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Questions and Answers
What are the sources of blood glucose?
What are the sources of blood glucose?
After you eat, glucose spikes and then declines, between meals glycogenolysis takes place, overnight gluconeogenesis occurs, and lactose, fatty acids, and triglycerides are converted into glucose.
What is so important about glycogen?
What is so important about glycogen?
It is a major storage form of glucose found in both liver and muscle, serving different purposes in each tissue.
Why is tissue distribution of carbohydrate energy reserves important?
Why is tissue distribution of carbohydrate energy reserves important?
Muscle has limited capacity to store glycogen despite having a large mass.
Describe the general structure of glycogen.
Describe the general structure of glycogen.
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What initiates glycogen synthesis?
What initiates glycogen synthesis?
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What is glycogenin?
What is glycogenin?
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What is the biological advantage of synthesizing glycogen with many branches?
What is the biological advantage of synthesizing glycogen with many branches?
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What happens at the reducing and non-reducing ends of glycogen?
What happens at the reducing and non-reducing ends of glycogen?
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What are the steps in glycogenesis?
What are the steps in glycogenesis?
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What is glycogen storage disease?
What is glycogen storage disease?
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What occurs in Pompe disease?
What occurs in Pompe disease?
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What is McArdle's disease?
What is McArdle's disease?
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What are some symptoms associated with glycogen storage diseases?
What are some symptoms associated with glycogen storage diseases?
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Von Gierke's disease is caused by a deficiency in glucose 6-phosphatase.
Von Gierke's disease is caused by a deficiency in glucose 6-phosphatase.
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What is the treatment approach for patients with glycogen storage diseases?
What is the treatment approach for patients with glycogen storage diseases?
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Von Gierke's disease primarily affects which enzyme connected to glucose metabolism?
Von Gierke's disease primarily affects which enzyme connected to glucose metabolism?
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What is a key characteristic of Cori disease?
What is a key characteristic of Cori disease?
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In muscle, G6P must undergo what process for energy utilization?
In muscle, G6P must undergo what process for energy utilization?
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Muscle can directly contribute to blood glucose levels.
Muscle can directly contribute to blood glucose levels.
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What initiates the hormonal regulation of glycogenolysis?
What initiates the hormonal regulation of glycogenolysis?
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What are the effects of glucagon on glycogen metabolism?
What are the effects of glucagon on glycogen metabolism?
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Which condition results in elevated lactate and urea levels?
Which condition results in elevated lactate and urea levels?
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Match the following conditions with their key characteristics:
Match the following conditions with their key characteristics:
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What does epinephrine do in the context of glycogen metabolism in muscle?
What does epinephrine do in the context of glycogen metabolism in muscle?
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In the liver, glycogen is located in the cytosol near folds of the ______.
In the liver, glycogen is located in the cytosol near folds of the ______.
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Glycogen metabolism is regulated only by hormonal signals.
Glycogen metabolism is regulated only by hormonal signals.
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What is the main consequence of Von Gierke's disease?
What is the main consequence of Von Gierke's disease?
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Study Notes
Sources of Blood Glucose
- Blood glucose levels rise post-meal due to glucose intake and decrease afterwards.
- Between meals, glycogenolysis occurs, breaking down glycogen to release glucose.
- Overnight, gluconeogenesis converts non-carbohydrate sources like lactate and fatty acids into glucose.
Importance of Glycogen
- Glycogen is the body's primary storage form of glucose, primarily found in the liver and muscle tissues.
- In the liver, glycogen maintains blood glucose levels during fasting; it depletes within 12-18 hours.
- Muscle glycogen is utilized for energy during prolonged exercise and is less affected by fasting.
Tissue Distribution of Glycogen
- Muscle potentially holds large stores of glycogen, but its capacity is less due to greater muscle mass.
General Structure of Glycogen
- Glycogen is a branched polymer of glucose with alpha 1-4 linkages in linear segments and alpha 1-6 linkages at branch points.
