Biochemistry of Glycogen and Gluconeogenesis

Questions and Answers

What is the primary role of glycogen in the body?

To facilitate protein synthesis

To provide a rapidly mobilizable storage form of glucose (correct)

To serve as a long-term energy source

To act as a buffer during exercise

Which of the following statements about glycogen structure is correct?

Branches in glycogen are formed by α(1→6) glycosidic bonds. (correct)

Glycogen consists solely of fructose units.

Glucose molecules are linked by β(1→4) glycosidic bonds.

Glycogen is entirely linear with no branches.

What hormone is primarily responsible for promoting glycogen synthesis?

Glucagon

Insulin (correct)

Cortisol

Adrenaline

Which process plays a key role in maintaining plasma glucose during fasting?

Gluconeogenesis Signup and view all the answers

What is the primary function of the Cori Cycle during fasting?

To recycle lactate into glucose Signup and view all the answers

What is a common characteristic of glycogen storage disorders?

Dysregulated carbohydrate digestion Signup and view all the answers

In which tissue is glycogen primarily stored for maintaining blood glucose levels?

Liver Signup and view all the answers

What is generated during the process of glycolysis from glucose?

Pyruvate and ATP Signup and view all the answers

What percentage of gluconeogenesis occurs in the liver?

90% Signup and view all the answers

Which hormones stimulate and inhibit gluconeogenesis, respectively?

Glucagon and insulin Signup and view all the answers

Which of the following is NOT a substrate for gluconeogenesis?

Acetyl-CoA Signup and view all the answers

How many enzymes are specifically required to reverse glycolysis in gluconeogenesis?

4 Signup and view all the answers

What is the primary goal of treatment for Von Gierke’s Disease?

Avoid low blood sugar Signup and view all the answers

Which of the following symptoms is associated with Von Gierke’s Disease?

Coma Signup and view all the answers

What is primarily measured in blood tests for diagnosing Von Gierke’s Disease?

Decreased glucose and increased uric acid Signup and view all the answers

Which type of glyogen storage disorder has an incidence of 1 in 100,000 births and is due to a deficiency of glucose-6-phosphatase?

GSD I – Von Gierke’s Disease Signup and view all the answers

What is the initial step in the conversion of glucose for glycogen synthesis?

Conversion of glucose to glucose-6-phosphate Signup and view all the answers

Which enzyme catalyzes the addition of glucose from UDP-glucose onto glycogenin?

Glycogenin Signup and view all the answers

What is the role of the debranching enzyme in glycogenolysis?

It cleaves α(1→6) glycosidic bonds. Signup and view all the answers

How does insulin affect glycogen metabolism during the fed state?

It activates glycogen synthase. Signup and view all the answers

What is the primary function of glycogen phosphorylase?

To cleave α(1→4) glycosidic bonds in glycogen. Signup and view all the answers

What triggers glycogenolysis to occur?

Decreased plasma glucose levels Signup and view all the answers

Which form of glycogen synthase is active?

Active 'a' form Signup and view all the answers

Which molecule primarily activates glycogen phosphorylase in muscle during exercise?

Ca2+ Signup and view all the answers

What happens to glucose-6-phosphate in the liver after glycogenolysis?

It is released into the bloodstream as glucose. Signup and view all the answers

Which of the following is a condition that promotes gluconeogenesis?

Fasting Signup and view all the answers

Which enzyme is responsible for converting glucose-1-phosphate into glucose-6-phosphate?

Phosphoglucomutase Signup and view all the answers

What is one of the enzymatic activities of the glycogen branching enzyme?

It transfers glucose to other glycogen chains. Signup and view all the answers

What primarily activates glycogen synthase during the fed state?

Insulin Signup and view all the answers

What is a byproduct of the reaction catalyzed by UDP-glucose pyrophosphorylase?

UDP Signup and view all the answers

What is the primary metabolic role of lactate in the Cori Cycle?

It is transported to the liver and converted to glucose. Signup and view all the answers

Which of the following enzymes is crucial for bypassing the irreversible step catalyzed by pyruvate kinase in gluconeogenesis?

