Metabolism of Fructose and Galactose — MCQs
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?
Maltase hydrolyzes maltose to form:
Galactosemia is due to deficiency of which enzyme?
History of dislike for sweet food items is typically present in:
Which enzyme is deficient in Galactosemia?
A cataract formation in both eyes was discovered in a 1-year-old child during a routine well-child visit, with blood tests showing elevated galactose and galactitol levels. To determine which enzyme might be defective in the child, which intracellular metabolite should be measured?
Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?
What are the immediate metabolic products formed during the conversion of Fructose 1,6-bisphosphate to two molecules of pyruvate?
Most Common enzyme deficient in galactosemics:
Fructose intolerance is due to deficiency of which enzyme?
Metabolism of Fructose and Galactose Indian Medical PG Practice Questions and MCQs
Question 1: 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?
- A. Glycolysis
- B. Pentose phosphate pathway (Correct Answer)
- C. TCA cycle
- D. Urea cycle
Explanation: ***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.
Question 2: Maltase hydrolyzes maltose to form:
- A. Glucose and fructose
- B. Galactose and fructose
- C. Two glucose molecules (Correct Answer)
- D. Glucose and galactose
Explanation: ***Two glucose molecules*** - Maltase is an enzyme that specifically breaks down **maltose**. - Maltose, a disaccharide, is composed of **two glucose units** linked by an α-1,4-glycosidic bond. *Glucose and fructose* - This is the hydrolysis product of **sucrose**, a disaccharide broken down by the enzyme **sucrase**. - Sucrose consists of one **glucose** and one **fructose** molecule. *Galactose and fructose* - This combination does not represent a common disaccharide hydrolysis product. - While galactose and fructose are monosaccharides, they do not form a common dietary disaccharide linked together. *Glucose and galactose* - This is the hydrolysis product of **lactose**, a disaccharide broken down by the enzyme **lactase**. - Lactose is composed of one **glucose** and one **galactose** molecule.
Question 3: Galactosemia is due to deficiency of which enzyme?
- A. Galactose-1-phosphate uridyltransferase (Correct Answer)
- B. HGPRT
- C. Galactokinase
- D. Epimerase
Explanation: ***Galactose-1-phosphate uridyltransferase*** - Deficiency of **galactose-1-phosphate uridyltransferase (GALT)** leads to the most severe form, **classic galactosemia**. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the Leloir pathway. *HGPRT* - **HGPRT** (hypoxanthine-guanine phosphoribosyltransferase) deficiency causes **Lesch-Nyhan syndrome**, a distinct metabolic disorder. - Lesch-Nyhan syndrome is characterized by **hyperuricemia**, neurological dysfunction, and self-mutilation, unrelated to galactose metabolism. *Galactokinase* - Deficiency of **galactokinase** causes Type II galactosemia, a milder form than classic galactosemia. - This defect primarily leads to **cataracts** due to galactitol accumulation but does not result in the severe systemic issues seen in classic galactosemia. *Epimerase* - Deficiency of **UDP-galactose-4'-epimerase** (GALE) causes Type III galactosemia, which has a variable clinical presentation from mild to severe. - While involved in galactose metabolism, it's not the primary enzyme deficient in the most common and severe form of **galactosemia**.
Question 4: History of dislike for sweet food items is typically present in:
- A. Glycogen storage disease
- B. Diabetes mellitus
- C. Galactosemia
- D. Hereditary fructose intolerance (Correct Answer)
Explanation: ***Hereditary fructose intolerance*** - Patients with hereditary fructose intolerance develop severe symptoms like **nausea, vomiting, abdominal pain, and hypoglycemia** after ingesting fructose, leading to an aversive response and **dislike for sweet food items**. - This aversion is a protective mechanism, as avoiding fructose-containing foods (including many sweets) prevents the accumulation of toxic metabolites due to a deficiency in **hepatic aldolase B**. *Glycogen storage disease* - While glycogen storage diseases can cause hypoglycemia, they typically do not lead to a specific **aversion to sweet foods**. - The primary defect is in **glycogen synthesis or breakdown**, leading to symptoms like hepatomegaly, muscle weakness, and exercise intolerance. *Diabetes mellitus* - Patients with diabetes mellitus often have a **craving for sweet foods** due to uncontrolled blood glucose levels and insulin resistance, rather than a dislike. - The condition is characterized by **hyperglycemia** and may involve polydipsia, polyuria, and polyphagia. *Galactosemia* - Galactosemia involves an inability to metabolize galactose, leading to symptoms such as **vomiting, lethargy, and jaundice** upon milk ingestion [1]. - While patients will avoid milk, their aversion is not generally to all sweet foods, as sweet foods do not always contain galactose [1].
