Glucose and Glycogen Regulation
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Questions and Answers

What role does insulin play in glycogen synthesis in myocytes?

  • It decreases hexokinase activity.
  • It promotes the relocation of GLUT4 to the plasma membrane. (correct)
  • It increases blood glucose levels.
  • It inhibits glycogen synthase.

Which of the following statements about carbohydrate regulation in liver cells is true?

  • Insulin and glucagon have the same effects on carbohydrate metabolism.
  • Epinephrine has an antagonistic effect to glucagon in liver carbohydrate metabolism. (correct)
  • Epinephrine only stimulates glycogen synthesis in liver cells.
  • Only insulin stimulates glucose production in liver cells.

What is the main difference between carbohydrate metabolism in muscles and liver cells?

  • Insulin does not affect muscle cells in carbohydrate metabolism.
  • Muscles contribute to blood glucose levels like liver cells.
  • Liver cells respond to glucagon while muscles do not. (correct)
  • Muscles lack the ability to synthesize glycogen.

What effect does epinephrine have on glycolysis in liver and muscle cells?

<p>It acts oppositely on glycolysis in liver compared to muscle cells. (B)</p> Signup and view all the answers

Which of the following best describes the simultaneous action of hormones such as insulin?

<p>They act on different pathways and enzymes to result in a cohesive metabolic effect. (B)</p> Signup and view all the answers

What is the most common glycogen storage disease?

<p>Von Gierke Disease (D)</p> Signup and view all the answers

What enzyme deficiency leads to Pompe Disease?

<p>α(14)-glucosidase (C)</p> Signup and view all the answers

Which process is primarily involved in elongating glycogen chains?

<p>α(1→4) linkage reaction (D)</p> Signup and view all the answers

Which condition is a result of deficiency in muscle glycogen phosphorylase?

<p>McArdle Syndrome (A)</p> Signup and view all the answers

What is a key characteristic of covalent modifications of enzymes?

<p>They often involve phosphorylation and dephosphorylation. (B)</p> Signup and view all the answers

Which component is responsible for priming glycogen synthesis?

<p>Glycogenin (B)</p> Signup and view all the answers

What happens when the rate of conversion from substrate to product is equal to the reverse rate?

<p>A steady state is maintained. (C)</p> Signup and view all the answers

The FDA approved a recombinant form of which enzyme in 2006 for treating Pompe Disease?

<p>α(14)-glucosidase (D)</p> Signup and view all the answers

What is the primary function of glycogen in the body?

<p>To store excess glucose for later use. (D)</p> Signup and view all the answers

Which type of bond links the glucose units in glycogen?

<p>α1→4 and α1→6 Glycosidic bond (D)</p> Signup and view all the answers

Which of the following correctly describes the process of phosphorolysis in glycogen degradation?

<p>It removes glucose residues from the non-reducing end. (B)</p> Signup and view all the answers

Where is glucose-6-phosphatase predominantly found?

<p>In liver and kidney cells. (C)</p> Signup and view all the answers

What is the outcome of the debranching process in glycogen degradation?

<p>Production of glucose. (C)</p> Signup and view all the answers

Which of the following statements is true regarding muscle and adipose tissue cells?

<p>They do not have glucose-6-phosphatase. (B)</p> Signup and view all the answers

Which enzyme is responsible for a continuous, small scale degradation of glycogen?

<p>Acid maltase (α(14)-glucosidase). (A)</p> Signup and view all the answers

Which process primarily consumes glucose-6-phosphate in the liver?

<p>All of the above. (D)</p> Signup and view all the answers

What is the primary effect of insulin on glycogen synthesis?

<p>Stimulates glycogen synthesis (C)</p> Signup and view all the answers

Which hormone is responsible for increasing cyclic-AMP levels in the body?

<p>Adrenaline (A), Glucagon (D)</p> Signup and view all the answers

Which of the following is directly inhibited by insulin during carbohydrate metabolism?

