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Questions and Answers
In McArdle disease, exercise intolerance is manifested by hyperglycemia and cramps in exercising muscles.
In McArdle disease, exercise intolerance is manifested by hyperglycemia and cramps in exercising muscles.
False (B)
Glycogen metabolism abnormalities are unrelated to clinical and biochemical features of glycogen storage diseases.
Glycogen metabolism abnormalities are unrelated to clinical and biochemical features of glycogen storage diseases.
False (B)
Glycogen stores are built up from fructose, not glucose, in the liver and muscle during the fed state.
Glycogen stores are built up from fructose, not glucose, in the liver and muscle during the fed state.
False (B)
Glycogen’s primary function in muscle tissue is to maintain blood glucose homeostasis.
Glycogen’s primary function in muscle tissue is to maintain blood glucose homeostasis.
Glycogen is stored in discrete cytoplasmic granules that remain constant in number regardless of metabolic state.
Glycogen is stored in discrete cytoplasmic granules that remain constant in number regardless of metabolic state.
Glycogen consists of glucose residues linked exclusively by $\alpha$-1,4 glycosidic bonds.
Glycogen consists of glucose residues linked exclusively by $\alpha$-1,4 glycosidic bonds.
Storing glucose as free molecules within cells has no effect on osmotic pressure.
Storing glucose as free molecules within cells has no effect on osmotic pressure.
A linear polymer of glucose is as efficient as glycogen for rapid glucose release during high energy demand.
A linear polymer of glucose is as efficient as glycogen for rapid glucose release during high energy demand.
Glycogen synthesis occurs mainly in the mitochondria of cells.
Glycogen synthesis occurs mainly in the mitochondria of cells.
The direct addition of glucose to glycogen is thermodynamically favorable without prior activation.
The direct addition of glucose to glycogen is thermodynamically favorable without prior activation.
UDP-glucose pyrophosphorylase catalyzes the degradation of UDP-glucose during glycogenolysis.
UDP-glucose pyrophosphorylase catalyzes the degradation of UDP-glucose during glycogenolysis.
Glycogenin functions solely as a structural component at the core of glycogen, without enzymatic activity.
Glycogenin functions solely as a structural component at the core of glycogen, without enzymatic activity.
Glycogen synthase catalyzes the removal of glucose from the non-reducing ends of glycogen.
Glycogen synthase catalyzes the removal of glucose from the non-reducing ends of glycogen.
Branching enzyme catalyzes the formation of $\alpha$-1,4 glycosidic bonds during glycogenesis.
Branching enzyme catalyzes the formation of $\alpha$-1,4 glycosidic bonds during glycogenesis.
The glucose residues within glycogen connect via $\alpha$-2,6 glycosidic bonds at junction points.
The glucose residues within glycogen connect via $\alpha$-2,6 glycosidic bonds at junction points.
Glycogen phosphorylase cleaves glucose residues by hydrolysis, releasing free glucose molecules.
Glycogen phosphorylase cleaves glucose residues by hydrolysis, releasing free glucose molecules.
Glycogenolysis is a reversal of the synthetic reaction.
Glycogenolysis is a reversal of the synthetic reaction.
Glycogen phosphorylase continues to degrade glycogen through branch points until the molecule is completely broken down.
Glycogen phosphorylase continues to degrade glycogen through branch points until the molecule is completely broken down.
The resulting structure after glycogen phosphorylase acts is amylose.
The resulting structure after glycogen phosphorylase acts is amylose.
The role of the debranching enzyme is to create more branches.
The role of the debranching enzyme is to create more branches.
Debranching enzyme is specific for breaking only $\alpha(1-6)$ linkages.
Debranching enzyme is specific for breaking only $\alpha(1-6)$ linkages.
Liver glycogen is broken down to provide glucose for energy within the liver cells themselves.
Liver glycogen is broken down to provide glucose for energy within the liver cells themselves.
Lysosomal degradation is the primary pathway for glycogen breakdown under normal metabolic conditions.
Lysosomal degradation is the primary pathway for glycogen breakdown under normal metabolic conditions.