Question 5: Which enzyme is deficient in Galactosemia?
- A. Hexosaminidase B
- B. Hexosaminidase A
- C. Galactose 1-phosphate uridyltransferase (Correct Answer)
- D. Glucocerebrosidase
Explanation: ***Galactose 1-phosphate uridyltransferase*** - **Classical galactosemia** (Type I) is caused by a deficiency in this enzyme, which converts **galactose-1-phosphate** and UDP-glucose into UDP-galactose and glucose-1-phosphate. - This deficiency leads to the accumulation of toxic galactose metabolites, such as **galactitol** and galactose-1-phosphate. *Hexosaminidase B* - Deficiency of this enzyme is seen in **Sandhoff disease**, a lysosomal storage disorder, which leads to the accumulation of **GM2 gangliosides** in neurons. - This enzyme is less commonly associated with the primary defect in **Tay-Sachs disease**, which is predominantly due to hexosaminidase A deficiency. *Hexosaminidase A* - A deficiency in **hexosaminidase A** causes **Tay-Sachs disease**, another lysosomal storage disorder, leading to the accumulation of **GM2 gangliosides** primarily in nerve cells. - This enzyme is not involved in the metabolism of galactose. *Glucocerebrosidase* - A deficiency in **glucocerebrosidase** causes **Gaucher disease**, which leads to the accumulation of **glucocerebroside** in macrophages and other cells. - This enzyme is not involved in the metabolic pathway of galactose.
Question 6: A cataract formation in both eyes was discovered in a 1-year-old child during a routine well-child visit, with blood tests showing elevated galactose and galactitol levels. To determine which enzyme might be defective in the child, which intracellular metabolite should be measured?
- A. Galactose
- B. Fructose
- C. Glucose
- D. Galactose-1-phosphate (Correct Answer)
Explanation: ***Galactose-1-phosphate*** - An elevation of **galactose-1-phosphate** in a patient with cataracts and elevated galactose and galactitol levels points to a deficiency in **galactose-1-phosphate uridyltransferase (GALT)**, indicating **classic galactosemia**. - The accumulation of **galactose-1-phosphate** is toxic and underlies the severe symptoms of classic galactosemia, including cataracts, liver damage, and intellectual disability. - Measuring this metabolite specifically identifies GALT deficiency and distinguishes it from other enzyme defects in galactose metabolism. *Galactose* - Elevated **galactose** is observed in **galactosemia**, but measuring galactose itself doesn't differentiate between the different enzyme deficiencies (e.g., GALT vs. GALK deficiency). - While elevated, it's the downstream metabolites like **galactose-1-phosphate** that are more specific for diagnosing the enzyme defect in classic galactosemia. *Fructose* - **Fructose** metabolism is distinct from galactose metabolism, and its levels would not be directly affected by defects in galactose-metabolizing enzymes. - Elevated fructose would suggest a different metabolic disorder, such as **hereditary fructose intolerance**, which has different clinical presentations. *Glucose* - **Glucose** levels are not specific for diagnosing enzyme defects in galactose metabolism. - While hypoglycemia can occur in severe galactosemia, measuring glucose doesn't identify which specific enzyme is deficient and is not the primary diagnostic metabolite.
Question 7: Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?
- A. 2,3-Bisphosphoglycerate (2,3-BPG)
- B. Fructose 2,6-bisphosphate (Correct Answer)
- C. Glucokinase
- D. Phosphoenolpyruvate (PEP)
Explanation: ***Fructose 2,6-bisphosphate*** - **Fructose 2,6-bisphosphate** is a potent **allosteric activator** of **phosphofructokinase-1 (PFK-1)**, increasing its affinity for fructose 6-phosphate and overcoming ATP inhibition. - Its synthesis is regulated by **insulin** (stimulating) and **glucagon** (inhibiting), linking glucose availability to glycolytic flux. *2,3-Bisphosphoglycerate (2,3-BPG)* - **2,3-BPG** is an important regulator of **hemoglobin oxygen affinity** in red blood cells. - It is not an allosteric activator of **PFK-1**; its primary role is in oxygen delivery. *Glucokinase* - **Glucokinase** is an **enzyme** in glycolysis, specifically catalyzing the phosphorylation of glucose to glucose 6-phosphate in the liver and pancreatic beta cells. - It is not an allosteric activator of **PFK-1** but rather an upstream enzyme in the pathway. *Phosphoenolpyruvate (PEP)* - **PEP** is an intermediate in glycolysis, formed from 2-phosphoglycerate and converted to pyruvate by pyruvate kinase. - It acts as an **allosteric inhibitor** of phosphofructokinase-1, signaling high energy status and slowing down glycolysis.
