Carbohydrate Metabolism Practice Questions

Q: During prolonged fasting, how does the liver maintain blood glucose levels?

Converts non-carbohydrate substrates to glucose. ***Converts non-carbohydrate substrates to glucose*** - During **prolonged fasting**, the liver maintains glucose homeostasis primarily through **gluconeogenesis**. - **Non-carbohydrate precursors** such as amino acids (e.g., alanine), lactate, and glycerol are converted to glucose to meet the body's energy demands, especially for the brain. - After **24-48 hours of fasting**, hepatic glycogen stores are depleted, making gluconeogenesis the predominant mechanism. *Breaks down stored hepatic glycogen to glucose* - **Hepatic glycogenolysis** does contribute to maintaining blood glucose, but only in the **early phases of fasting** (first 24-48 hours). - During **prolonged fasting**, liver glycogen stores become **depleted**, and this mechanism can no longer sustain blood glucose levels. - Gluconeogenesis becomes the primary pathway after glycogen depletion. *Increases FA oxidation to glucose* - **Fatty acid oxidation (beta-oxidation)** generates acetyl-CoA, which can enter the citric acid cycle for energy production. - However, there is **no net conversion of fatty acids to glucose** in humans because acetyl-CoA cannot be converted back to pyruvate for gluconeogenesis (pyruvate dehydrogenase is irreversible). - Only the **glycerol backbone** from triglycerides can be used for gluconeogenesis, not the fatty acid chains. *Promotes lipogenesis to store glucose* - **Lipogenesis** is the process of synthesizing fatty acids and triglycerides from glucose and other precursors for energy storage. - This process occurs primarily in the **fed state** when there is an abundance of glucose, not during prolonged fasting when glucose is scarce and must be produced, not stored.

Q: A child with hepatomegaly, hypoglycemia, and normal glycogen structure is likely to have a deficiency in which enzyme?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - A deficiency in **glucose-6-phosphatase** (Type I glycogen storage disease or von Gierke's disease) leads to the inability to convert **glucose-6-phosphate** to glucose. - This results in the accumulation of **glycogen** in the liver (hepatomegaly) and severe **hypoglycemia** due to impaired glucose release, even though glycogen structure is normal. *Lysosomal α-glucosidase* - Deficiency in **lysosomal α-glucosidase** (Pompe disease, Type II GSD) leads to accumulation of glycogen in **lysosomes** in various tissues, notably cardiac and skeletal muscle. - While it causes **hepatomegaly** and cardiomegaly, it typically does not present with hypoglycemia because the primary defect is in lysosomal degradation, not glucose release from storage. *Glycogen synthase* - Deficiency in **glycogen synthase** (Glycogen storage disease Type 0) results in an inability to synthesize glycogen effectively. - This leads to very little glycogen storage, which would cause **hypoglycemia** but would not result in **hepatomegaly** due to glycogen accumulation. *Debranching enzyme* - Deficiency in the **debranching enzyme** (Cori disease, Type III GSD) leads to the accumulation of **abnormal glycogen** with short outer branches. - It presents with **hepatomegaly** and **hypoglycemia**, but the key differentiating factor is the presence of **abnormal glycogen structure**, which is stated as normal in the question.

Q: A 4-year-old boy exhibits symptoms of vomiting, lethargy, and hepatomegaly after ingesting fructose. Which enzyme is likely to be deficient?

Aldolase B. ***Aldolase B*** - **Hereditary Fructose Intolerance** (HFI) is caused by a deficiency of **aldolase B**, leading to the accumulation of **fructose-1-phosphate** after fructose ingestion. - The accumulation of **fructose-1-phosphate** is toxic to liver cells, causing symptoms such as **vomiting**, **lethargy**, **hepatomegaly**, and can lead to **hepatic and renal failure** if untreated. *Hexokinase* - **Hexokinase** phosphorylates glucose to **glucose-6-phosphate** and can also phosphorylate fructose, though **fructokinase** is the primary enzyme for fructose. - A deficiency in hexokinase would primarily affect **glucose metabolism**, not specifically cause adverse reactions to fructose ingestion. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is a key regulatory enzyme in **glycolysis**, converting **fructose-6-phosphate** to **fructose-1,6-bisphosphate**. - A deficiency in PFK-1 (Tarui's disease) primarily causes **muscle cramps** and **hemolytic anemia**, and does not involve specific intolerance to fructose. *Fructokinase* - **Fructokinase** (also known as ketohexokinase) is the first enzyme in fructose metabolism, converting **fructose** to **fructose-1-phosphate**. - A deficiency in fructokinase causes **essential fructosuria**, a benign condition where fructose accumulates in the blood and urine, but does not lead to **vomiting**, **lethargy**, or **hepatomegaly**.

