Glycogen Metabolism and Disorders Quiz

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Questions and Answers

What is glycogen primarily composed of?

Galactose units

Unbranched polymer of glucose

Fructose units

Branched polymer of glucose (correct)

What is the main outcome of glycogen degradation via phosphorolysis?

Lactate

Glucose-1-phosphate (correct)

Glucose-6-phosphate

Glucose

Where is glycogen mainly stored in the human body?

Liver and skeletal muscles (correct)

Brain and lungs

Heart and adipose tissues

Pancreas and kidneys

Which enzyme converts glucose-1-phosphate to glucose-6-phosphate?

Phosphoglucomutase Signup and view all the answers

Which process primarily occurs in the liver to regenerate glucose from glucose-6-phosphate?

Gluconeogenesis Signup and view all the answers

What happens to glucose when it is in excess in the blood?

It is stored as glycogen. Signup and view all the answers

Which of the following cells lack glucose-6-phosphatase?

Muscle cells Signup and view all the answers

What role does acid maltase (α(14)-glucosidase) play in glycogen metabolism?

It degrades glycogen continuously. Signup and view all the answers

What condition is characterized by α(14)-glucosidase deficiency?

Pompe Disease Signup and view all the answers

Which glycogen storage disease is the most common?

Von Gierke Disease Signup and view all the answers

What process ensures priming during glycogen synthesis?

Glycogenin action Signup and view all the answers

How does a glycogen branching enzyme modify glycogen structure?

It creates α1→6 linkages Signup and view all the answers

Which of the following statements about McArdle Syndrome is correct?

It results in muscle glycogen phosphorylase deficiency. Signup and view all the answers

What is the effect of phosphorylation on enzymes during metabolic regulation?

It alters the enzyme's electrostatics. Signup and view all the answers

Which enzyme is responsible for the phosphorylation of glucose during glycogen synthesis?

Hexokinase Signup and view all the answers

What signals often lead to covalent modifications of enzymes?

Extracellular signals Signup and view all the answers

What hormone primarily promotes the relocation of GLUT4 transporters to the plasma membrane in muscle cells?

Insulin Signup and view all the answers

Which of the following statements correctly describes the role of epinephrine in carbohydrate metabolism?

Epinephrine has opposite effects on glycolysis in liver versus muscle. Signup and view all the answers

What is the primary effect of glucagon on liver cells?

Stimulates glucose production Signup and view all the answers

Why do muscle cells not contribute to blood glucose levels?

They lack glucose-6-phosphatase. Signup and view all the answers

How does insulin affect glycogen synthase in muscle cells?

By activating its activity. Signup and view all the answers

What effect does insulin have on glycogen degradation?

It inhibits glycogen degradation. Signup and view all the answers

Which hormone primarily promotes gluconeogenesis?

Glucagon Signup and view all the answers

What is the primary target organ for glucagon's action?

Liver Signup and view all the answers

Which of the following is NOT affected by insulin?

Gluconeogenesis Signup and view all the answers

How does insulin affect the levels of cyclic AMP?

It decreases cyclic AMP levels. Signup and view all the answers

What effect do glucagon and epinephrine have on fructose-2,6-bisphosphate levels?

Both decrease fructose-2,6-bisphosphate levels. Signup and view all the answers

Which of the following statements about phosphatase is correct?

Phosphatase is lacking in muscles. Signup and view all the answers

Which hormone is released from the adrenal medulla?

Epinephrine Signup and view all the answers

What characterizes the regulation of hexokinase IV (glucokinase) compared to hexokinases I-III?

It operates at a higher Vmax during high blood glucose levels. Signup and view all the answers

Which of the following statements is true regarding pyruvate kinase regulation?

It is activated by glucagon through phosphorylation. Signup and view all the answers

What role does fructose 2,6-bisphosphate play in metabolism?

It activates glycolysis and inhibits gluconeogenesis. Signup and view all the answers

Which statement accurately describes the function of glycogen phosphorylase in the liver?

It converts phosphorylase a to its less active form in response to high glucose. Signup and view all the answers

How does ATP influence the activity of phosphofructokinase-1 (PFK-1)?

High ATP concentrations inhibit PFK-1. Signup and view all the answers

What distinguishes the activity of glycogen synthase a from glycogen synthase b?

Glycogen synthase a is active and not phosphorylated. Signup and view all the answers

In terms of glycolysis regulation, what is the primary function of ATP and citrate?

They inhibit glycolysis and enhance gluconeogenesis. Signup and view all the answers

What is the impact of glucagon on pyruvate kinase in the liver?

