Carbohydrate Metabolism Practice Questions

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.

Q: A patient with hemolytic anemia has a defect in the enzyme glucose-6-phosphate dehydrogenase. Which of the following pathways is directly affected by this defect?

Pentose phosphate pathway. ***Pentose phosphate pathway*** - **Glucose-6-phosphate dehydrogenase (G6PD)** is the **rate-limiting enzyme** in the **pentose phosphate pathway (PPP)**, initiating the oxidative phase. - Deficiency in G6PD impairs the production of **NADPH**, which is crucial for reducing **oxidative stress** in red blood cells. *Glycolysis* - This pathway metabolizes glucose to pyruvate for **ATP production** and does not directly involve G6PD. - While G6P is an intermediate in both pathways, its conversion in glycolysis is catalyzed by phosphoglucose isomerase, not G6PD. *TCA cycle* - The **tricarboxylic acid (TCA) cycle** is a central metabolic pathway for energy production occurring in the **mitochondria**. - It involves the oxidation of acetyl-CoA and does not directly utilize G6PD. *Urea cycle* - The **urea cycle** is responsible for detoxifying ammonia by converting it into urea, primarily occurring in the **liver**. - This pathway is unrelated to glucose metabolism or G6PD activity.

Q: A patient with pyruvate dehydrogenase deficiency exhibits lactic acidosis. What metabolic shift is primarily responsible for this condition?

Increased glycolysis. ***Increased glycolysis*** - Pyruvate dehydrogenase (PDH) deficiency blocks the conversion of **pyruvate to acetyl-CoA**, preventing pyruvate from entering the TCA cycle. - The accumulated pyruvate is **shunted to lactate** via lactate dehydrogenase to regenerate NAD+ and maintain cellular redox balance. - This metabolic shift towards **lactate production** leads to **lactic acidosis**. - Additionally, cells may upregulate glycolysis to compensate for impaired oxidative metabolism, further increasing pyruvate (and subsequently lactate) production. *Accumulation of acetyl-CoA* - PDH deficiency actually causes a **decrease in acetyl-CoA** production from pyruvate, as the enzyme's role is to convert pyruvate into acetyl-CoA. - An accumulation of acetyl-CoA would inhibit glycolysis through negative feedback rather than promote lactate production. *Increased fatty acid oxidation* - While cells may increase fatty acid oxidation to compensate for reduced glucose oxidation, this produces **acetyl-CoA** for the TCA cycle but does not directly cause lactic acidosis. - Increased fatty acid oxidation would provide alternative energy but would not address the pyruvate accumulation or its conversion to lactate. *Enhanced gluconeogenesis* - Gluconeogenesis synthesizes glucose from non-carbohydrate precursors and would **consume pyruvate**, thereby reducing its buildup. - Enhanced gluconeogenesis would actually counteract lactic acidosis by reducing pyruvate availability for lactate production.

Q: A 25-year-old male presents with muscle cramps after prolonged exercise. Which enzyme deficiency is most likely involved in impaired muscle glycogen metabolism?

Glycogen phosphorylase. ***Glycogen phosphorylase*** - Deficiency of **glycogen phosphorylase** (McArdle disease, Type V glycogen storage disease) directly impairs the breakdown of **glycogen** into glucose-1-phosphate, leading to insufficient ATP production during exercise. - This inability to mobilize glycogen stores results in **muscle cramping**, pain, and fatigue, often presenting as "second wind" phenomenon where symptoms improve after resting as **free fatty acids** become the primary energy source. *Phosphofructokinase* - Deficiency of **phosphofructokinase** (Tarui disease, Type VII glycogen storage disease) affects glycolysis downstream of glycogenolysis, leading to similar symptoms of exercise intolerance. - However, unlike glycogen phosphorylase deficiency, **glycogen levels in muscle are elevated** in phosphofructokinase deficiency, and patients often show **hemolytic anemia** in addition to muscle symptoms. *Hexokinase* - **Hexokinase** is the enzyme primarily responsible for the first step of glycolysis, phosphorylating glucose to glucose-6-phosphate; its deficiency is rare. - Hexokinase deficiency primarily affects **red blood cells**, leading to **hemolytic anemia**, and typically does not cause exercise-induced muscle cramps associated with glycogen metabolism. *Pyruvate kinase* - **Pyruvate kinase** is the final enzyme in glycolysis, converting phosphoenolpyruvate to pyruvate. - Deficiency of **pyruvate kinase** is a common cause of **chronic hemolytic anemia** but does not directly impair glycogen breakdown or present with exercise-induced muscle cramps from insufficient glycogenolysis.

Question 1

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 2

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 3

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 4

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 5

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 6

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 7

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

Question 8

A patient with hemolytic anemia has a defect in the enzyme glucose-6-phosphate dehydrogenase. Which of the following pathways is directly affected by this defect?

Accepted Answer

Pentose phosphate pathway

Answer

Glycolysis

Answer

TCA cycle

Answer

Urea cycle

Question 9

A patient with pyruvate dehydrogenase deficiency exhibits lactic acidosis. What metabolic shift is primarily responsible for this condition?

Accepted Answer

Increased glycolysis

Answer

Accumulation of acetyl-CoA

Answer

Increased fatty acid oxidation

Answer

Enhanced gluconeogenesis

Question 10

A 25-year-old male presents with muscle cramps after prolonged exercise. Which enzyme deficiency is most likely involved in impaired muscle glycogen metabolism?

Accepted Answer

Glycogen phosphorylase

Answer

Phosphofructokinase

Answer

Hexokinase

Answer

Pyruvate kinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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