Question 8: What are the immediate metabolic products formed during the conversion of Fructose 1,6-bisphosphate to two molecules of pyruvate?
- A. Glyceraldehyde-3-phosphate and 1,3-bisphosphoglycerate
- B. Dihydroxyacetone phosphate and 1,3 bisphosphoglycerate
- C. Glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate (Correct Answer)
- D. 3-phosphoglycerate and 1,3 bisphosphoglycerate
Explanation: ***Glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate*** - Fructose-1,6-bisphosphate is cleaved by the enzyme **aldolase** into two 3-carbon isomers: **dihydroxyacetone phosphate (DHAP)** and **glyceraldehyde-3-phosphate (GAP)**. - This reaction is a key step in **glycolysis**, occurring after the phosphorylation of fructose-6-phosphate. *Glyceraldehyde-3-phosphate and 1,3-bisphosphoglycerate* - **1,3-bisphosphoglycerate** is formed later in glycolysis from glyceraldehyde-3-phosphate, catalyzed by **glyceraldehyde-3-phosphate dehydrogenase**, not directly from Fructose-1,6-bisphosphate cleavage. - The initial products of Fructose-1,6-bisphosphate cleavage are two distinct three-carbon compounds, not one three-carbon compound and one already modified intermediate. *Dihydroxyacetone phosphate and 1,3 bisphosphoglycerate* - Similar to the above, **1,3-bisphosphoglycerate** is a product of an oxidation and phosphorylation step following the initial cleavage, not an immediate product of Fructose-1,6-bisphosphate breakdown. - The enzyme **triose phosphate isomerase** quickly interconverts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate, ensuring that both can proceed through the subsequent steps of glycolysis. *3-phosphoglycerate and 1,3 bisphosphoglycerate* - **3-phosphoglycerate** is formed from 1,3-bisphosphoglycerate by the enzyme **phosphoglycerate kinase**, a later step in glycolysis where ATP is generated. - Neither of these molecules are the direct products of the enzymatic cleavage of Fructose-1,6-bisphosphate by aldolase.
Question 9: Most Common enzyme deficient in galactosemics:
- A. Galactosidase
- B. UDP galactose epimerase
- C. Galactokinase
- D. Galactose-1-phosphate uridyl transferase/GALT (Correct Answer)
Explanation: ***Galactose-1-phosphate uridyl transferase/GALT*** - **GALT deficiency** is the most common cause of **classic galactosemia** (Type I), a severe inherited metabolic disorder. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the main pathway of galactose metabolism. - Accounts for approximately **95%** of all galactosemia cases. *Galactosidase* - **Galactosidase** enzymes are involved in the hydrolysis of galactose-containing oligosaccharides or glycoconjugates but are not the primary enzymes deficient in classic galactosemia. - This enzyme is not part of the Leloir pathway of galactose metabolism, which is the pathway affected in galactosemia. *UDP galactose epimerase* - Deficiency of **UDP galactose epimerase** (GALE) causes a milder form of galactosemia (Type III), but it is much less common than GALT deficiency. - GALE is involved in the interconversion of UDP-galactose and UDP-glucose. - This is the rarest form of galactosemia. *Galactokinase* - **Galactokinase deficiency** (GALK) causes a different, milder form of galactosemia (Type II), characterized by **cataracts** as the primary symptom. - It prevents the initial phosphorylation of galactose to galactose-1-phosphate. - This accounts for less than 5% of galactosemia cases.
Question 10: Fructose intolerance is due to deficiency of which enzyme?
- A. Aldolase B (Correct Answer)
- B. Aldolase A
- C. Fructokinase
- D. Triokinase
Explanation: ***Aldolase B*** - **Hereditary fructose intolerance** is a genetic disorder caused by a deficiency in the enzyme **aldolase B**. - This deficiency leads to an accumulation of **fructose-1-phosphate** in the liver, kidneys, and small intestine, causing **hypoglycemia**, **vomiting**, and **liver damage** upon exposure to fructose. *Fructokinase* - A deficiency in **fructokinase** causes **essential fructosuria**, a benign metabolic disorder. - This condition is asymptomatic because **fructose** simply accumulates in the blood and urine without causing significant clinical problems. *Triokinase* - **Triokinase**, also known as **glycerol kinase**, is involved in glycerol metabolism, converting glycerol to **glycerol-3-phosphate**. - Its deficiency is not directly linked to fructose intolerance and typically presents with **hyperglycerolemia**. *Aldolase A* - **Aldolase A** is one of the three aldolase isoenzymes (A, B, and C) and is primarily involved in **glycolysis**, specifically in the breakdown of **fructose-1,6-bisphosphate**. - A deficiency in aldolase A can lead to **hemolytic anemia** and **myopathy**, not directly fructose intolerance.
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