Glycogen Metabolism Flashcards
What is the main storage form of glucose?
Glycogen
Where are the largest amounts of glycogen found?x
Liver and skeletal muscle
Does glycogenesis require energy? Glycogenolysis?
Yes, no
What is glycogen made of?
Alpha 1-4 and alpha 1-6 glycosidic linkages
Where are branches formed?
At alpha-1,6-bonds
What is attached at the reducing end of each glycogen molecule?
Glycogenin
Can glycogen be degraded rapidly?
Yes, enzymes are present and can work on several chains simultaneously
What is the purpose of liver glycogen? Skeletal muscle glycogen?
Glucose for blood; glucose for ATP use in muscle contraction
Does muscle have G6Pase?
No, it doesn’t need it, only uses glucose for ATP, does not have gluconeogenesis
What provides energy for glycogenesis?
UTP
What is the glucose donor in glycogen synthesis?
UDP-Glucose
What enzyme catalyzes synthesis of glycogen in alpha 1,4 linkages? 1,6?
Glycogen synthase, amylo 4-6 transferase
What is the regulated step in glycogen degradation?
Glycogen phosporylase
What removes 3 glucosyl residues from glycogen (debranching enzyme)
Transferase
What hydrolyzes alpha 1,6 glycosidic linkages ?
A-1-6- Glucosidase
T or F: Insulin/glucagon ratio is low glycogen is degraded
True
When you phosphorylate glycogen phosphorylase is it active? What about glycogen synthase?
Yes, No
What removes phosphates from glycogen phosphorylase & glycogen synthase?
Hepatic protein phosphatase 1
In fasting is the answer to #423 active?
no (leaves P on glycogen synthase making it inactive and P on glycogen phosphorylase making it active glycogen breakdown and no glycogen synthesis)
Does AMP activate liver glycogen phosphorylase?
No, Only in muscle
Does glucose inhibit glycogen phosphorylase in muscle?
No, Only in liver
Describe GSD Type I along with location
Glucose 6 phosphatase deficiency (Von Gierke’s); hepatomegaly, normal glycogen structure
Location: liver and kidneys
Describe GSD Type II along with location
Alpha 1,4 glucosidase deficiency (Pompe’s); cardiomegaly, debranching enzyme deficiency
Location: heart, muscle, liver
Describe GSD Type III along with location
Debranching deficiency (Cori’s disease), hepatomegaly, dx. By excessive branched glycogen in urine Location: liver, skeletal muscle, heart
Describe GSD Type IV along with location
Branching deficiency; dx. By glycogen that has no branches in urine
Location: liver, muscle
Describe GSD Type V along with location
Muscle phosphorylase deficiency (McArdles); accumulation of glycogen in muscle, reduced ability to degrade muscle glycogen, lactate not produced in exercise, dx. By measuring lactate deficiency
Location: skeletal muscle
Describe GSD Type VI along with location
Liver phosphorylase deficiency
Describe GSD Type VII along with location
Muscle PFK-1 deficiency (elevated F26BP and G6P)
Describe GSD Type IX along with location
Liver phosphorylase kinase defect
Describe GSD Type X along with location
PKA, cAMP dependent defect
What pathways are affected when glucagon is released? How?
increase in glycogenolysis, gluconeogenesis, lipolysis
Decrease in liver glycolysis
What pathways are affected when insulin is released? How?
