Carbohydrate Metabolism Practice Questions

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 of the following compounds is the PRIMARY allosteric inhibitor of phosphofructokinase-1 in glycolysis?

ATP. ***ATP*** - **ATP** acts as a primary **allosteric inhibitor** of phosphofructokinase-1 (PFK-1), signaling abundant energy and reducing the need for further glycolysis. - High concentrations of **ATP** bind to an allosteric site on **PFK-1**, decreasing its affinity for fructose-6-phosphate. *AMP* - **AMP** is an **allosteric activator** of **PFK-1**, indicating a low energy state and promoting glycolysis. - Low ATP levels and high AMP levels signal the cell's need for more energy, driving glycolysis forward. *F-2,6-BP* - **Fructose-2,6-bisphosphate (F-2,6-BP)** is a potent **allosteric activator** of **PFK-1**, especially in the liver. - Its presence overrides **ATP** inhibition, ensuring glycolysis proceeds even when ATP levels are high, particularly during fed states. *Citrate* - **Citrate** is an **allosteric inhibitor** of **PFK-1**, indicating that precursors for the **Krebs cycle** are abundant. - While an inhibitor, it signals that the cell has enough building blocks and energy, thereby slowing down glycolysis.

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 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 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.

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: Which enzyme is deficient in McArdle's disease, resulting in muscle pain and weakness after exercise?

Muscle phosphorylase. ***Muscle phosphorylase*** - **McArdle's disease** (Glycogen Storage Disease Type V) is characterized by a deficiency in **muscle glycogen phosphorylase**, also known as **myophosphorylase**. - This enzyme is crucial for breaking down **glycogen in muscle cells** to release glucose for energy during exercise, leading to **muscle pain and weakness** when deficient. *Glucose-6-phosphatase* - A deficiency in **glucose-6-phosphatase** causes **Von Gierke's disease** (Glycogen Storage Disease Type I), which primarily affects the liver and kidneys. - This deficiency results in **hepatomegaly**, **hypoglycemia**, and **lactic acidosis**, not predominantly muscle pain and weakness after exercise. *Liver phosphorylase* - A deficiency in **liver phosphorylase** (or phosphorylase kinase) causes **Hers' disease** (Glycogen Storage Disease Type VI). - This condition mainly affects the **liver**, leading to **hepatomegaly** and mild **hypoglycemia**, with less significant muscle symptoms. *Debranching enzyme* - A deficiency in the **debranching enzyme** causes **Cori's disease** (Glycogen Storage Disease Type III). - This leads to **abnormal glycogen structure**, affecting both **liver and muscle**, causing **hepatomegaly**, **hypoglycemia**, and **muscle weakness**, but typically presenting differently than McArdle's.

Question 1

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 2

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 3

Which of the following compounds is the PRIMARY allosteric inhibitor of phosphofructokinase-1 in glycolysis?

Accepted Answer

ATP

Answer

AMP

Answer

F-2,6-BP

Answer

Citrate

Question 4

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 5

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 6

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 7

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 8

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

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

Which enzyme is deficient in McArdle's disease, resulting in muscle pain and weakness after exercise?

Accepted Answer

Muscle phosphorylase

Answer

Glucose-6-phosphatase

Answer

Liver phosphorylase

Answer

Debranching enzyme

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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