Q: Which enzyme is responsible for converting pyruvate to acetyl-CoA, thereby linking glycolysis and the TCA cycle?

Pyruvate dehydrogenase. ***Pyruvate dehydrogenase*** - The **pyruvate dehydrogenase complex** catalyzes the oxidative decarboxylation of **pyruvate** to **acetyl-CoA**, releasing carbon dioxide. - This crucial reaction is the committed step that links glycolysis (which produces pyruvate) to the **TCA cycle** (which consumes acetyl-CoA). *Pyruvate carboxylase* - This enzyme converts **pyruvate** to **oxaloacetate**, an important anaplerotic reaction that replenishes intermediates of the **TCA cycle**. - It does not produce acetyl-CoA and is typically involved in **gluconeogenesis**. *Pyruvate kinase* - **Pyruvate kinase** is the final enzyme in **glycolysis**, catalyzing the transfer of a phosphate group from **phosphoenolpyruvate (PEP)** to ADP, generating ATP and pyruvate. - It is an ATP-generating step within glycolysis and does not convert pyruvate to acetyl-CoA. *Lactate dehydrogenase* - This enzyme catalyzes the reversible conversion of **pyruvate** to **lactate**, primarily under anaerobic conditions. - It regenerates NAD+ for glycolysis to continue in the absence of oxygen but does not link glycolysis to the TCA cycle.

Q: During a prolonged fast, which enzyme is essential for maintaining blood glucose levels through gluconeogenesis in the liver?

Pyruvate carboxylase. ***Pyruvate carboxylase*** - This enzyme catalyzes the conversion of **pyruvate to oxaloacetate**, a crucial first step in **gluconeogenesis** in the liver. - During a prolonged fast, **pyruvate carboxylase** is essential for utilizing non-carbohydrate precursors like amino acids to synthesize new glucose. *Hexokinase* - **Hexokinase** phosphorylates glucose to **glucose-6-phosphate**, trapping glucose within the cell for glycolysis or glycogen synthesis. - Its activity would **lower blood glucose** by consuming it, which is contrary to the goal of maintaining blood glucose during a fast. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is a key regulatory enzyme in **glycolysis**, catalyzing the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. - Its activation promotes **glucose breakdown**, which counteracts the need for new glucose production during a fast. *Lactate dehydrogenase* - **Lactate dehydrogenase** interconverts **pyruvate and lactate**, allowing for regeneration of NAD+ during anaerobic conditions. - While lactate can be a gluconeogenic precursor, **lactate dehydrogenase** itself does not initiate the main gluconeogenic pathway; it primarily facilitates the anaerobic metabolism of glucose.

Q: Which enzyme converts ribose-5-phosphate to ribulose-5-phosphate in the pentose phosphate pathway?

Ribose-5-phosphate isomerase. ***Ribose-5-phosphate isomerase*** - This enzyme *catalyzes the interconversion* of the aldose sugar **ribose-5-phosphate** and the ketose sugar **ribulose-5-phosphate**. - This isomerization reaction is crucial for the **non-oxidative phase** of the pentose phosphate pathway, producing precursors for nucleotide synthesis. *Transketolase* - This enzyme transfers a **two-carbon unit** from a ketose sugar to an aldose sugar. - It is involved in later steps of the **non-oxidative phase** of the pentose phosphate pathway, converting xylulose-5-phosphate and ribose-5-phosphate into sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate. *Transaldolase* - This enzyme transfers a **three-carbon unit** from a ketose sugar to an aldose sugar. - It also participates in the **non-oxidative phase**, converting sedoheptulose-7-phosphate and glyceraldehyde-3-phosphate into fructose-6-phosphate and erythrose-4-phosphate. *Glucose-6-phosphate dehydrogenase* - This is the **rate-limiting enzyme** of the pentose phosphate pathway's **oxidative phase**. - It catalyzes the conversion of **glucose-6-phosphate to 6-phosphoglucono-δ-lactone**, producing NADPH in the process.