- The branching structure allows simultaneous addition or removal of glucose molecules, enhancing synthesis and degradation rates.
Glycogen Synthesis Initiation
- Glycogen synthesis relies on pre-existing glycogen molecules.
- Glycogenin functions as a primer for glycogen synthesis through autoglucosylation.
Glycogenin
- Glycogenin's anomeric carbon binds to glucose, forming a glycosidic bond and serving as a critical component in glycogen synthesis.
Biological Advantage of Glycogen Branching
- The highly branched structure of glycogen increases solubility and provides numerous non-reducing ends for enzyme action, facilitating faster synthesis and breakdown.
Glycogen Metabolism Overview
- Glycogen conversion to glucose occurs during fasting, stress, or exercise.
- Important to know the detailed steps and enzymes involved in glycogen metabolism.
Regulation of Glycogen
- Simultaneous regulation includes activating glycogen synthase and inactivating glycogen phosphorylase.
- Enzymatic actions are interdependent to prevent simultaneous synthesis and breakdown.
Steps in Glycogenesis
- Conversion from glucose to glucose-6-phosphate (G6P) initiates glycogenesis.
- G6P further converts to glucose-1-phosphate, leading to synthesis of high-energy UDP-glucose.
- Glycogen synthase catalyzes the formation of alpha-1,4 linkages during glycogen elongation.
Action of Branching Enzyme
- The branching enzyme cleaves alpha-1,4 linkages and forms new alpha-1,6 linkages to create branches in glycogen structure.
Glycogen Storage Diseases (GSDs)
- Over 10 inherited metabolic disorders affecting glycogen synthesis or breakdown.
- Symptoms vary but can be life-threatening, primarily manifesting in childhood.
GSD 0
- Caused by glycogen synthase deficiency, leading to low glycogen levels in the liver.
- Symptoms include fasting hypoglycemia and growth delays, with dietary management required.
Anderson's Disease (GSD IV)
- Resulting from branching enzyme deficiency, leading to abnormal glycogen structure and severe liver damage.
- Symptoms include hypotonia, failure to thrive, and a high risk of liver cirrhosis.
Hormonal Regulation of Glycogenesis
- Insulin stimulates glycogen synthesis while suppressing the breakdown by activating phosphatases.
- Increased glucose levels correlate with higher insulin and lower glucagon.
Insulin's Effect on Glycogen Metabolism
- Insulin enhances glycogen synthesis and promotes glucose uptake in muscles.
- It inactivates glycogen phosphorylase, reducing glycogen breakdown.
Allosteric Regulation by Glucose-6-Phosphate (G6P)
- Elevated G6P levels, particularly post-meal, stimulate glycogen synthesis by activating glycogen synthase.
- G6P serves as a key intermediate in glycolysis, influencing metabolic pathways based on glucose demand.
Glycogenolysis
- Triggered by low glucose levels, physical activity, and stress responses to maintain blood glucose.
- Involves systematic breakdown of glycogen to release glucose for energy.
Debranching Enzymes
- Essential for removing glucose residues from glycogen branches, allowing for comprehensive breakdown of the stored polysaccharide.
Lysosomal Degradation of Glycogen
- A small percentage of glycogen is degraded by lysosomal enzymes, crucial for maintaining cellular energy metabolism.
Specific Glycogen Storage Diseases
- Pompe disease: Affects muscle function due to alpha(1--> 4)-glucosidase deficiency.
- McArdle disease: Muscle phosphorylase deficiency causing exercise intolerance and high glycogen levels.
- Von Gierke's disease: Results from glucose-6-phosphatase deficiency leading to severe hypoglycemia and hepatomegaly.
- Cori disease: Debranching enzyme deficiency causing shorter glycogen branches and mild symptoms.
Fate of Glucose-1-Phosphate
- Converted to glucose-6-phosphate, precursor for energy production in muscle or glucose release in the liver.
