Carbohydrate Metabolism & Glycogen Synthesis

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Questions and Answers

What is the primary energy source used by muscle during physical activity?

Glucose derived from dietary intake

Glucose exported from blood

Ketone bodies

Its own glycogen (correct)

Which of the following correctly describes the composition of glycogen?

A polypeptide of amino acids

A homopolymer of glucose linked by α-1,4 and α-1,6 bonds (correct)

A polymer of lipids connected by ester bonds

A mixture of glucose and fructose

Which substrate is NOT required for the synthesis of glycogen?

UDP-glucose

AMP (correct)

Glucose-6-P

UTP

What is the role of glycogenin in glycogen synthesis?

It initiates the chain of glycogen synthesis by attaching a glucose molecule Signup and view all the answers

What enzyme is responsible for cleaving α-1,4 bonds during glycogenolysis?

Glycogen phosphorylase Signup and view all the answers

How does the branching enzyme contribute to glycogen structure?

It creates branches by breaking and reattaching glucose residues Signup and view all the answers

What is a key characteristic of glucose utilization in the brain?

It requires glucose and can utilize ketone bodies later Signup and view all the answers

Which of the following statements about glycogen storage is true?

Muscle stores glycogen in a readily mobilizable form Signup and view all the answers

Which substrates are used in gluconeogenesis?

Lactate, glycerol, and amino acids Signup and view all the answers

What is a shared enzyme in both gluconeogenesis and glycolysis?

Fructose-1,6-bisphosphatase Signup and view all the answers

Which of the following steps in gluconeogenesis is irreversible?

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

What happens to lactate produced by exercising muscle cells in the Cori cycle?

It is converted to pyruvate before gluconeogenesis Signup and view all the answers

Which of the following describes the primary issue in glycogen storage disorders (GSDs)?

Deficiency of enzymes for glycogen synthesis or breakdown Signup and view all the answers

Which tissue specifically requires glucose and cannot directly utilize fatty acids?

Brain Signup and view all the answers

What is the effect on the body when glycogen stores are depleted?

Need for gluconeogenesis to synthesize glucose Signup and view all the answers

Which of the following enzymes is not associated with gluconeogenesis?

Phosphofructokinase Signup and view all the answers

What is the primary function of glycogen synthesis?

Storage of glucose for future energy use Signup and view all the answers

Which hormone is primarily responsible for glycogen synthesis during the fed state?

Insulin Signup and view all the answers

In the fasting state, which of the following processes occurs in the liver?

Glycogenolysis Signup and view all the answers

What role does gluconeogenesis play in the body during fasting?

Production of glucose from non-carbohydrate sources Signup and view all the answers

What is the primary energy source utilized by muscle during fasting?

Fatty acids and ketone bodies Signup and view all the answers

Which disorder is characterized by glycogen storage abnormalities?

Von Gierke's Disease Signup and view all the answers

During the fed state, which metabolic response occurs in adipose tissue?

Triglyceride synthesis Signup and view all the answers

What causes the switch in metabolism between fed and fasting states?

Hormonal regulation Signup and view all the answers

What happens to insulin levels during the fasting state?

They decrease Signup and view all the answers

What is the main purpose of the Cori Cycle?

To convert lactic acid back to glucose Signup and view all the answers

What is the primary direct product of glycogen breakdown?

Glucose-1-phosphate Signup and view all the answers

Which enzymatic activity is responsible for converting glucose-1-phosphate to glucose-6-phosphate?

Phosphoglucomutase Signup and view all the answers

How does glucose-6-phosphate regulate glycogen metabolism in the liver?

It activates glycogen synthase Signup and view all the answers

What effect does glucagon have on glycogen metabolism when blood sugar is low?

Activates glycogen phosphorylase Signup and view all the answers

Which of the following statements about muscle cells is correct?

They do not release glucose to the bloodstream. Signup and view all the answers

Which enzyme is NOT involved in the debranching of glycogen?

Phosphoglucomutase Signup and view all the answers

During fasting, what happens to glycogen synthesis?

It decreases Signup and view all the answers

What is the role of protein phosphatase 1 (PP1) in glycogen metabolism?

It activates glycogen synthase Signup and view all the answers

Which of the following is a consequence of AMP accumulation in muscle cells?

Activation of glycogen phosphorylase Signup and view all the answers

Cortisol has what type of effect on glycogen metabolism?

It increases synthesis in the liver while promoting breakdown in muscle Signup and view all the answers

What is the primary role of glycogen in the liver?

To maintain blood glucose levels Signup and view all the answers

Which bond types are found in the structure of glycogen?

α-1,4 and α-1,6 glycosidic bonds Signup and view all the answers

What initiates the synthesis of glycogen chains?

Glycogenin Signup and view all the answers

Which of the following correctly describes the action of the branching enzyme in glycogen synthesis?

It cleaves α-1,6 bonds to form branches Signup and view all the answers

What is the immediate product generated from glycogen during glycogenolysis?

Glucose-1-phosphate Signup and view all the answers

Why does muscle tissue not export glucose to the bloodstream during energy usage?