Q: A patient with a deficiency in aldolase B presents with hypoglycemia and vomiting after consuming fruit. Which condition is this patient likely to have?

Fructose intolerance. ***Fructose intolerance*** - A deficiency in **aldolase B** is the hallmark of **hereditary fructose intolerance (HFI)**, preventing the metabolism of fructose-1-phosphate. - Ingestion of fructose, commonly found in fruit, leads to the accumulation of **fructose-1-phosphate**, causing **hypoglycemia** (due to inhibition of glycogenolysis and gluconeogenesis) and **vomiting**. *Lactose intolerance* - This condition results from a deficiency in **lactase**, an enzyme that breaks down **lactose** (milk sugar), leading to gastrointestinal symptoms like bloating and diarrhea. - It is unrelated to aldolase B deficiency or the metabolism of fructose. *Sucrase-isomaltase deficiency* - This involves a deficiency in enzymes required to digest **sucrose** and **isomaltose**, causing symptoms like diarrhea and abdominal pain after consuming these sugars. - It is not associated with aldolase B deficiency or the specific metabolic pathway of fructose. *Galactosemia* - Galactosemia is an inability to metabolize **galactose**, typically due to a deficiency in **galactose-1-phosphate uridyltransferase (GALT)**. - It primarily causes symptoms upon consumption of milk products containing galactose, presenting differently from fructose intolerance.

Q: Which of the following is the primary effect of insulin on glucose metabolism?

Increase glycogenesis. ***Increase glycogenesis*** - **Insulin** is an **anabolic hormone** that promotes the storage of glucose in the form of **glycogen** in the liver and muscles. - By stimulating **glycogenesis**, insulin helps to lower blood glucose levels after a meal. *Increase gluconeogenesis* - **Gluconeogenesis** is the process of synthesizing glucose from non-carbohydrate precursors, primarily in the liver. - Insulin **inhibits** gluconeogenesis, as its role is to lower blood glucose, not raise it. *Increase lipolysis* - **Lipolysis** is the breakdown of triglycerides into fatty acids and glycerol. - Insulin **inhibits** lipolysis, promoting fat storage and preventing the release of fatty acids into circulation. *Increase glycogenolysis* - **Glycogenolysis** is the breakdown of stored glycogen into glucose. - Insulin **inhibits** glycogenolysis, preventing the release of glucose from storage and thus helping to lower blood glucose.

Q: A 45-year-old man presents with muscle weakness and cramps. Blood tests reveal hypokalemia. Which enzyme deficiency could result in this condition?

Pyruvate kinase. ***Pyruvate kinase*** - **Pyruvate kinase deficiency** causes **chronic hemolytic anemia** with compensatory increased erythropoiesis and high metabolic demand. - In the context of chronic hemolysis, patients may develop **secondary hypokalemia** due to: increased renal potassium losses from chronic illness, poor nutrition, or associated gastrointestinal losses. - The **muscle weakness and cramps** result from both the chronic anemia (tissue hypoxia, reduced ATP) and the electrolyte disturbance (hypokalemia affects muscle membrane potential). - Among the glycolytic enzyme deficiencies listed, pyruvate kinase deficiency is most commonly associated with chronic systemic complications. *Phosphofructokinase* - **Phosphofructokinase deficiency** (Tarui's disease, glycogenosis type VII) causes **exercise-induced muscle symptoms** including myalgia, cramps, and fatigue. - Results from impaired glycolysis in muscle with glycogen accumulation. - Does not typically cause hypokalemia; may show elevated lactate and myoglobinuria after exercise. *Glucose-6-phosphate dehydrogenase* - **G6PD deficiency** causes **episodic hemolytic anemia** triggered by oxidative stress (infections, drugs, fava beans). - Presents with acute hemolytic crises rather than chronic muscle weakness. - Not associated with chronic hypokalemia as a primary feature. *Aldolase* - **Aldolase deficiency** is extremely rare and can cause **hemolytic anemia**. - May present with myopathy in some variants, but hypokalemia is not a characteristic feature. - Clinical presentation varies depending on which aldolase isoenzyme is affected.

Q: A patient with muscle weakness and cardiomyopathy is diagnosed with Pompe disease. What is the biochemical basis for the symptoms observed in Pompe disease?