It inactivates pyruvate kinase through phosphorylation. Signup and view all the answers

What factor is critical for determining the fate of pyruvate in the cell?

The availability of acetyl-CoA. Signup and view all the answers

When glucose levels fall, which hormone is primarily responsible for stimulating gluconeogenesis?

Glucagon Signup and view all the answers

What is the role of Casein Kinase II in glycogen regulation?

Priming for glycogen synthase activation. Signup and view all the answers

Which statement best describes the interaction between hormones and fructose-2,6-bisphosphate?

Insulin activates PFK-2 while glucagon activates FBPase-2. Signup and view all the answers

What happens to the activity of glycolysis if fructose-2,6-bisphosphate levels increase?

Glycolysis will be stimulated. Signup and view all the answers

What is the effect of high glucose levels on glycogen availability in muscles?

High glucose levels stimulate glycogen synthesis. Signup and view all the answers

Study Notes

Glucose and Glycogen Regulation

Glucose levels in the blood (glycemia) are tightly controlled through various processes that produce or consume glucose.
Excess glucose is stored as glycogen, which is released when needed to maintain normal blood glucose levels.

Glucose Dynamics in Living Organisms

Key processes involved in glucose regulation include glycogenesis, glycogenolysis, glycolysis, and gluconeogenesis.
Glycogen is a branched polymer of glucose, stored primarily in the liver and skeletal muscles.

Glycogen Degradation

Glycogen degradation involves removing glucose residues from the non-reducing end of the glycogen molecule through phosphorolysis.
Glycogen phosphorylase catalyzes the breakdown of glycogen.
A debranching enzyme is also needed for complete glycogen breakdown.

Glycogen Degradation: Debranching

The debranching process involves two steps:
- Transferase activity: moves a block of oligoglucose from a branched point to a linear chain
- α(1→6) glucosidase activity: hydrolyzes the remaining α1→6 linked glucose.
Outcome of debranching is glucose-1-phosphate.

Glycogen Degradation and Regulation of Glucose Level

Glucose-1-phosphate is converted to glucose-6-phosphate.
Glucose-6-phosphate can undergo glycolysis, the pentose pathway, or gluconeogenesis.
Gluconeogenesis regenerates glucose using glucose-6-phosphatase, primarily in liver and kidney cells.
Muscle and adipose tissue lack glucose-6-phosphatase.

Glycogen Storage Diseases

Several diseases are possible from glycogen breakdown issue due to various enzyme deficiencies.
Acid maltase (α(1,4)-glucosidase) deficiency is known as Pompe disease.
McArdle syndrome is caused by a deficiency in glycogen phosphorylase in muscle tissue.
Von Gierke disease is the most common glycogen storage disease, caused by glucose-6-phosphatase deficiency.

Glucose 6-Phosphatase Deficiency: Von Gierke Disease

Von Gierke disease is characterized by glucose-6-phosphatase deficiency.
Symptoms include fasting hypoglycemia, fatty liver, hepatomegaly, and other issues.
Treatment often includes frequent glucose administration.

Glycogen Synthesis

Glucose-6-phosphate is converted to glucose-1-phosphate for glycogen storage synthesis.
Glycogenin initiates glycogen synthesis by attaching glucose units.

Glycogen Synthesis: Chain Elongation

UDP-glucose is used as a precursor for glucose attachment to the growing glycogen chain.
Glycogen synthase catalyzes the elongation process.

Glycogen Synthesis: Branching

Glycogen branching enzyme creates α(1→6) linkages, making the glycogen molecule more branched.

Glycogen Synthesis Priming

Glycogenin starts the polymerization.

Regulation of Metabolic Pathways

Cells and organisms maintain a dynamic steady state.
Enzyme activity is regulated by various factors, including association with regulatory proteins and compartmentation.

Covalent Modifications of Enzymes

Phosphorylation/dephosphorylation are common means of enzyme regulation, modifying their activity.
Kinases add phosphate groups, while phosphatases remove them.

Assessment of Individual Enzyme Contribution to a Pathway

The contribution of each enzyme to the metabolic flux can be determined experimentally.

Coordinated Regulation of Glycolysis and Gluconeogenesis

Coordinated regulation between glycolysis and gluconeogenesis is critical.
Regulation is achieved at the level of the three exergonic steps in both pathways.
F-2,6-BP is a key regulator.

Regulation of Glycolysis at Hexokinase Level

Important isozymes, hexokinase IV (glucokinase), have distinctive features.
Different isozymes have varying Km, Vmax and regulation mechanisms.
Glucokinase is not inhibited by glucose-6-phosphate.