Increase in glycogen synthesis, FA synthesis, Triglyceride synthesis, liver glycolysis
Glycogen is a branched chain homopolymer made up of glucose residues linked where? Where do the branches form?
alpha-1,4- and alpha-1,6-bonds
at the alpha-1,6-bonds
Why do branched structures allow rapid degradation and synthesis of glycogen?
enzymes can work on several chains simultaneously and it enhances the solubility of the molecule
What produced the branches of glycogen?
the branching enzyme => glucosyl 4,6 transferase
What is the precursor for glycogen synthesis? Name its action and place of action on the glycogen
UDP glucose supplies glucose moieties that are added to the nonreducing ends of a glycogen primer by glycogen synthase
What does glycogen degradation produce?
glucose 1-phosphate as major product but also free glucose
What supplies glucose for ATP generation when demands are high? What stimulates this? Why is this necessary?
skeletal muscle
epinephrine, bc muscle does not have glc-6-P
What probides glc-6-P for hydrolysis to maintain blood glucose? What stimulates the breakdown?
liver
glucagon and epinephrine
T/F synthetic pathway requires energy from ATP
false, UTP
What is the common intermediate for glycogenesis(glycogen synthesis) and glycogenolysis (degradation)?
glucose -1- Phosphate
What are the stages of glycogen synthesis?
- synthesis of UDP glucose
- action of glycogen synthase
- formation of branches (4:6 transferase)
- growth of glycogen chains (UDP glucose/glycogen synthase)
When lengthening the polysaccharide chains of a pre-existing glycogen, what is the regulated step?
glycogen synthase adding to the glucosyl residue from UDP-glc to chain
Describe the primer used in glycogen synthase activity
glycogenin is the protein which glycogen is attached and has autoglycosylation abilities
- catalyzes add’n of glucosyl resude from UDG gluc to a tyrosine residue
- extend the chain until it is long enough to serve as a primer
- De novo glycogen synthesis for addition of glucosyl residue
What are the steps in glycogen degradation?
- action of glycogen phosphorylase
- removal of branches (4:4 transferase as debranching enzyme)
- degradation of glycogen chains
- fate of glucosyl units released from glycogen
What is the regulated step of glycogenolysis?
glycogen phosphorylase
What regulates the glycogenolysis and glycogenesis?
insulin/glucagon ratio and blood glucose levels
If the ratio of insulin/glucagon decreases, what occurs?
phosphorylation of glycogen phosphorylase to active state and the P of glycogen synthase to inactive state
What molecule is the key regulator for the direction of gluconeogenesis?
cAMP
T/F glycogen degradation is stimulated and synthesis is inhibited when the enzymes of glycogen metabolism are phosphorylated
true
Once cAMP activates PKA, what can occur wrt to glycogen synthesis and phosphorylase kinase?
If PKA phosphorylates glycogen synthase then glycogen synthesis is decreased
If PKA phosphorylates phosphorylase kinase then phoshphorylase b is activated to phosphorylase a which cleaves glucose residues from nonreducing ends of glycogen chains
What is produced when PKA phosphorylates phosphorylase kinase?
glucose 1-phosphate which is oxidized in the liver and converted to blood glucose
What is the major phosphatase involved in glycogen metabolism? describe its action and when it is used
Hepatic protein phosphatase 1 => removes phosphate groups from glycogen phosphorylase and glycogen synthase
-fasting => hepatic PP-1 is inactivated
What is the principle regulator of glycogen synthesis and degradation?
insulin
As neural signals to release epinephrine from the adrenal medulla, what will be the difference bw Beta receptors and alpha receptors?
Stimulation through beta receptors is similar to glucagon which activates PKA
stimulation through alpha receptors stimulates glycogenolysis in liver by increasing Ca levels
Describe the regulation of glycogenolysis in skeletal muscle
related to availability of ATP and only made when need for ATP is high
Why is skeletal muscle committed to the glycolytic pathway?
it has no glucose-6-phosphatase so G-6-P is committed to the glycolytic pathway
T/F AMP activates muscle glycogen phosphorylase but not liver glycogen phosphorylase
True
Describe the relationships from the stimuli of muscle contraction, nerve impulse and epinephrine
muscle: ATP => AMP => glycogen phosphorylase b => activated by phosphorylase kinase to yield => glycogen phosphorylase a
Nerve: Ca released => Ca-calmodulin => phosphorylase kinase
epinephrine=> cAMP => PKA => phosphorylase kinase