Muscle is designed solely for localized energy use Signup and view all the answers

Which substrate is necessary for the conversion of glucose to glucose-6-phosphate during glycogenesis?

ATP Signup and view all the answers

How does glycogen serve as a dynamic resource in the body?

It is rapidly synthesized and degraded as needed Signup and view all the answers

What is one of the main functions of gluconeogenesis in the body?

To produce glucose when glycogen stores are depleted Signup and view all the answers

Which substrate is primarily utilized in the Cori cycle for producing glucose?

Lactate Signup and view all the answers

Which enzyme is specifically involved in the conversion of lactate to pyruvate in the Cori cycle?

Lactate dehydrogenase Signup and view all the answers

What characterizes glycogen storage diseases (GSD)?

Deficiency of enzymes involved in glycogen synthesis or breakdown Signup and view all the answers

Which of the following enzymes is NOT a shared enzyme between gluconeogenesis and glycolysis?

Glycogen phosphorylase Signup and view all the answers

What is a consequence of glycogen depletion in the body?

Increased reliance on gluconeogenesis Signup and view all the answers

What metabolic cycle is involved in transporting lactate to the liver for gluconeogenesis?

Cori cycle Signup and view all the answers

Which tissues specifically require glucose and cannot use fatty acids directly?

Red blood cells and brain Signup and view all the answers

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

Phosphoglucomutase Signup and view all the answers

How does adrenaline affect glycogen metabolism during exercise?

It activates glycogen phosphorylase, promoting glycogen breakdown. Signup and view all the answers

What is the primary allosteric regulator of glycogen phosphorylase in muscle cells during contraction?

Ca2+ Signup and view all the answers

What role does glucose-6-phosphatase play in glycogen metabolism?

It catalyzes the hydrolysis of glucose-6-phosphate to release free glucose. Signup and view all the answers

Which hormonal regulator decreases glycogen synthesis when blood sugar is low?

Glucagon Signup and view all the answers

During which metabolic state is glycogen synthase primarily activated?

Fed state Signup and view all the answers

What effect does AMP have on glycogen metabolism in muscle cells?

It activates glycogen phosphorylase. Signup and view all the answers

Which of the following correctly describes the function of protein kinase A in glycogen metabolism?

It inactivates glycogen synthase and activates glycogen phosphorylase. Signup and view all the answers

How does insulin affect glycogen metabolism in muscle and liver cells?

It promotes glycogen synthesis by activating glycogen synthase. Signup and view all the answers

Which enzyme involved in glycogenolysis is responsible for cleaving the α-(1,6) bond at branch points?

Debranching enzyme Signup and view all the answers

What is primarily responsible for the synthesis of glycogen during the fed state?

Insulin Signup and view all the answers

Which process is primarily utilized by the liver during the fasting state to maintain plasma glucose levels?

Glycogenolysis Signup and view all the answers

What is the primary role of gluconeogenesis during fasting?

To synthesize glucose from non-carbohydrate sources Signup and view all the answers

Which hormone decreases its levels during the fasting state?

Insulin Signup and view all the answers

How does the body respond in terms of muscle metabolism during a fasting state?

Increased proteolysis Signup and view all the answers

What condition exemplifies a glycogen storage disorder affecting glucose metabolism?

Von Gierke's disease Signup and view all the answers

What is the role of adrenaline during the change from the fed to fasting state?

Increases gluconeogenesis Signup and view all the answers

What is a key characteristic of the Cori cycle during fasting?

Transfer of lactate from muscle to liver for gluconeogenesis Signup and view all the answers

In what state would the body primarily be in a catabolic process, utilizing stored nutrients?

Fasting state Signup and view all the answers

Which metabolic pathway is least active during the fed state?

Glycogenolysis Signup and view all the answers

Study Notes

Carbohydrate Metabolism & Glycogen Synthesis

Breakdown of glucose in the body is a key metabolic process
Glucose is stored in a readily mobilizable form – glycogen
- In Liver (~100g) [and muscle (~400g)]

Glycogen Formation and Breakdown

Glycogenesis is the synthesis of glycogen from glucose
- Occurs in the cytosol
- Requires ATP and uridine triphosphate (UTP)
Glycogenolysis is the breakdown of glycogen into glucose
- The main direct product of glycogen breakdown is glucose-1-phosphate
- Glucose-6-phosphatase releases free glucose
  - This can only occur in the liver

Regulation of Glycogen Metabolism

Allosteric regulation:
- In the liver Glucose-6-P allosterically
  - activates Glycogen Synthase
  - inhibits Glycogen Phosphorylase
- Ca2+ and AMP activate Glycogen Phosphorylase in muscle
Hormonal Regulation:
- Insulin
  - Secreted from the pancreas when blood sugar is high
  - Activates PP1, which activates glucagon synthase and inactivates phosphorylase
  - Promotes glycogen synthesis (storage)
- Glucagon
  - Secreted from the pancreas when blood sugar is low
  - Activates protein kinase A (PKA)
  - Inactivates PP-1, which inactivates glycogen synthase and activates phosphorylase
  - Promotes glycogen breakdown and glucose release
- Adrenaline
  - Released from the adrenal glands during exercise
  - Activates protein kinase A (PKA)
  - Inactivates PP1
  - Promotes glycogen breakdown and glucose release