In Pompe disease, glycogen accumulation occurs within the cytosol of liver cells.
In Pompe disease, glycogen accumulation occurs within the cytosol of liver cells.
Individuals with Pompe disease commonly exhibit hyperglycemia due to impaired glycogen breakdown.
Individuals with Pompe disease commonly exhibit hyperglycemia due to impaired glycogen breakdown.
The kidneys store the most glycogen relative to the liver and skeletal muscle.
The kidneys store the most glycogen relative to the liver and skeletal muscle.
Brain tissue utilizes glycogen stores to maintain glucose homeostasis.
Brain tissue utilizes glycogen stores to maintain glucose homeostasis.
Glycogen synthesis begins with glycogen phosphorylase binding directly to free glucose molecules.
Glycogen synthesis begins with glycogen phosphorylase binding directly to free glucose molecules.
Hexokinase functions in glycogenolysis.
Hexokinase functions in glycogenolysis.
Glycogen synthase catalyzes glucose addition at both the reducing and non-reducing ends of glycogen.
Glycogen synthase catalyzes glucose addition at both the reducing and non-reducing ends of glycogen.
Liver glycogenolysis is activated during periods of high blood glucose and insulin secretion.
Liver glycogenolysis is activated during periods of high blood glucose and insulin secretion.
Glycogen branching increases its solubility and compactness, which is critical for its function.
Glycogen branching increases its solubility and compactness, which is critical for its function.
Without hydrolysis, Glycogen phosphorylase can act effectively to release Glucose-1-Phosphate
Without hydrolysis, Glycogen phosphorylase can act effectively to release Glucose-1-Phosphate
Phosphorolysis is not a tightly regulated enzyme, so the energy can easy go from the body.
Phosphorolysis is not a tightly regulated enzyme, so the energy can easy go from the body.
Glycogen is not broken down by the enzyme Glycogen Phosphorylase.
Glycogen is not broken down by the enzyme Glycogen Phosphorylase.
A lysosomal defect is associated with hipoglycemia, including muscular hipotomia and cardiomyopathy.
A lysosomal defect is associated with hipoglycemia, including muscular hipotomia and cardiomyopathy.
The glicosidase activity makes the enzyme easier to the energy mobilization.
The glicosidase activity makes the enzyme easier to the energy mobilization.
Glycogen storage and breakdown has the same enzymes for synthesis and release.
Glycogen storage and breakdown has the same enzymes for synthesis and release.
Liver is the responsible to regulates the energy during the hipoglycemia.
Liver is the responsible to regulates the energy during the hipoglycemia.
Flashcards
Glycogen storage
Glycogen storage
Excess glucose is stored in the liver and skeletal muscle.
Liver glycogen function
Liver glycogen function
Liver glycogen maintains blood glucose homeostasis.
Skeletal muscle Function
Skeletal muscle Function
Muscle glycogen provides energy for strenuous exercise.
Glycogen storage
Glycogen storage
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Structure of glycogen
Structure of glycogen
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Glycosidic Bonds
Glycosidic Bonds
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Glycogen Synthase
Glycogen Synthase
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Branching Enzyme
Branching Enzyme
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UDP-glucose
UDP-glucose
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Glycogenin function
Glycogenin function
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Glycogen synthase
Glycogen synthase
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Branches
Branches
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location of Glycogenesis
location of Glycogenesis
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Glycogen Phosphorylase
Glycogen Phosphorylase
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Glycogen Phosphorylase limits
Glycogen Phosphorylase limits
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Debranching enzyme
Debranching enzyme
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Lysosomal degradation of glycogen
Lysosomal degradation of glycogen
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Irreversibility
Irreversibility
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Study Notes
- Excess glucose is stored as glycogen mainly in the liver and skeletal muscle.
- Liver glycogen stores increase.
- Muscle mass is higher than the liver, so glycogen stores are greater in the liver.
Glycogen stores are found in...
- Liver glycogen helps to maintain glucose homeostasis in the body during the early stages of fasting.