PEP carboxykinase Signup and view all the answers

The accumulation of which substance is primarily responsible for the hepatomegaly seen in Von Gierke's Disease?

Glucose-6-phosphate Signup and view all the answers

What non-carbohydrate source primarily contributes lactate to gluconeogenesis during the fasting state?

Muscle protein breakdown Signup and view all the answers

Which of the following conditions would NOT typically trigger increased gluconeogenesis?

High-carbohydrate meal Signup and view all the answers

Which mechanism primarily regulates glycogen phosphorylase activity in response to physiological demand?

Allosteric activation by AMP Signup and view all the answers

What is the primary biochemical consequence of a deficiency in glucose-6-phosphatase?

Inability to convert glucose-6-phosphate to glucose Signup and view all the answers

What event is triggered by glucagon during fasting to maintain blood glucose levels?

Promotion of glycogenolysis Signup and view all the answers

What is the role of glucagon in glycogen metabolism during fasting?

Stimulates glycogen breakdown by activating glycogen phosphorylase Signup and view all the answers

Which activity of the debranching enzyme is responsible for releasing glucose from the glycogen branch?

Glucosidase activity Signup and view all the answers

How does insulin promote glycogenesis in the liver after a meal?

By activating protein phosphatase 1 to dephosphorylate glycogen synthase Signup and view all the answers

Which is the correct sequence of events during glycogenesis starting from glucose?

Glucose → G6P → G1P → UDP-glucose → Glycogen Signup and view all the answers

What consequence arises from the action of catecholamines in muscle during stress?

Promotion of glycogen breakdown for immediate energy Signup and view all the answers

Which compound is used by the enzyme UDP-glucose pyrophosphorylase to synthesize UDP-glucose?

UTP Signup and view all the answers

What effect does activation of protein kinase A (PKA) have on glycogen metabolism?

Promotes glycogen breakdown by phosphorylating glycogen phosphorylase Signup and view all the answers

In the fasting state, what is the primary fate of glucose-6-phosphate in the liver?

Released as free glucose into the bloodstream Signup and view all the answers

What is the primary product of glycolysis?

Two molecules of pyruvate Signup and view all the answers

Which of the following categories contain carbohydrates? (Select all that apply)

Complex carbohydrates (starch and glycogen) and natural disaccharides (lactose and sucrose) Signup and view all the answers

What happens to pyruvate during pyruvate oxidation?

It is converted into acetyl CoA Signup and view all the answers

Where does Acetyl CoA enter after its formation from pyruvate?

Krebs cycle (Citric acid cycle) Signup and view all the answers

In oxidative phosphorylation, which molecules enter the electron transport chain?

NADH and FADH2 Signup and view all the answers

Glycogen consists of:

Branched polymers of glucose Signup and view all the answers

Glycogen is primarily synthesized and stored in which tissues?

Liver and skeletal muscle Signup and view all the answers

What is the role of liver glycogen?

To maintain blood glucose levels Signup and view all the answers

How are glucose molecules linked linearly in glycogen?

α(1→4) glycosidic bonds Signup and view all the answers

Branches in glycogen are formed by which type of glycosidic bond?

α(1→6) Signup and view all the answers

The enzyme responsible for converting glucose to glucose-6-phosphate in glycogen synthesis is:

Glucokinase (Hexokinase) Signup and view all the answers

What is the role of glycogen phosphorylase in glycogen breakdown (glycogenolysis)?

Cleaves α(1→4) glycosidic bonds to release glucose-1-phosphate Signup and view all the answers

What is the role of glycogen phosphorylase in glycogen breakdown (glycogenolysis)?

Cleaves α(1→4) glycosidic bonds to release glucose-1-phosphate Signup and view all the answers

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

Phosphoglucomutase Signup and view all the answers

Glycogen breakdown is triggered when:

Plasma glucose levels decrease Signup and view all the answers

The primary regulators of glycogen metabolism include which of the following enzymes?

Glycogen phosphorylase and glycogen synthase Signup and view all the answers

In allosteric regulation, which molecule activates glycogen synthase in the liver?

