Chapter 25: Glycogen Synthesis Flashcards
Glycogen degradation yields…
Glucose 1-Phosphate
Glycogen synthesis requires…
UDP-Glucose
What is UDP-Glucose?
Give:
- full name
- purpose
- how it is synthesized
- how it is rendered irreversible
- Uridine diphosphate-glucose (activated form of glucose)
- Is the glucose donor in glycogen synthesis.
- Synthesized by UDP-Glucose pyrophosphorylase
- The reaction is rendered irreversible by the hydrolysis of pyrophosphate
What catalyzes the transfer of glucose from UDP-Glucose to a growing chain?
Glycogen Synthase
Activated glucosyl unit of UDP-glucose is attached to glycogen by _____ catalyzed by _____.
Attached by a-1,4 glycosidic bond
Catalyzed by glycogen synthase
Glycogen synthesis requires an oligosaccharide of glucose residues as _____.
Primer (Need a chain of 7 or 8)
What synthesizes glycogen synthase’s primer?
Glycogenin - generates an oligosaccharide of glucose residues 8 molecules long. Considered a glucosyl transferase
Glycogen synthase can only synthesize _____.
a-1,4 linkages
A branching enzyme forms _____. What does this do?
a-1,6 linkages
Branching of glycogen increases its solubility and the rate of its synthesis and degradation.
Glycogen synthesis is regulated by?
Reversible phosphorylation
What is the key regulatory enzyme in glycogen synthesis?
Glycogen synthase
*Similar to glycogen phosphorylase but has the opposite effect
Glycogen synthase is _____ when in phosphorylated b form?
Inactive
Glycogen synthase is _____ when in unphosphorylated a form?
Active
Glycogen is an effective _____ of glucose.
Storage form
Only _____ is required to incorporate glucose 6-phosphate into glycogen.
One molecule of ATP
The complete oxidation of glucose derived from glycogen yields _____.
31 molecules of ATP
The overall efficiency of storage is nearly _____.
97%
Glycogen synthesis is inhibited by the same _____ and _____ that stimulate glycogen breakdown.
Glucagon and epinephrine signaling pathways
What stimulates glycogen breakdown and how?
Protein Kinase A (PKA) stimulates by phosphorylating and activating phosphorylase kinase
How does PKA inhibit glycogen synthesis?
Inhibits by inactivating glycogen synthase by phosphorylation
Glycogen synthase kinase also…
Phosphorylates and inhibits glycogen synthase
Glycogen Degradation is _____ by phosphorylation.
Activated
Glycogen Synthesis is _____ by phosphorylation.
Inhibited
What reverses the regulatory effects of kinases on glycogen metabolism?
Protein Phosphatase 1 (PP1)
PP1 _____ glycogen synthesis.
PP1 activates
PP1 _____ glycogen degradation.
PP1 inhibits
Insulin _____ glycogen synthesis. How?
Stimulates by inactivating glycogen synthase kinase (it’s kept unphosphorylated)
In summary, glucagon and epinephrine regulate glycogen metabolism by:
G&E: Stimulating glycogen degradation
G&E: Inhibiting glycogen synthesis
In summary, insulin regulates glycogen metabolism by:
I: Stimulating glycogen synthesis
I: Inhibiting glycogen degradation
Low blood glucose levels means _____. This requires…
Hypoglycemia
Glucagon (starved or fasting state)
High blood glucose levels means _____. This requires…
Hyperglycemia
Insulin (fed state)
Diabetes is characterized by…
The presence of excess glucose and underutilization of glucose. This excess glucose is excreted in the urine.
What happens with type 1 diabetes?
Insulin is not produced (10% of diabetic patients)
What happens will type 2 diabetes?
Insulin is produced, but the insulin-signaling pathway is not responsive (insulin resistance)
During diabetes, _____ is produced in excess.
Glucagon
What is von Gierke disease (type I)?
- Massive enlargement of the liver and hypoglycemia due to the lack of glucose 6-phosphatase from the liver.
- Mutations in the gene encoding of the glucose 6-phosphate transporter also cause von Gierke disease
- Cannot get into the ER lumen to produce glucose
What is Cori disease (type III)?
Structure of liver and muscle glycogen abnormal and glycogen amount increased due to lack of debraching enzyme
What is McArdle disease (type V)?
- Results from defective muscle glycogen phosphorylase (Not able to degrade glycogen - losing ATP source)
- Painful cramps occur when patients with McArdle disease exercise
- Even when walking, they have muscle cramps due to lack of ATP
A deficiency in the glycogen phosphorylase in liver would result in:
- hemolytic anemia.
- hyperglycemia.
- hypoglycemia.
- lactic acidosis.
- muscle cramps.
Hypoglycemia
The glycogen phosphorylase b in muscle is allosterically inhibited by high levels of:
- AMP
- NADH
- fructose 2,6-bisphosphate
- UDP-glucose
- glucose 6-phosphate
Glucose 6-Phosphate
T or F: Active protein phosphatase 1 (PP1) inhibits glycogen synthesis and stimulates glycogen degradation.
FALSE
T or F: Glycogen synthesis requires UDP-glucose, an activated form of glucose.
TRUE
T or F: Insulin stimulates glycogen synthesis by preventing the phosphorylation of glycogen synthase.
True
T or F: Glycogen synthase requires an oligosaccharide of glucose residues as a primer, which is synthesized by the enzyme glycogenin.
True
Diabetes mellitus results from _______________ insufficiency and _______________ excess.
- epinephrine; insulin
- glucagon; epinephrine
- glucagon; insulin
- leptin; epinephrine
- insulin; glucagon
Insulin; glucagon