Muscle and Liver Glucose Management
- Muscle does not release glucose directly into the bloodstream, whereas the liver releases glucose from G6P to regulate blood glucose levels during fasting states.### Hormonal Regulation of Glycogenolysis
- Decreased glucose leads to decreased insulin, increased glucagon, and increased epinephrine, activating kinase enzymes.
- Regulation relies on phosphorylation and dephosphorylation, primarily triggered by glucagon.
Gluconeogenesis and Glycogenolysis
- Glycogenesis and glycogenolysis utilize different, non-reversible enzymes, with tightly controlled regulation.
- The binding of glucagon and epinephrine triggers cyclic AMP (cAMP), a key second messenger in glycogen breakdown.
Response Mechanisms
- Glycogenolysis responds to stress, exercise, and blood loss to maintain blood glucose levels.
- Under these conditions, glycogen is mobilized to ensure adequate glucose supply in the liver.
Hormonal Effects on Glycogenolysis
- Insulin promotes glycogen synthesis; glucagon primarily affects the liver, inhibiting glycogen breakdown.
- Epinephrine rapidly stimulates glycogenolysis in both liver and muscle tissues.
Mechanism of Action
- Protein Kinase A phosphorylates glycogen synthase to inactivate it and activates glycogen phosphorylase through phosphorylation.
- This process ensures glycogen degradation and promotes glycogenolysis while inhibiting glycogenesis.
Muscle Regulation Differentiation
- Muscle tissues do not express glucagon receptors; they rely on epinephrine and norepinephrine for regulation.
- During glycogenolysis in muscle, epinephrine activates pathways similar to those in the liver but through different receptors.
Allosteric Regulation in Muscle
- AMP levels increase during muscle activity, activating glycogen phosphorylase, essential for energy during contractions.
- Calcium ions further enhance the activity of glycogen phosphorylase kinase, promoting glycogenolysis.
Glycolytic Enzyme Activity in Muscle
- Muscle glycolytic enzymes remain active during glycogenolysis due to epinephrine mimicking glucagon's effects.
- Hexokinase and Phosphofructokinase-1 (PFK-1) promote metabolism by responding to AMP, providing a rapid energy supply.
Bifunctional Protein Regulation
- PFK-2/FBPase-2 gets phosphorylated under low blood sugar conditions, decreasing fructose 2,6-bisphosphate levels.
- This phosphorylation reduces glycolysis and enhances gluconeogenesis, vital for maintaining blood glucose levels.
ATP and AMP Concentrations
- ATP concentrations are typically higher than AMP, making AMP a sensitive indicator of energy status.
- Small fluctuations in ATP levels trigger significant variations in AMP, modulating various regulatory processes.
Allosteric Interaction of Glycogen Metabolism
- Glucose-6-phosphate (G6P) activates glycogen synthase but inhibits glycogen phosphorylase, balancing energy production and storage.
Glycogen Storage Diseases (GSDs)
- GSDs can affect liver, muscle, or red blood cells, often resulting in hypoglycemia and hepatomegaly when the liver is involved.
Pyruvate Kinase Variability
- Muscle isoenzyme of pyruvate kinase remains unphosphorylated and active despite hormonal signaling, unlike its liver counterpart.
McArdle's Disease
- McArdle's disease features glycogen phosphorylase deficiency in skeletal muscle, leading to cramping and weakness during exertion due to limited glycogen utilization.
Gierke's Disease
- Gierke's disease results from glucose-6-phosphatase deficiency, causing severe hypoglycemia and liver enlargement due to glycogen accumulation.
- The body compensates by relying on fatty acids and ketone bodies, often leading to metabolic acidosis.
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Test your knowledge of glycogen and its role in glucose metabolism with these flashcards. Explore important concepts such as glycogenolysis and gluconeogenesis, and understand the significance of glycogen storage in the liver and muscles.