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate sources
- This occurs when glycogen stores are depleted
- Shared enzymes with glycolysis
- Irreversible steps of glycolysis require different enzymes in gluconeogenesis

Substrates for Gluconeogenesis

Lactate
- Released into blood by exercising muscle
- The Cori Cycle converts lactate back into glucose
Glycerol
- From triglyceride breakdown in adipose tissue
Amino acids
- From protein breakdown in tissues

The Cori Cycle

Metabolic pathway named after its discoverers, Carl and Gerty Cori
Lactate produced by muscle cells during metabolism is converted into glucose
Occurs in the Liver
Liver exports glucose into the bloodstream, where it can be taken up by muscle

Glycogen Storage Disorders

Group of inherited metabolic disorders caused by a deficiency of enzymes required for glycogen synthesis or breakdown in muscle or liver
Results in chronic hypoglycaemia
- also enlarged liver, muscle fatigue, cramps
12 types of GSD (I-XII)
- Varying severity depending on the particular enzyme affected and its tissue distribution

Post-absorption Processing of Carbohydrates

Glycogen synthesis and metabolism: Glucose is stored as glycogen in the liver and muscle. Glycogen synthesis occurs primarily in the fed state, when glucose levels are high. Glycogen breakdown (glycogenolysis) occurs during fasting or exercise, when glucose levels are low.
Hormonal regulation: Insulin promotes glycogen synthesis, while glucagon and adrenaline promote glycogen breakdown.
Cori Cycle: The Cori Cycle is a metabolic pathway where lactate produced by muscle cells during exercise is transported to the liver and converted back into glucose. This glucose can then be used by muscle cells for energy.
Gluconeogenesis: Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate sources (lactate, glycerol, amino acids). This occurs when glycogen stores are depleted. Gluconeogenesis is essential for maintaining blood glucose levels during fasting or prolonged exercise.
Glycogen Storage Disorders: These are genetic disorders that affect glycogen metabolism, leading to an inability to properly store or break down glycogen.

Glycogen Metabolism

Structure of glycogen: Glycogen is a branched polymer of glucose, with α-1,4 and α-1,6 glycosidic bonds.
Glycogen synthesis (glycogenesis):
- Occurs in the cytosol.
- Requires ATP and UTP.
- Glucose is converted to glucose-6-phosphate, which is then converted to glucose-1-phosphate.
- UDP-glucose is synthesized and used as a substrate for glycogen synthase, which catalyzes the formation of α-1,4 linkages between glucose molecules.
- Branching enzyme creates α-1,6 linkages, increasing the number of non-reducing ends and allowing for more rapid glycogen synthesis.
Glycogen degradation (glycogenolysis):
- Glycogen phosphorylase breaks down glycogen by cleaving α-1,4 bonds, producing glucose-1-phosphate.
- Debranching enzyme removes the remaining glucose residues from branch points.
- Muscle cells do not have glucose-6-phosphatase and cannot directly release glucose into the bloodstream.
- Liver cells have glucose-6-phosphatase, allowing for the release of free glucose into the bloodstream.

Regulation of Glycogen Metabolism

Allosteric regulation: Glucose-6-phosphate activates glycogen synthase in the liver but inhibits glycogen phosphorylase. Muscle contraction (increased Ca2+ and AMP) activates glycogen phosphorylase.
Hormonal regulation:
- Insulin: Activates protein phosphatase 1 (PP1), which activates glycogen synthase (promoting glycogen synthesis).
- Glucagon and adrenaline: Activate protein kinase A, which inactivates PP1 and activates glycogen phosphorylase (promoting glycogen breakdown).

Gluconeogenesis

Importance: Gluconeogenesis is crucial for maintaining blood glucose levels during fasting or prolonged exercise, as it provides glucose for tissues like the brain and red blood cells.
Substrates: Lactate from muscle, glycerol from fat, and amino acids from protein breakdown can be used as substrates for gluconeogenesis.
Irreversible steps: Gluconeogenesis bypasses three irreversible steps in glycolysis:
- Pyruvate kinase is bypassed by pyruvate carboxylase and PEP-carboxykinase.
- Phosphofructokinase is bypassed by fructose-1,6-bisphosphatase.
- Hexokinase/glucokinase is bypassed by glucose-6-phosphatase

Glycogen Storage Disorders (GSDs)

Cause: A deficiency of enzymes involved in glycogen synthesis or breakdown.
Consequences: Can lead to chronic hypoglycemia, enlarged liver, muscle fatigue, and cramps.
Types: There are 12 known types of GSDs, classified by the specific enzyme affected and its tissue distribution.

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Description

This quiz covers key concepts related to carbohydrate metabolism, including glycogen synthesis and breakdown. It addresses processes such as glycogenesis and glycogenolysis, as well as their regulation by allosteric and hormonal mechanisms. Test your understanding of how glucose is metabolized and stored in the body.