Module 10: Carbohydrate Metabolism - Glycogen Metabolism (Part 03) Flashcards
This process is known as the biosynthesis of glycogen.
Glycogenesis
This process is known as the breakdown of glycogen to glucose 1 phosphate.
Glycogenolysis
This phenomenon is anaerobically generated, which uses glycogen as a rapid energy source.
Glycogenolysis
In Glycogenolysis, how many atp is generated per glucose.
3 ATP
In Glycogenolysis, how many atp is generated per non-glycogen glucose?
2 ATP
What are the disadvantages of glycogenolysis?
(1) Low ATP/Mass
(2) Limited Storage
This provides a rapid source of energy.
Muscle
This provides maintains blood glucose levels.
Liver
This provides the storage form of glucose that can be used by the cell if glucose is low or deficient. A branched polymer of α-D-glucose linked by a-1,4 glycosidic linkage
Glycogen
What linkage does glycogen have?
α-D-glucose linked by a-1,4 glycosidic linkage
where does glycogen arise?
Branches arise from frequent a-1,6 glycosidic linkages (every 4th glucosyl residue). Occurs in all tissues but predominantly in liver and muscles
This is the largest capacity to store glycogen. This stores and exports glucose to maintain blood glucose between meals via the enzyme glucose-6-phosphatase which converts glucose-6-phosphate to glucose
Liver
The liver stores and exports glucose to maintain blood glucose between meals via the enzyme __________________-which converts glucose-6-phosphate to glucose
glucose-6-phosphatase
Glycogen store is almost depleted after ______________of fasting
12-18 hours
Glycogen is _______of the liver weight (well fed)
10%
This has greater total glycogen concentration due to greater mass than liver. It provides a readily available source of glucose for glycolysis (fuel reserve)
Muscle
About _______________of total body glycogen is in the muscle
three quarters
This mobilizes muscle glycogen thereby releasing ATP.
Exercise
Why doesn’t muscle directly yield free glucose?
Does not directly yield free glucose due to absence of glucose-6-phosphatase
Glycogen is _______of the muscle weight (well fed)
2 %
Why do we store glycogen rather than glucose?
(1) Glucose is osmotically active
(2) Glycogen is more inert (can be stored)
(3) Highly soluble which is a ready source of fuel
(4) Easier to be released than fat
(5) Can be metabolized even in anaerobic conditions
What happens in step 1 in glycogen synthesis?
Glucose priming through phosphorylation
What enzymes facilitate glucose priming through phosphorylation?
(1) Glucokinase (hexokinase IV in the liver
(2) Hexokinase (Hexokinase I and II in the muscle)
what enzyme is responsible for the formation of glucose 1 phosphate?
phosphoglucomutase (isomerase)
What happens in step 3 in glycogen synthesis?
Formation of UDP glucose
What happens in step 4 of glycogen synthesis?
Transfer of glucose to a non reducing glycogen
What enzyme facilitates the Transfer of glucose to a non reducing glycogen ?
Glycogen synthase (where the free energy of glucose 1 phosphate is highly favorable)
What happens in step 5 of glycogen synthesis?
Branch formation where 6 to 7 from the 11 residues built is transferred to a branch. if the branching solubility increases, the non reducing ends also increase.
What enzyme is responsible for the glycogen branching?
Glycogen branching enzyme or amylo transglycolase
What does glycogen synthase require?
Requires a primer - glycogenin
This catalyzes two distinct reactions. Initial attack by the hydroxyl group of Tyr194 on C-1 of the glucose moiety of UDP-glucose results in a glycosylated Tyr residue.
Glycogenin
what happens in glycogen synthesis?
The C-1 of another UDP-glucose molecule is now attacked by the C-4 hydroxyl group of the terminal glucose, and this sequence repeats to form a nascent glycogen molecule of eight glucose residues attached by (a-1→4) glycosidic linkages.
This cannot initiate synthesis of a new glycogen chain de novo. It requires a primer, a preformed (a 1 -> 4) polyglucans chain or branch having at least 8 glucose residues
Glycogen synthase
This acts both as a primer and a self-glycosylating enzyme by adding glucose to itself from UDP-glucose.
Glycogenin
What are the two reactions catalyzed by glycogenin?
(1) Attack by –OH group of tyrosine on C1 of UDP-glucose
(2) C1 of another UDP-glucose is now attacked by C-4 OH of terminal glucose residues lengthening the chain by 1 glucose unit. This is repeated until about 8 glucose units are added then glycogen synthase takes over.
This adds glucose units from UDP-glucose to a non-reducing end of glycogen
Glycogen Synthase
This catalyzes transfer of 6-7 glucose residues to C6 OH of glucose of another chain, creating a new branch.
Glycogen Branching Enzyme
What is the biologic effect of branching?
(1) Increase solubility of glycogen
(2) Increases the number of non-reducing ends increasing the sites for the action of glycogen phosphorylase and glycogen synthase
(3) To make synthesis and breakdown of glycogen occur more rapidly
How is the regulation of glycogenesis promoted?
when the blood glucose is high after a meal
This enzyme promotes glycogenesis.
Insulin
This enzyme subject to regulation. It is active when dephosphorylated but inactive when phosphorylated.
Glycogen Synthase
This is known as the breakdown of glycogen to glucose or glucose-1-P
Glycogenolysis
Where does glycogenolysis occur?
Occur abundantly in muscle and liver but virtually absent in liver after 24 hours of fasting or heavy exercise
This catalyzes phosphorolysis of glycogen, a Pi is used to cleave an α-1,4 glycosidic linkage to form glucose1-Phosphate
Glycogen Phosphorylase
Where does Glycogen Phosphorylase occur?
Non reducing ends
Glycogen phosphorylase requires this as a cofactor.
pyridoxal phosphate, PLP
Liver glycogen can be depleted in __________________.
12 to 24 hrs.
What are the three enzymes that facilitates glycogenolysis?
(1) Glycogen phosphorylase - releases glucose 1 phosphate
(2) Debranching Enzyme
(3) Phosphoglucomutase - converts G1P to G6P
What does the action of glycogen phosphorylase do
Prevents cleaved glucose from leaving the cell
What are the two (2) debranching enzymes that has catalytic actvities?
(1) Oligo-1,4 to 1,4- glucan transferase
(2) Amylo-1,6-glucosidase
How many net yield of glucose are there from glycogen?
Note: if glucose comes from glycogen when it enters glycolysis, it is already phosphorylated. Saves one ATP. Net yield of glucose from glycogen is 3 ATPs instead of 2.
This enzyme subject to regulation by allosteric effectors that signal the energy status of the cell.
Glycogen phosphorylase
Glycogen phosphorylase is active or responsive to hormones like what?
Insulin, Glucagon, Norepinephrine
When is glycogen phosphorylase active?
It is active when phosphorylated and inactive when dephosphorylated.
These activates glycogen phosphorylase by phosphorylation
Glucagon and epinephrine
