Carbohydrate Metabolism Practice Questions

Q: Which of the following cell pathways occurs exclusively in cell cytoplasm?

Glycolysis. ***Glycolysis***- Glycolysis is the metabolic pathway converting **glucose** to **pyruvate** and occurs entirely within the **cytoplasm** (cytosol) [1].- This pathway is crucial for producing ATP both in the presence (aerobic) and absence (anaerobic) of oxygen [3].*Beta oxidation*- **Beta oxidation**, the breakdown of fatty acids, occurs primarily within the **mitochondrial matrix**.- Long-chain fatty acid activation occurs in the cytosol, but the subsequent oxidative steps are confined to the **mitochondria**.*TCA cycle*- The **TCA cycle** (Krebs cycle) is located exclusively within the **mitochondrial matrix** in eukaryotic cells.- It is responsible for the complete oxidation of acetyl-CoA, producing electron carriers like **NADH** and **FADH2**.*Urea cycle*- The **urea cycle** occurs across **two distinct cellular compartments**: reactions take place in both the **mitochondrial matrix** and the **cytosol** [2].- Specifically, the synthesis of **carbamoyl phosphate** and citrulline occurs in the mitochondria [2].

Q: Cancer cells take up excess glucose because?

Lactate is produced even in the presence of Oxygen. ***Lactate is produced even in the presence of Oxygen*** - This phenomenon is known as the **Warburg effect** (or aerobic glycolysis). Cancer cells preferentially ferment glucose to lactate, even when adequate **oxygen** is available, bypassing efficient oxidative phosphorylation. - This inefficient use of glucose allows rapid generation of metabolic intermediates (e.g., carbon backbones) required for the synthesis of **lipids**, proteins, and nucleic acids needed for rapid cell proliferation. *High NADH/NAD ratio* - A **high NADH/NAD+ ratio** signals abundance of reducing equivalents, which would typically inhibit glycolysis and favor oxidative phosphorylation. - Rapid glycolysis, as seen in cancer (Warburg effect), requires the constant regeneration of **NAD+** from NADH via lactate dehydrogenase for the pathway to continue. *High GLUT2* - While cancer cells increase glucose uptake by overexpressing glucose transporters, the typically overexpressed transporter in many solid tumors is **GLUT1**, not GLUT2. - **GLUT2** is primarily found in the liver, kidney, and pancreatic beta cells and is less commonly the primary high-affinity transporter driving the intense uptake seen in malignant cells. *Absence of Oxygen* - The characteristic metabolic change in cancer is that glucose uptake and lactate production occur despite the **presence of oxygen** (aerobic glycolysis). - If oxygen were truly absent (anaerobic conditions), all cell types would produce lactate; hence, the defining feature of cancer is the metabolic shift occurring in an **oxygenated environment**.

Q: A child presents with fatigue and hepatomegaly. Liver enzymes (ALT, AST) are elevated. Ketosis was significant. Liver biopsy shows excess glycogen accumulation. After feeding, blood glucose levels rise, but there is no rise in glucose after overnight fasting. Which of the following enzyme deficiencies is most likely responsible for this presentation?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - This deficiency is characteristic of Glycogen Storage Disease type I (**GSD I** or **Von Gierke disease**), which prevents the final step of both glycogenolysis and gluconeogenesis (conversion of **Glucose-6-Phosphate** to free glucose). - The inability to release free glucose from the liver, especially during fasting, causes severe **fasting hypoglycemia**, significant **ketosis**, and massive **hepatomegaly** due to trapped glycogen and fat accumulation. *Liver phosphorylase* - Deficiency in **Liver phosphorylase** (GSD VI or Hers disease) impairs the breakdown of glycogen but does not affect the gluconeogenesis pathway. - While it leads to hepatomegaly and hypoglycemia, the symptoms are generally **milder** than GSD I, and the profound metabolic derangements (severe ketosis, lactic acidosis) seen in this case are typically less pronounced. *Muscle phosphorylase* - This enzyme deficiency, known as **GSD V** or **McArdle disease**, primarily affects the skeletal muscle, leading to muscle pain, cramps, and **exercise intolerance**. - It does not cause hepatomegaly or issues with hepatic glucose release and therefore is an **unlikely** cause of fasting hypoglycemia in a child. *Phosphofructokinase* - Phosphofructokinase (GSD VII or **Tarui disease**) deficiency affects both muscle and erythrocytes, presenting similar to McArdle disease with **exercise intolerance** and often mild hemolysis. - PFK deficiency primarily affects glycolysis and does not directly impair the liver's ability to release glucose via **gluconeogenesis** or the final G6Pase step during fasting.