<p>Phosphorylase kinase (D)</p> Signup and view all the answers

In which tissue does glucagon primarily exert its effects?

<p>Liver (A)</p> Signup and view all the answers

What is the effect of glucagon on gluconeogenesis?

<p>Stimulates gluconeogenesis (A)</p> Signup and view all the answers

Which of the following is NOT an effect of insulin?

<p>Stimulates gluconeogenesis (C)</p> Signup and view all the answers

What is the general role of phosphoprotein phosphatase related to insulin action?

<p>It dephosphorylates glucose transporter (D)</p> Signup and view all the answers

Which pathway is primarily stimulated by epinephrine in muscle tissue?

<p>Glycogen breakdown (C)</p> Signup and view all the answers

What is the primary regulatory mechanism of PFK-1 in glycolysis?

<p>Allosteric regulation by ATP and citrate (A)</p> Signup and view all the answers

Which hexokinase isozyme is primarily found in the liver?

<p>Hexokinase IV (glucokinase) (A)</p> Signup and view all the answers

What effect does fructose-2,6-bisphosphate have on glycolysis and gluconeogenesis?

<p>Activates glycolysis and inhibits gluconeogenesis (A)</p> Signup and view all the answers

How does the activity of glucokinase change when blood glucose levels increase?

<p>Is activated and increases (A)</p> Signup and view all the answers

What is the effect of ATP on pyruvate kinase activity in glycolysis?

<p>Inhibition when energy is abundant (C)</p> Signup and view all the answers

Which enzyme initiates the gluconeogenesis pathway from pyruvate?

<p>Pyruvate carboxylase (C)</p> Signup and view all the answers

What role does insulin play in the regulation of glycogen metabolism?

<p>Promotes glycogen synthesis (B)</p> Signup and view all the answers

Which of the following is an allosteric inhibitor of PFK-1?

<p>Citrate (A)</p> Signup and view all the answers

What is the function of glycogen phosphorylase in the liver?

<p>Catalyze the breakdown of glycogen (B)</p> Signup and view all the answers

Which enzyme is responsible for converting fructose-6-phosphate to fructose-1,6-bisphosphate?

<p>Phosphofructokinase (D)</p> Signup and view all the answers

What happens to glycogen phosphorylase when high levels of glucose are detected?

<p>Converts from active form a to inactive form b (B)</p> Signup and view all the answers

What is the consequence of acetyl-CoA on the regulation of pyruvate?

<p>Allosterically regulates both glycolysis and gluconeogenesis (C)</p> Signup and view all the answers

Which molecule serves as a signal of high-energy status to inhibit glycolysis?

<p>Citrate (B)</p> Signup and view all the answers

Flashcards

Glycogen Storage

Excess glucose is stored as glycogen, a branched glucose polymer, in the liver and muscles to maintain blood glucose levels.

Glycogen Degradation

Process of breaking down glycogen into glucose units for energy or to maintain blood sugar levels.

Phosphorolysis

The process of removing glucose units from glycogen using inorganic phosphate.

Debranching

Removal of branches from glycogen to release free glucose.

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Glucose-6-Phosphate

Intermediate product formed after breaking down glycogen; a key molecule for energy or glucose production.

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Glucose-6-phosphatase

Enzyme that converts glucose-6-phosphate back into glucose for release into the bloodstream; located in liver cells.

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Glycogen Storage Diseases

Genetic disorders where glycogen cannot be properly broken down or stored, leading to health problems.

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Blood Glucose Regulation

Tight control of glucose levels in the blood through processes of production and consumption.

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Pompe Disease

A genetic disorder caused by a deficiency of α(14)-glucosidase, leading to glycogen accumulation in muscles.

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McArdle Syndrome

A glycogen storage disease caused by a deficiency in muscle glycogen phosphorylase, leading to exercise intolerance and muscle cramps.

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Von Gierke Disease

The most common glycogen storage disease, caused by a deficiency in glucose 6-phosphatase, disrupting glucose release from the liver.