Glucose-6-phosphate (G6P) Signup and view all the answers

Hormonal regulation of glycogen synthesis involves: A) Glucagon stimulating glycogen synthesis B) Insulin inhibiting glycogenesis C) Protein kinase A phosphorylating enzymes to regulate activity D) Glucose directly binding to enzymes

Protein kinase A phosphorylating enzymes to regulate activity Signup and view all the answers

Which hormone is secreted in response to low blood sugar, leading to glycogen breakdown?

Glucagon Signup and view all the answers

The Cori Cycle converts lactate into glucose in which organ?

Liver Signup and view all the answers

Von Gierke’s disease (GSD I) is caused by a deficiency in which enzyme?

Glucose-6-phosphatase Signup and view all the answers

Glycogen phosphorylase enzyme cleaves which bonds in glycogen?

α(1→4) glycosidic bonds Signup and view all the answers

Which enzyme is responsible for converting Glucose-1-Phosphate (G1P) to Glucose-6-Phosphate (G6P)?

Phosphoglucomutase Signup and view all the answers

Glycogen synthesis is regulated primarily by which two enzymes?

Glycogen synthase and Glycogen phosphorylase Signup and view all the answers

What happens to glycogen synthase when it is phosphorylated?

It becomes inactive Signup and view all the answers

In response to low blood sugar, which hormone activates Protein Kinase A (PKA), leading to glycogen breakdown?

Glucagon Signup and view all the answers

During muscle contraction, which molecule activates Glycogen Phosphorylase?

AMP and Ca²⁺ Signup and view all the answers

In muscle cells, glucose-6-phosphate (G6P) enters directly into which metabolic pathway?

Glycolysis Signup and view all the answers

Which enzyme converts Glucose-6-Phosphate (G6P) to glucose in the liver for release into the bloodstream?

Glucose-6-phosphatase Signup and view all the answers

What is the function of the Debranching enzyme in glycogenolysis?

Both B and C Signup and view all the answers

What happens to glycogen synthase when it is dephosphorylated by phosphatases?

It becomes active Signup and view all the answers

Which of the following is activated by insulin to promote glycogen synthesis?

Protein Phosphatase 1 (PP1) Signup and view all the answers

What is the primary energy-consuming step in gluconeogenesis that bypasses irreversible steps of glycolysis?

Glucose-6-Phosphatase activity Signup and view all the answers

Which of the following processes is inhibited by insulin?

Glycogenolysis Signup and view all the answers

Which is a common treatment strategy for Von Gierke’s Disease?

Glucose-rich diet Signup and view all the answers

Which enzyme catalyzes the conversion of glucose to glucose-6-phosphate in the first step of glycogenesis?

Glucokinase (in liver) / Hexokinase (in muscle) Signup and view all the answers

What is the role of glycogen phosphorylase in glycogenolysis?

Cleaves α(1→4) glycosidic bonds in glycogen Signup and view all the answers

Glycogen synthase is responsible for which of the following actions?

Adding glucose units to the growing glycogen chain in α(1→4) linkage Signup and view all the answers

Which enzyme is responsible for forming the α(1→6) linkages that create branches in glycogen?

Glycogen branching enzyme Signup and view all the answers

What effect does insulin have on glycogen metabolism?

Stimulates glycogen synthesis by activating glycogen synthase Signup and view all the answers

Which enzyme converts glucose-6-phosphate to free glucose in the liver, allowing glucose to enter the bloodstream?

Glucose-6-phosphatase Signup and view all the answers

What is the regulatory effect of calcium (Ca²+) in muscle glycogenolysis?

Activates glycogen phosphorylase Signup and view all the answers

What role does protein kinase A (PKA) play in glycogen metabolism regulation?

Activates glycogen phosphorylase by phosphorylation Signup and view all the answers

Which enzyme catalyzes the conversion of glucose-1-phosphate to UDP-glucose in glycogen synthesis?

UDP-glucose pyrophosphorylase Signup and view all the answers

During fasting or exercise, which hormone primarily activates glycogenolysis in the liver?

Glucagon Signup and view all the answers

Which enzyme removes the phosphate group from glycogen phosphorylase, deactivating it?

Protein phosphatase 1 (PP1) Signup and view all the answers

AMP, generated during ATP depletion in muscles, activates which enzyme involved in glycogen breakdown?

