Brenda - Glycogenolysis Flashcards
What happens in the early stages of fasting?
(5)
Liver glycogen stores are broken down
The liver plays a great role in maintaining blood glucose levels in the first 24 hours
After glycogen stores are depleted the body uses energy stores from adipose tissue
Fatty acids are the major source of energy after 24 hours
Ketone bodies are produced from fatty acids when liver glycogen is entirely depleted
What happens in the body after an extended period of fasting?
(2)
The body begins breaking down proteins into amino acids and releasing them into the bloodstream
This allows for production of more glucose by gluconeogenesis
Write a note on starch
(3)
Two forms: unbranched glucose polymer ‘amylose’ and branched polymer ‘amylopectin’
In both forms glucose residues are linked by alpha (1-4) glycosidic bonds
Amylopectin also has alpha (1->6) glycosidic bonds (branch points)
Write a note on glycogen
(3)
A branched polymer of glucose residues
Contains same types of bonds as amylopectin a(1->6) but branches are smaller and more frequent
It accounts for up to 10% of mass of the liver, 2% mass of muscle in mammals
What is the function of glycogen in the liver?
(3)
Glycogen synthesis
Glycogen breakdown
Both are regulated to maintain blood glucose levels
What is the function of glycogen in the muscle?
Regulation of synthesis and breakdown of glycogen to meet the energy requirements of the muscle cell
What is glycogenolysis?
The degradation of stored glycogen
What enzyme degrades glycogen?
Glycogen phosphorylase
How does glycogen phosphorylase degrade glycogen?
By cleaving sequential phospholytic cleavage of a(1->4) bonds
What product is formed from the degradation of glycogen?
Glucose1Phosphate
What is G1P converted into ?
Glucose-6-phosphate
What enzyme converts G1P to G6P?
Phosphoglucomutase
What is G6P converted into?
Glucose
What enzyme converts G6P to glucose?
Glucose-6-phosphatase
Where is glucose-6-phosphatase found?
Liver
Kidney
Intestine
(not in muscle)
List the molecules in order of glycogenolysis
Glycogen
Glucose-1-phosphate
Glucose-6-phosphate
Glucose
List the enzyme in order of glycogenolysis
Glycogen phosphorylase
Phosphoglucomutase
Glucose-6-phosphatase
How does glycogen phosphorylase convert glycogen to glucose-1-phosphate?
(3)
It catalyses the sequential removal of glucose residues from non-reducing ends of glycogen chains (a1->4) bonds
It stops 4 residues from branch point (a1->6) glucosidic bond leaving limit dextrin
Limit dextrin is further degraded by debranching enzymes
How does glycogen phosphorylase convert glucose-1-phosphate to glucose-6-phosphate?
(3)
This is a rate controlling step in glycogen breakdown
Two forms:
- phosphorylase a (phosphorylate/active form)
- phosphorylase b (dephosphorylated/less active form)
What are the two debranching enzymes?
Glucanotransferase
Amylo-1,6-glucosidase
What does glucanotransferase do?
It catalyses the relocation of a chain of 3 glucose from a branch to a free OH end of glycogen
What does amylo-1,6-glucosidase do?
Catalyses the hydrolytic removal of remaining alpha (1->6) linked glucose
What are the produces of the two debranching enzymes?
1 free glucose + chain (substrate for glycogen phosphorylase)
Where is glycogen stored
Liver
Muscle
What does the liver do when there is low blood glucose?
This triggers glycogen breakdown to G6P that is hydrolysed to glucose and supplies in the blood stream
What does muscle do when there is low blood glucose?
Muscle needs ATP for movement and converts glycogen to G6P for glycolysis
What happens when liver glucose increases?
Glycogen phosphorylase decreases
Glycogen synthase increases
What regulates the activity of Glycogen phosphorylase and glycogen synthase?
Phosphorylation/dephosphorylation
What main hormone promotes glycogenolysis in muscle?
Adrenaline/Noradrenaline
What main hormone promotes glycogenolysis in liver?
Glucagon
How does a hormone activate glycogenolysis?
(6)
The hormone binds at the membrane
cAMP is synthesised
cAMP activates protein kinase (PK)
PK catalyses phosphorylation of phosphorylase b kinase (PBK)
PBK catalyses phosphorylation of phosphorylase b to a
Phosphorylase a catalyses glycogen breakdown
What does cAMP do?
Activates protein kinase
What does protein kinase do?
Catalyses the phosphorylation of phosphorylase b kinase (PBK)
What does phosphorylase b kinase do?
Catalyses the phosphorylation of phosphorylase b to a
What does phosphorylase a do?
Catalyses glycogen breakdown
What does the glycogenolysis cascade depend on?
(2)
Intracellular calcium
cAMP levels
What is calmodulin?
A calcium modulating protein with 4 calcium binding sites
Name two other proteins involved in the glycogenolytic cascade
Calmodulin
Phosphorylase b kinase multisubunits
What three other nonhormonal ways are there to control glycogenolysis?
Glycogen phosphorylase
Allosteric inhibitors
Allosteric activators
Name two allosteric inhibitors of glycogenolysis?
ATP
Glucose-6-phosphate
Name an allosteric activator of glycogenolysis
AMP
What two hormones stimulate the breakdown of glycogen?
Adrenalin
Glucagon
What hormone stimulates the synthesis of glycogen?
Insulin
What does the breakdown of glycogen result from?
Kinase cascade
Where is GLUCAGON made?
Alpha cells of islets of langerhans in the pancreas
These cells sense blood glucose and release hormones in response to low levels
What does glucagon do?
It increases liver cAMP so that metabolic cascades promote glycogenolysis
This also inhibits glycogen synthase
What is the main energy source for resting muscle?
Fatty acids
What is the main energy source for exerted muscle?
Glucose
Early exertion (glucose from muscle’s glycogen reserves)
Late exertion (glucose from fatty acid reserves)
How does adrenaline work in muscle
(3)
Adrenaline activates adenylate cyclase
It activates glycogenolysis and the inhibition of glycogen synthesis
Promotes glycogen mobilisation
How does glucagon affect the liver?
Stimulates synthesis of cAMP by membrane bound adenylate cyclase through action of a G protein
What are the two types of receptors for epinephrine?
B adrenergic receptors
A adrenergic receptors
What are B adrenergic receptors linked to?
To the adenylate cyclase system
What are a adrenergic receptors?
Receptors whose second messenger causes intracellular Ca2+ to increase
What type of adrenergic receptors do muscles have?
Beta
What does increased intracellular Ca2+ do?
It reinforces cells response to cAMP