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Glycogen Synthesis

Building glycogen from glucose molecules. This process involves elongation and branching of the glucose chains.

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Glycogen branching

The creation of branches in the glycogen molecule. An enzyme cuts linear chains and reattaches them at position 6.

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Glycogenin

A protein that initiates glycogen synthesis by attaching the first few glucose units to itself, providing a starting point for the chain.

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Metabolic Pathway Homeostasis

Maintaining a stable internal environment by regulating the rates of chemical reactions in cells; when enzymes' forward and reverse activity are balanced, the amounts of intermediate components remain constant.

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Enzyme Modification (Phosphorylation/Dephosphorylation)

A quick way to change enzyme activity by adding or removing phosphate groups; this affects enzyme's charges and shape.

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Muscle Glycogen Synthesis

Muscle cells synthesize glycogen from blood glucose, driven by insulin, by relocating GLUT4 and activating hexokinase. This increases glucose uptake and glycogen production.

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Liver Carbohydrate Control

Liver carbohydrate regulation is governed by opposing actions of insulin and glucagon. Insulin promotes glycogen synthesis while glucagon promotes glucose release.

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Hormonal Pathway Effects

Hormones often act on multiple pathways and enzymes simultaneously to elicit a specific cellular effect.

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Muscle vs. Liver Glucose

Muscle cells do not directly contribute to blood glucose; they use glucose for their own energy, but lack glucose-6-phosphatase. The liver releases glucose into the blood, regulated by glucagon and epinephrine (opposite effects on liver glycolysis vs. muscle glycolysis).

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Epinephrine's Dual Role

Epinephrine stimulates glycogen breakdown in the liver but inhibits glycolysis in muscles. The effect is different in different tissues.

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Conformational changes in enzymes

Enzyme shape changes affecting activity (e.g., Vmax, Km).

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Active site modification

Altering an enzyme's active site, changing its activity

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Enzyme contribution to pathway

Determining how much each enzyme affects metabolic pathways

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Coordinated regulation glycolysis/gluconeogenesis

Controlling both pathways in balance by regulating exergonic steps

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Irreversible steps

Steps with high energy change that are hard to reverse in pathways

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Futile cycle

When opposite pathways run wasting energy

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Hexokinase isozymes

Different forms of hexokinase enzyme with specific roles in tissues

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Glucokinase (hexokinase IV)

Liver enzyme; high Km, high Vmax; handles high blood glucose

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Hexokinase regulation

Isozymes I-III inhibited by product G-6-P; isozyme IV (glucokinase) is not

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PFK-1 regulation

Allosterically regulated by ATP & citrate; affects glycolysis

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PFK-1

Phosphofructokinase-1; an important enzyme in the glycolysis pathway

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Pyruvate kinase regulation

Inhibited by energy abundance signals (ATP, acetyl-CoA); activated by F1,6BP

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Gluconeogenesis regulation

First enzymes regulated by acetyl-CoA; cell energy directs fate of pyruvate

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Fructose 2,6-bisphosphate

Allosteric regulator; activates glycolysis; inhibits gluconeogenesis

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PFK-2/FBPase-2

Enzyme with dual activity regulating F26BP; controlled by hormones.

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Glucose Homeostasis Pathways

The processes that maintain stable blood glucose levels.

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Glycogen Degradation Enzymes

Enzymes that break down glycogen to glucose.

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Glycogen Synthase

Enzyme that synthesizes glycogen.

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Metabolic Regulation in Vivo

Methods for altering pathway speeds in living organisms.

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Enzyme Chemical Modification

Altering enzyme activity by adding/removing chemical groups(e.g., phosphorylation).

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Insulin's effect on cAMP

Insulin lowers cyclic-AMP levels and inhibits glycogen breakdown.

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Glucagon's effect on Glycogen

Glucagon promotes glycogen breakdown.

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Glucose-6-phosphatase

Enzyme critical for glucose release from liver glycogen.

