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Questions and Answers
What is the effect of elevated insulin levels on glycogen metabolism?
What is the effect of elevated insulin levels on glycogen metabolism?
- Decreased both glycogen synthesis and degradation
- Increased glycogenolysis and decreased glycogenesis
- Activation of glycogen phosphorylase
- Increased glycogenesis and decreased glycogenolysis (correct)
What role does Ca2+ play during exercise in muscle glycogen metabolism?
What role does Ca2+ play during exercise in muscle glycogen metabolism?
- It phosphorylates glycogen phosphorylase
- It activates phosphorylase kinase (correct)
- It activates protein phosphatase-1
- It activates glycogen synthase directly
Which of the following statements is true regarding glycogen phosphorylase and glycogen synthase following stimulation by epinephrine?
Which of the following statements is true regarding glycogen phosphorylase and glycogen synthase following stimulation by epinephrine?
- Glycogen phosphorylase is phosphorylated and active, while glycogen synthase is phosphorylated and inactive (correct)
- Glycogen phosphorylase is phosphorylated and inactive
- Glycogen synthase remains unphosphorylated and active
- Both are phosphorylated and active
Which enzyme deficiency results in exercise intolerance without elevation in blood lactate levels?
Which enzyme deficiency results in exercise intolerance without elevation in blood lactate levels?
What is the primary consequence of glucose 6-phosphatase deficiency?
What is the primary consequence of glucose 6-phosphatase deficiency?
In the well-fed state, what is the effect of glucose 6-phosphate on glycogen metabolism?
In the well-fed state, what is the effect of glucose 6-phosphate on glycogen metabolism?
What happens to glycogen metabolism when both epinephrine and glucagon are present?
What happens to glycogen metabolism when both epinephrine and glucagon are present?
What is characterized by generalized accumulation of glycogen and severe hypotonia?
What is characterized by generalized accumulation of glycogen and severe hypotonia?
The phosphorylation of glycogen synthase leads to which state of the enzyme?
The phosphorylation of glycogen synthase leads to which state of the enzyme?
What is the primary glycosidic bond in glycogen?
What is the primary glycosidic bond in glycogen?
Which enzyme is responsible for synthesizing UDP-glucose?
Which enzyme is responsible for synthesizing UDP-glucose?
Which enzyme cleaves the α(1→4) bonds in glycogen to produce glucose 1-phosphate?
Which enzyme cleaves the α(1→4) bonds in glycogen to produce glucose 1-phosphate?
What is produced after the cleavage of a limit dextrin?
What is produced after the cleavage of a limit dextrin?
What happens to glucose 6-phosphate in the muscle cells?
What happens to glucose 6-phosphate in the muscle cells?
Which condition results from a deficiency of glucose 6-phosphatase?
Which condition results from a deficiency of glucose 6-phosphatase?
What role does glycogen serve in liver cells?
What role does glycogen serve in liver cells?
What type of linkage forms branches in glycogen?
What type of linkage forms branches in glycogen?
Which enzyme transfers glucosyl residues to another glycogen chain during the formation of branches?
Which enzyme transfers glucosyl residues to another glycogen chain during the formation of branches?
What remains after the sequential degradation of glycogen by phosphorylase?
What remains after the sequential degradation of glycogen by phosphorylase?
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Study Notes
Glycogen Overview
- Major glycogen storage sites: skeletal muscle (fuel for ATP during contraction) and liver (regulation of blood glucose during fasting).
- Glycogen structure: highly branched polymer of α-D-glucose with α(1→4) linkages and branches connected by α(1→6) linkages.
Glycogen Synthesis
- Key building block: uridine diphosphate (UDP)-glucose, formed from glucose 1-phosphate and UTP via UDP-glucose pyrophosphorylase.
- Glycogen synthase catalyzes the addition of glucose from UDP-glucose to glycogen chains by forming α(1→4) linkages.
- Branching occurs through glucosyl 4:6 transferase, transferring groups of six to eight glucosyl residues.
Glycogen Degradation
- Glycogen phosphorylase breaks α(1→4) bonds, generating glucose 1-phosphate.
- Limit dextrin structure remains when four glucosyl units are left at branch points.
- Debranching enzyme processes limit dextrin via glucosyl 4:4 transferase and amylo-(1→6) glucosidase, releasing free glucose.
- Glucose 1-phosphate converts to glucose 6-phosphate by phosphoglucomutase; enters glycolysis in muscles or released as free glucose in liver.
Glycogen Storage Disease (GSD)
- Type Ia (Von Gierke disease): glucose 6-phosphatase deficiency leads to severe fasting hypoglycemia and inability of liver to provide glucose.
- Type VI: liver phosphorylase deficiency results in mild hypoglycemia and ability to produce glucose via gluconeogenesis.
Hormonal Regulation
- Insulin increases glycogenesis (activates glycogen synthase) and decreases glycogenolysis.
- Glucagon and epinephrine increase glycogenolysis (activate phosphorylase) and decrease glycogenesis via phosphorylation of enzymes.
- cAMP-dependent protein kinases and protein phosphatase-1 play crucial regulatory roles.
Enzyme Regulation
- Glycogen synthase is activated by glucose 6-phosphate in a fed state; inhibited by ATP.
- Phosphorylase activity is increased by Ca2+ in muscle, activating phosphorylase kinase.
- AMP also acts as an activator for glycogen phosphorylase in muscle.
Study Questions Summary
- Myophosphorylase deficiency leads to exercise intolerance, preventing muscle glycogen degradation.
- Branching enzyme deficiency results in glycogen with long outer chains and solubility issues.
- Acid maltase deficiency affects multiple tissues, particularly heart, causing glycogen accumulation.
- Glucose 6-phosphatase deficiency results in severe fasting hypoglycemia and metabolic abnormalities.
- Glycogen metabolism in liver is affected by phosphorylation: phosphorylase active and synthase inactive when signals from epinephrine/glucagon are present.
- Rapid calcium release in muscle activates phosphorylase kinase, enhancing glycogen breakdown.
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