
It cuts a straight chain and re-attaches it as a branch.
How is glycogen synthase activated/deactivated?
What enzymes are involved in the process?
Glycogen synthase is active when NOT phosphorylated.
To inactivate it, protein KINASE A + ATP are required to phosphorylate it.
To activate it, protein phosphotase + H2O are required to dephosphorylate it.
How does insulin work with respect to glycogen synthase?
It phosphorylates and therefore activates glycogen synthase phosphatase (the dephosphorylating enzyme for glycogen synthase) which as a result activates glycogen synthase and causes glycogenesis.
What is the reaction of glygogenolysis?
What are all the molecules/enzymes involved?
Glycogen + glycogen phosphorylase + Pi —> glycogen(n-1) + glucose-1-phosphate.
Which enzyme is involved in removing glucose units from a 1,6-glycosidic bond where the glycogen is branched during glycogenolysis?
Glycogen debranching enzyme.
How is glycogen phosphorylase activated/deactivated?
What enzymes are involved in the process?
Glycogen phosphorylase is active when it IS phosphorylated.
To activate it, protein KINASE A + ATP are required to phosphorylate it.
To inactivate it, protein phosphotase + H2O are required to dephosphorylate it.
Why is it important that glycogen synthase and glycogen phosphorylase are BOTH phosphorylated by protein kinase A?
Because due to them being active in opposing states (one when phosphorylated, one when not) they are stopped from both acting at once.
Which hormone activates protein phosphotase?
What is the result of this?
Insulin.
Glycogen synthase = active
Glycogen phosphorylase = inactive
Which hormone activates protein kinase A?
What is the result of this?
Glucagon (and to some extent adrenaline).
Glycogen synthase = inactive
Glycogen phosphorylase = active
What is important to remember about protein kinase A in these examples?
Increases in PKA do not directly result in the phosphorylation of glycogen synthase and glycogen phosphorylase, is works via the intermediate enzymes:
1) glycogen synthase kinase
2) glycogen phosphorylase kinase
How does glucose self regulate?
It binds to glycogen phosphorylase to inhibit it, thereby reducing its own release.