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Study Notes

Glucose and Glycogen Regulation

  • Glucose levels in the blood (glycemia) are tightly controlled through various processes involving glucose production and consumption.
  • Excess glucose is stored as glycogen, which cannot be stored in large quantities due to its osmotic nature.
  • Glycogen is a branched polymer of glucose linked together through α(1→4) and α(1→6) linkages.
  • Glycogen is primarily stored in the liver (up to 10% weight) and skeletal muscles (up to 1-2% weight).

Glycogen Degradation

  • Glucose residues are removed from the non-reducing ends via phosphorolysis
  • Glycogen phosphorylase catalyzes the removal of glucose-1-phosphate from glycogen.
  • Debranching enzyme is involved in the removal of branches from the glycogen molecule.
  • Glucose-1-phosphate is converted to glucose-6-phosphate.
  • Glucose-6-phosphate can be used in glycolysis, the pentose pathway, or gluconeogenesis to regenerate glucose.
  • Liver cells contain glucose-6-phosphatase, which is absent in muscle cells. This allows for the release of glucose into the bloodstream.

Glycogen Storage Diseases

  • Glycogen storage diseases result from deficiencies in enzymes involved in glycogen metabolism.
  • Pompe disease is a lysosomal (1→4)-glucosidase deficiency leading to glycogen accumulation in lysosomes.
  • McArdle syndrome is caused by a deficiency in skeletal muscle glycogen phosphorylase.
  • Von Gierke disease is the most common glycogen storage disease, resulting from a glucose-6-phosphatase deficiency.

Regulation of Glycolysis and Gluconeogenesis

  • Glycolysis and gluconeogenesis are regulated to maintain a dynamic steady state. Regulation happens at the level of the three irreversible steps.
  • Key enzymes in these pathways (e.g., PFK-1, pyruvate kinase, glucose-6-phosphatase) are regulated allosterically and/or via covalent modification.
  • Fructose-2,6-bisphosphate plays a crucial regulatory role, activating glycolysis and inhibiting gluconeogenesis.
  • ATP and citrate inhibit glycolysis.
  • ADP and AMP activate glycolysis and inhibit gluconeogenesis.
  • These pathways are regulated as a coordinated system to prevent futile cycles.

Regulation of Glycolysis at Hexokinase Level

  • Hexokinase IV (glucokinase) in liver cells has a higher Km for glucose and is not inhibited by glucose 6-phosphate.
  • This allows the liver to uptake and metabolize glucose even at low blood glucose concentrations.

Regulation of Glycolysis at PFK Level

  • PFK-1 is allosterically regulated by ATP and citrate.
  • Its activity is higher when ATP levels are low and citrate levels are high.

Regulation of Glycolysis at Pyruvate Kinase Level

  • Pyruvate kinase is allosterically regulated by ATP, acetyl-CoA, fatty acids, and alanine.
  • Insulin promotes the activation of pyruvate kinase in the liver.

Regulation of Glycogen Synthesis and Breakdown

  • Glycogen synthase is activated by insulin and de-activated by glucagon.
  • Glycogen phosphorylase is activated by glucagon, epinephrine, and AMP and inactivated by insulin and PP1.

Hormonal Regulation of Carbohydrate Metabolism

  • Insulin stimulates glycogen synthesis and inhibits glycogen breakdown.
  • Glucagon and epinephrine stimulate glycogen breakdown and inhibit glycogen synthesis.
  • They have opposing effects on glycolysis and gluconeogenesis and on the different tissues.

Summary of Glycogen Regulation

  • Coordinated regulation of glycolysis and gluconeogenesis is achieved to prevent futile cycles and maintain glucose homeostasis.

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Glucose and Glycogen Regulation (Lehninger Chapter 15) PDF

Description

Test your knowledge on the regulation of glucose and glycogen metabolism, including their storage, degradation, and biochemical pathways. This quiz covers key processes like glycolysis, gluconeogenesis, and the role of enzymes in glucose metabolism.

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