Accumulation of glycogen in lysosomes. ***Accumulation of glycogen in lysosomes*** - Pompe disease is an **autosomal recessive lysosomal storage disorder** caused by a deficiency in acid alpha-glucosidase (GAA), an enzyme responsible for breaking down glycogen in the lysosomes. - The **accumulation of undigested glycogen** within lysosomes, particularly in muscle cells, leads to widespread tissue damage, explaining the patient's muscle weakness and cardiomyopathy. *Accumulation of sphingomyelin* - This is characteristic of **Niemann-Pick disease**, another lysosomal storage disorder, caused by a deficiency in the enzyme **sphingomyelinase**. - While Niemann-Pick disease can affect various organs, its clinical presentation and the specific stored substrate are different from Pompe disease. *Defective β-oxidation of fatty acids* - This defect is associated with conditions like **fatty acid oxidation disorders** (e.g., MCAD deficiency), which can cause muscle weakness and cardiomyopathy but involve problems with fatty acid metabolism, not glycogen. - The primary issue in Pompe disease is the inability to degrade **glycogen**, not fatty acids. *Impaired gluconeogenesis* - Impaired gluconeogenesis leads to **hypoglycemia**, especially during fasting, and is seen in various metabolic disorders affecting liver glucose production. - While some glycogen storage diseases can affect glucose homeostasis, the primary defect in Pompe disease is **lysosomal glycogen degradation**, not the synthesis of glucose from non-carbohydrate sources.

Question 1

During prolonged fasting, how does the liver maintain blood glucose levels?

Accepted Answer

Converts non-carbohydrate substrates to glucose

Answer

Breaks down stored hepatic glycogen to glucose

Answer

Increases FA oxidation to glucose

Answer

Promotes lipogenesis to store glucose

Question 2

A child with hepatomegaly, hypoglycemia, and normal glycogen structure is likely to have a deficiency in which enzyme?

Accepted Answer

Glucose-6-phosphatase

Answer

Lysosomal α-glucosidase

Answer

Glycogen synthase

Answer

Debranching enzyme

Question 3

A 4-year-old boy exhibits symptoms of vomiting, lethargy, and hepatomegaly after ingesting fructose. Which enzyme is likely to be deficient?

Accepted Answer

Aldolase B

Answer

Hexokinase

Answer

Phosphofructokinase

Answer

Fructokinase

Question 4

Which enzyme is responsible for converting pyruvate to acetyl-CoA, thereby linking glycolysis and the TCA cycle?

Accepted Answer

Pyruvate dehydrogenase

Answer

Pyruvate carboxylase

Answer

Pyruvate kinase

Answer

Lactate dehydrogenase

Question 5

During a prolonged fast, which enzyme is essential for maintaining blood glucose levels through gluconeogenesis in the liver?

Accepted Answer

Pyruvate carboxylase

Answer

Hexokinase

Answer

Phosphofructokinase

Answer

Lactate dehydrogenase

Question 6

Which enzyme converts ribose-5-phosphate to ribulose-5-phosphate in the pentose phosphate pathway?

Accepted Answer

Ribose-5-phosphate isomerase

Answer

Transketolase

Answer

Transaldolase

Answer

Glucose-6-phosphate dehydrogenase

Question 7

A patient with a deficiency in aldolase B presents with hypoglycemia and vomiting after consuming fruit. Which condition is this patient likely to have?

Accepted Answer

Fructose intolerance

Answer

Sucrase-isomaltase deficiency

Answer

Lactose intolerance

Answer

Galactosemia

Question 8

Which of the following is the primary effect of insulin on glucose metabolism?

Accepted Answer

Increase glycogenesis

Answer

Increase gluconeogenesis

Answer

Increase lipolysis

Answer

Increase glycogenolysis

Question 9

A 45-year-old man presents with muscle weakness and cramps. Blood tests reveal hypokalemia. Which enzyme deficiency could result in this condition?

Accepted Answer

Pyruvate kinase

Answer

Glucose-6-phosphate dehydrogenase

Answer

Aldolase

Answer

Phosphofructokinase

Question 10

A patient with muscle weakness and cardiomyopathy is diagnosed with Pompe disease. What is the biochemical basis for the symptoms observed in Pompe disease?

Accepted Answer

Accumulation of glycogen in lysosomes

Answer

Accumulation of sphingomyelin

Answer

Defective β-oxidation of fatty acids

Answer

Impaired gluconeogenesis

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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