Q: What is the cycle shown below called?

Cori cycle. ***Cori cycle*** - The diagram illustrates the **Cori cycle**, showing **lactate produced in the muscle** during anaerobic glycolysis being transported to the liver. - In the liver, this **lactate is converted back to glucose** via gluconeogenesis, which can then be returned to the muscle. *Embden-Meyerhof pathway* - The **Embden-Meyerhof pathway** is an older name for **glycolysis**, which is only one part of the cycle shown in the diagram (specifically, the conversion of glucose to lactate in the muscle). - It does not encompass the entire pathway of lactate transport to the liver and its conversion to glucose. *Pentose phosphate pathway* - The **pentose phosphate pathway** is a metabolic pathway parallel to glycolysis that generates **NADPH** and the precursor for nucleotide biosynthesis. - It is not depicted in this diagram, which focuses on glucose-lactate interconversion between muscle and liver. *Pyruvate decarboxylation* - **Pyruvate decarboxylation** is the conversion of pyruvate to acetyl-CoA, primarily occurring when oxygen is available for aerobic respiration. - The cycle shown specifically highlights the **anaerobic metabolism** in muscle producing lactate and its subsequent processing.

Q: Name the antigen marked as X determining blood group A.

N-Acetyl-Galactosamine. ***N-Acetyl-Galactosamine*** - Blood group A antigens are formed by the addition of **N-acetylgalactosamine** to the H antigen precursor molecule on the surface of red blood cells. - This sugar modification is catalyzed by the **A transferase enzyme**, which is specific for N-acetylgalactosamine. *N-Acetyl-Glucosamine* - While N-acetylglucosamine is a component of many glycans, it is not the terminal sugar that defines the **blood group A antigen**. - **N-acetylglucosamine** is a key building block for the H antigen and other blood group precursors, but not the specific modifying sugar for A. *Dermatan sulphate* - **Dermatan sulfate** is a **glycosaminoglycan** primarily found in connective tissues, skin, and blood vessels. - It plays a role in wound healing and coagulation, but is not involved in **ABO blood group determination**. *Keratan sulfate* - **Keratan sulfate** is another **glycosaminoglycan** found in cartilage, cornea, and bone. - It contributes to tissue hydration and structural integrity, but it is not part of the **ABO blood group antigens**.

Q: The enzyme catalyzing this pathway is absent in?

Muscle. The image depicts the conversion of **Glucose-6-phosphate** to **Glucose**, catalyzed by the enzyme **Glucose-6-phosphatase**. The question asks where this enzyme is *absent*. ***Correct: Muscle*** - **Muscle tissue lacks glucose-6-phosphatase**, ensuring that glucose-6-phosphate formed within muscle cells is committed to intracellular use (glycolysis or glycogen synthesis) - This prevents muscle from releasing free glucose into the bloodstream, which is physiologically appropriate since muscle primarily uses glucose for its own energy needs - The absence of this enzyme in muscle is a key concept in understanding **tissue-specific glucose metabolism** and why muscle glycogen cannot directly contribute to blood glucose levels *Incorrect: Liver* - Liver is one of the **primary sites where glucose-6-phosphatase is highly expressed** - This enzyme is essential for hepatic glucose production during **gluconeogenesis** and **glycogenolysis** - Allows the liver to release free glucose into the bloodstream to maintain blood glucose homeostasis *Incorrect: Kidney* - Kidney also **contains glucose-6-phosphatase**, particularly in the renal cortex - The kidney contributes to glucose homeostasis through **renal gluconeogenesis**, especially during prolonged fasting - Can release free glucose into the circulation *Incorrect: Intestine* - Small intestine (particularly during fed state) **expresses glucose-6-phosphatase** - Plays a role in glucose production, though to a lesser extent than liver and kidney - Can contribute to postprandial glucose homeostasis **Clinical Relevance:** Deficiency of glucose-6-phosphatase causes **Von Gierke disease (Glycogen Storage Disease Type I)**, characterized by severe hypoglycemia, hepatomegaly, and lactic acidosis.