Hexokinase Isozyme Regulation

Isozymes I-III have low Km and are inhibited by glucose-6-phosphate, functioning primarily in muscle tissue.
Glucokinase activity increases with increasing blood glucose to regulate its uptake.

Regulation of Glycolysis at PFK Level

This is the key regulatory step in glycolysis.
PFK-1 is allosterically regulated by ATP, and enhanced by citrate.
Lower ATP stimulates PFK-1 activity.

Regulation of Glycolysis at Pyruvate Kinase Level

Pyruvate kinase is allosterically regulated by ATP, acetyl-CoA, fatty acids and alanine.
A cAMP-dependent protein kinase (PKA) phosphorylates Pyruvate kinase (liver) inhibiting it.

Coordinated Regulation of Gluconeogenesis

Pyruvate carboxylase is regulated allosterically by acetyl-CoA.

Coordinated Regulation of Glycolysis and Gluconeogenesis

ATP and citrate inhibit glycolysis.
ADP and AMP activate glycolysis and inhibit gluconeogenesis.
Fructose 2,6-bisphosphate is an important regulator

Fructose 2,6-bisphosphate

Fructose 2,6-bisphosphate is an allosteric regulator of PFK-1 (glycolysis) and FBPase-1 (gluconeogenesis).
High levels of F2,6-BP activates PFK-1 and inhibits FBPase-1.

PFK-2/FBPase-2

The PFK-2/FBPase-2 regulates F26BP levels and thus influences glycolysis and gluconeogenesis.
Insulin stimulates its kinase activity, while glucagon stimulates the phosphatase activities.

Coordinated Regulation at the level of F6P/F1,6bisP: overview

Shows the regulation by allosteric effectors of the enzymes at the glycolysis/gluconeogenesis junction and the role of cAMP and hormones.

Coordinated regulation of glycogen synthesis and breakdown: Control of glycogen breakdown

Glycogen phosphorylase exists in two forms; a (active) and b (less active).
Phosphorylation (by phosphorylase kinase) transforms phosphorylase b into phosphorylase a.

Coordinated regulation of glycogen synthesis and breakdown: Control of glycogen synthesis

Glycogen synthase exists as a (active) and b (less active) forms.
Activation of Glycogen synthase b (inactive) is catalyzed by PP1 (phosphatase).
Glycogen synthesis and breakdown are reciprocally regulated.

Control of glycogen synthesis from blood glucose in myocytes

GLUT4 translocation to the plasma membrane facilitates glucose uptake in muscle cells upon insulin signaling.
Hexokinase, once stimulated properly, begins phosphorylation and subsequent steps towards glycogen synthesis.

Carbohydrate Regulation in Liver Cells (Overview)

Insulin and glucagon have antagonistic effects on carbohydrate metabolism in liver cells.
Insulin promotes glycogen synthesis and inhibits glycogen breakdown.
Glucagon promotes glycogen breakdown and inhibits glycogen synthesis and activates gluconeogenesis.

Hormones and Their Effects

Specific hormones regulate pathways by impacting enzyme activity in different cells.
Insulin promotes glycogen synthesis and inhibits glycogen breakdown.
Glucagon promotes glycogen breakdown and inhibits glycogen synthesis.

Differences in Carbohydrate Metabolism in Liver and Muscles

These are distinct (different) processes in terms of their regulation, especially in terms of the availability of glucose.
Muscle cells do not produce blood glucose, therefore they do not need a blood glucose regulatory system (phosphatase).

Effects of Insulin

Insulin inhibits cAMP-dependent protein kinase (PKA)
Insulin stimulates glycogen synthesis
Insulin inhibits glycogen degradation
Insulin stimulates phosphoprotein phosphatase

Effects of Glucagon, Epinephrine and Insulin

These hormones have different effects on liver and muscle cells' carbohydrate metabolism.
Glucagon and epinephrine promote glycogen breakdown.
Insulin promotes glycogen synthesis.

Metabolic Characteristics of Glycolysis

Glycolysis occurs in several stages and is influenced by different factors & metabolites (NADH, Lactate, etc...).

Drugs and diseases

Certain diseases can be caused by issues in glycogen metabolism or hormone regulation.
Some drugs can enhance specific biochemical reactions and/or help with disease management.

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Description

Test your knowledge on glycogen metabolism, storage, and related diseases with this comprehensive quiz. Explore the biochemical pathways, enzyme functions, and clinical implications associated with glycogen in the human body. Perfect for students studying biochemistry or related fields.