- In the liver, when glycogen is broken down it is released into circulation
- Muscle acts as a fuel reserve/energy for strenuous exercise versus rest.
- When glycogen is broken down in the muscle, it is not released into circulation; it is used as energy by the muscle.
- Glycogen stores are also found in the kidneys, heart, and brain.
- Stored in discrete cytoplasmic granules that are spherical and stained.
- Fed states increase the state number of granules
Structure of Glycogen
- Branched-chain polysaccharide (polymer of glucose)
- Contains 10,000-50,000 glucose residues per molecule.
- Branches help the body mobilize or breakdown glycogen rapidly for energy.
- Lack of branching would reduce solubility and accessibility, making it harder for enzymes to reach the glucose units.
- A highly branched polysaccharide made of glucose units linked by α-1,4 glycosidic bonds in the linear chains and α-1,6 glycosidic bonds at the branch points.
- Branching occurs roughly every 8-12 glucose units.
Structure of the Building Block of Glycogen
- Made exclusively from a-D glucose.
Glycogenesis
- Requires formation of uridine di-phosphate glucose (UDP-Glucose).
- Enzymes should be in the cytosol.
- Glucose must be activated first before glycogenesis, and is activated by ATP and UTP
UDP-Glucose in Glycogen Synthesis
- Key for specificity, glycogen synthase recognizes UDP-glucose
- UDP-glucose is the active form of glucose, ready to be added to the growing glycogen chain.
- High-energy ensures reactions happen and are unique for glycogenesis
Glycogen Synthesis Initiation
- Requires a fragment of glycogen/glucosyl chain produced by glycogenin that is the enzyme acceptor of glucose residues from UDP glucose
- Glycogenin extends the chain by 6-7 glucosyl units by forming α(1-4) linkages-glucan primer
- Glycogenin- autoglucosylation, is at the center of glycogen and becomes the enzyme
Elongation of Glycogen Chains
- Transfer of glucose from UDP glucose to nonreducing end of growing chain
- Glucose residues in glycogen connect via the alpha-1,4 glycosidic bonds in linear strands, catalyzed by glycogen synthase-rate-limiting step
Formation of Branches
- Branches located at intervals of 8 glucosyl residues apart
- Glucose residues connect via alpha-1,6 glycosidic bonds at junction points via the action of branching enzyme
- Branching enzyme removes a set of 6-8 glucosyl residues from the nonreducing end of the glycogen chain.
- Attaches it to non-terminal glucosyl residue by an α 1-6 linkage
Glycogenolysis
- Occurs mainly in the cytosol, little in lysosome, and is not a reversal of the synthetic reaction
Shortening of Chains
- Glycogen phosphorylase sequentially removes glucosyl residues at the nonreducing ends of the glycogen chains.
- Stops when 4 glycosyl units remain from the branch point
- Resulting structure after glycogen phosphorylase action now known as limit dextrin
- Phosphorylase cannot degrade past limit dextrin
Glycogen Phosphorylase
- Catalyzes the breakdown of glycogen and breaks the α(1-4) glycosidic bonds
Removal of Branches
- Transferase shifts 3 glycosyl units to the core from branched chain to main chain by breaking and making its alpha link
- An α-1-6 Glucosidase Hydrolyzes the single 1,6 glucose unit to free glucose.
Fate of Glucose-1-Phosphate
- Breakdown through phosphorolysis mechanism producing glucose-1-phosphate rather than by removing glucose one by one as free glucose by hydrolysis.
- It needs to be phosphorylated in some other steps and so it uses a molecule, or there would be no use of ATP as it would be already phosphorylated
Lysosomal Degradation of Glycogen
- 1-3% of glycogen is degraded by α (1-4) glucosidase (acid maltase) with a lysosomal enzyme
- In enzyme deficiencies there is an accumulation of glycogen in vacuoles in the lysosomes
- Enzyme deficiency is known as Glycogen storage disease type 2 (Pompe disease)
- Characterized by Lysosomal defect and associated with skeletal myopathy, muscular hypotonia and cardiomyopathy
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