Q: A 2-week-old neonate is brought with complaints of recurrent vomiting, prolongation of physiological jaundice. Urine tests negative for sugar by dipstick but Benedict's test is positive. Which of the following enzymatic defect is most likely responsible for this presentation?

1. ***1*** - The enzymatic defect at position 1 represents **galactokinase deficiency**. This leads to the accumulation of **galactose** in the blood and urine. - While urinary dipstick tests for glucose will be negative, the Benedict's test, which detects all reducing sugars, will be **positive due to galactose**, explaining the presentation of recurrent vomiting and prolonged jaundice. *2* - The enzyme at position 2 is **galactose-1-phosphate uridylyltransferase (GALT)**. A deficiency in GALT causes classic **galactosemia**, a more severe condition. - Patients with classic galactosemia typically present with **hepatic dysfunction**, **cataracts**, and **developmental delay**, in addition to the symptoms mentioned, which are more pronounced due to the accumulation of toxic intermediates like galactose-1-phosphate. *3* - The enzyme at position 3 is **UDP-galactose 4'-epimerase (GALE)**. A deficiency in this enzyme is known as **epimerase deficiency galactosemia**. - This condition has variable severity; milder forms might be asymptomatic, while severe forms can present similarly to classic galactosemia, including **jaundice**, **vomiting**, and **cataracts**. However, it is less common than galactokinase deficiency. *4* - The enzyme at position 4 is **UDP-glucose pyrophosphorylase**, which is involved in the synthesis of UDP-glucose. - A defect in this enzyme would primarily affect glucose metabolism and glycogen synthesis, presenting with **hypoglycemia** and related symptoms, rather than the primary accumulation of galactose or its metabolites.

Q: A floppy infant was brought with difficulty in breathing. CXR was performed. Family history was positive for death of sibling at 1-year of age. Which of the following enzyme deficiency will lead to this presentation?

Acid maltase deficiency. ***Acid maltase deficiency*** - This presentation describes **Pompe disease (Glycogen Storage Disease Type II)**, caused by a deficiency of **acid alpha-glucosidase (acid maltase)**, leading to glycogen accumulation in lysosomes. - The "floppy infant" (generalized hypotonia), difficulty breathing due to **cardiomyopathy** and **respiratory muscle weakness**, and the CXR showing **cardiomegaly** (enlarged heart) are classic findings of infantile-onset Pompe disease. The family history of sibling death supports a genetic cause. *Muscle phosphorylase deficiency* - This deficiency causes **McArdle disease (Glycogen Storage Disease Type V)**, primarily affecting **skeletal muscle**. - Symptoms usually appear in adolescence or adulthood, characterized by exercise intolerance, muscle pain, and cramps, not typically severe hypotonia and respiratory distress in infancy. *Glucose-6-phosphatase deficiency* - This is **Von Gierke disease (Glycogen Storage Disease Type I)**, affecting the **liver and kidneys**. - It presents with severe **hypoglycemia** (especially fasting), hepatomegaly, lactic acidosis, and hyperlipidemia, but not primary muscle weakness or cardiomyopathy. *Arylsulfatase B deficiency* - This deficiency causes **Maroteaux-Lamy syndrome (Mucopolysaccharidosis Type VI)**, a lysosomal storage disorder. - It primarily affects connective tissues, leading to skeletal deformities (**dysostosis multiplex**), coarse facial features, and corneal clouding, but not the severe infantile hypotonia or cardiomegaly seen in Pompe disease.

Q: Which transporter helps in the improvement of insulin resistance in type 2 diabetes mellitus (DM2) with regular exercise and physical activities?