Glycogen phosphorylase Signup and view all the answers

Study Notes

Here's the revised summary with your requested point on the glycogen branching enzyme added where it applies in the flow:

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### 1. Glycogen Synthesis (Glycogenesis) and Metabolism in Fed and Fasting States

Fed State:
- After eating, blood glucose rises, leading to increased glycogen synthesis.
- Glycogenesis is the process where glucose is converted into glycogen for storage in the liver and muscle.
- Key steps:
  
  1. Glucose is converted into glucose-6-phosphate (G6P).
  
  2. G6P is converted into glucose-1-phosphate (G1P).
  
  3. G1P reacts with uridine triphosphate (UTP) to form UDP-glucose, the building block for glycogen.
  
  4. Glycogen synthase extends the glycogen chain.
  
  5. Glycogen branching enzyme introduces branches by transferring a segment of the glycogen chain and attaching it via an α(1→6) glycosidic bond. This enzyme has two catalytic activities:
  - Transferase activity: Transfers a portion of the glycogen chain to create a branch by forming an α(1→6) bond.
  - Glycosidase activity: Cleaves α(1→4) bonds to free the glycogen segment for transfer.
  - Branching is crucial as it increases the solubility of glycogen and enhances the speed of glycogen breakdown, allowing for rapid glucose release during energy demand.
- Insulin activates this process by promoting glycogen synthase activity.
Fasting State:
- Blood glucose levels drop, prompting glycogen breakdown (glycogenolysis).
- Glycogen phosphorylase cleaves α(1→4) bonds, releasing glucose-1-phosphate. However, it stops 4 glucose residues before each branch point, requiring the debranching enzyme to remove the remaining residues:
  - Debranching enzyme has two activities:
    
    1. Transferase: Transfers three glucose residues from a branch to a nearby chain.
    
    2. Glucosidase: Cleaves the remaining glucose molecule.
- Phosphoglucomutase converts glucose-1-phosphate into glucose-6-phosphate (G6P).
- In the liver, glucose-6-phosphatase converts G6P to glucose for release into the bloodstream, while in muscles, G6P enters glycolysis to produce energy.
- Glucagon and adrenaline stimulate glycogenolysis by activating glycogen phosphorylase.

### 2. Hormonal Regulation of Glycogen by Insulin, Glucagon, and Catecholamines

Insulin: Promotes glycogenesis by activating glycogen synthase and inhibiting glycogen phosphorylase.
Glucagon: Stimulates glycogenolysis by activating glycogen phosphorylase and inhibiting glycogen synthase.
Catecholamines (Adrenaline): Increases glycogen breakdown during exercise by activating protein kinase A, which stimulates glycogen phosphorylase.

### 3. The Cori Cycle and Its Role in the Fasting State

Cori Cycle converts lactate from muscles into glucose in the liver during fasting or exercise, helping maintain blood glucose levels.

### 4. Gluconeogenesis in Maintaining Plasma Glucose

When glycogen stores are depleted, gluconeogenesis uses lactate, glycerol, and amino acids to produce glucose, maintaining blood sugar levels during prolonged fasting.

### 5. Glycogen Storage Disorders (GSD): Von Gierke’s Disease

Von Gierke's Disease is caused by a deficiency in glucose-6-phosphatase, leading to hypoglycemia and glycogen buildup in the liver.

### 6. Carbohydrate Digestion and Absorption

Carbohydrates are broken down into monosaccharides, absorbed in the small intestine, and used for energy.

### 7. Energy Production from Glucose (Glycolysis, TCA Cycle, ETC)

Glycolysis breaks down glucose into pyruvate, producing ATP, which enters the citric acid cycle and oxidative phosphorylation to generate more ATP through the electron transport chain.

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This revised summary includes the branching enzyme's activities and their importance, as well as a smooth integration of the related concepts throughout glycogen metabolism.

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Description

This quiz covers key aspects of glycogen synthesis and metabolism during fed and fasting states, as well as hormonal regulation by insulin and glucagon. Additionally, learn about the Cori Cycle, gluconeogenesis, and pathological conditions such as Glycogen Storage Disorders, including Von Gierke's Disease.