GLUT 4. ***GLUT 4*** - **GLUT 4** is the **insulin-sensitive glucose transporter** predominantly found in **skeletal muscle** and **adipose tissue**. Regular exercise and physical activity increase the expression and translocation of GLUT 4 to the cell membrane, enhancing glucose uptake independent of insulin, thereby improving **insulin sensitivity** and reducing insulin resistance in DM2. - Exercise also stimulates **AMPK (AMP-activated protein kinase)**, which promotes GLUT 4 translocation to the cell surface, facilitating glucose uptake and utilization in active muscles. *GLUT 1* - **GLUT 1** is a basal glucose transporter found in nearly all cells, responsible for **basal glucose uptake** to meet basic metabolic needs, especially in **red blood cells** and the **blood-brain barrier**. - Its activity is largely **insulin-independent** and does not significantly contribute to the exercise-induced improvement in insulin sensitivity in skeletal muscle. *GLUT 2* - **GLUT 2** is primarily found in the **liver, pancreatic beta cells, kidneys, and small intestine**. It has a **low affinity** but **high capacity** for glucose transport, serving as a glucose sensor in beta cells and allowing efficient glucose uptake/release in the liver. - It plays a role in glucose homeostasis but is **not directly involved in the exercise-mediated improvement of insulin resistance** in peripheral tissues like muscle. *GLUT 3* - **GLUT 3** is a **high-affinity glucose transporter** primarily expressed in **neurons** and the **placenta**. It is crucial for providing a constant supply of glucose to the brain, even at low glucose concentrations. - Like GLUT 1 and GLUT 2, its activity is largely **insulin-independent** and does not play a significant role in improving insulin resistance through exercise in DM2.

Q: Corneal Transparency is maintained by which of the following GAGs?

Keratan Sulphate. ***Keratan Sulphate*** - **Keratan sulfate** is a major glycosaminoglycan (GAG) found in the **cornea**, where its specific highly hydrated structure and arrangement help maintain corneal transparency. - The uniform spacing of collagen fibrils, maintained by keratan sulfate, is crucial for minimizing light scattering and allowing light to pass through the cornea. *Dermatan Sulphate* - **Dermatan sulfate** is primarily found in **skin, blood vessels, and heart valves**, contributing to tissue strength and elasticity. - It plays a significant role in wound healing and cardiovascular function, but not directly in maintaining corneal transparency. *Heparan Sulphate* - **Heparan sulfate** is ubiquitously found on **cell surfaces and in the extracellular matrix**, particularly in the basement membranes. - It is involved in cell adhesion, growth factor binding, and anticoagulant activity, but is not the primary GAG responsible for corneal transparency. *Chondroitin Sulphate* - **Chondroitin sulfate** is abundant in **cartilage, bone, and connective tissues**, providing compressive strength and elasticity. - While present in some ocular tissues, it is not the dominant GAG responsible for the unique transparent properties of the cornea.

Question 1

Which of the following cell pathways occurs exclusively in cell cytoplasm?

Accepted Answer

Glycolysis

Answer

TCA cycle

Answer

Urea cycle

Answer

Beta oxidation

Question 2

Cancer cells take up excess glucose because?

Accepted Answer

Lactate is produced even in the presence of Oxygen

Answer

High NADH/NAD ratio

Answer

High GLUT2

Answer

Absence of Oxygen

Question 3

A child presents with fatigue and hepatomegaly. Liver enzymes (ALT, AST) are elevated. Ketosis was significant. Liver biopsy shows excess glycogen accumulation. After feeding, blood glucose levels rise, but there is no rise in glucose after overnight fasting. Which of the following enzyme deficiencies is most likely responsible for this presentation?

Accepted Answer

Glucose-6-phosphatase

Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Phosphofructokinase

Question 4

What is the cycle shown below called?

Accepted Answer

Cori cycle

Answer

Embden-Meyerhof pathway

Answer

Pentose phosphate pathway

Answer

Pyruvate decarboxylation

Question 5

Name the antigen marked as X determining blood group A.

Accepted Answer

N-Acetyl-Galactosamine

Answer

N-Acetyl-Glucosamine

Answer

Dermatan sulphate

Answer

Keratan sulfate

Question 6

The enzyme catalyzing this pathway is absent in?

Accepted Answer

Muscle

Answer

Liver

Answer

Kidney

Answer

Intestine

Question 7

A 2-week-old neonate is brought with complaints of recurrent vomiting, prolongation of physiological jaundice. Urine tests negative for sugar by dipstick but Benedict's test is positive. Which of the following enzymatic defect is most likely responsible for this presentation?

Accepted Answer

1

Answer

2

Answer

3

Answer

4

Question 8

A floppy infant was brought with difficulty in breathing. CXR was performed. Family history was positive for death of sibling at 1-year of age. Which of the following enzyme deficiency will lead to this presentation?

Accepted Answer

Acid maltase deficiency

Answer

Muscle phosphorylase deficiency

Answer

Glucose-6-phosphatase deficiency

Answer

Arylsulfatase B deficiency

Question 9

Which transporter helps in the improvement of insulin resistance in type 2 diabetes mellitus (DM2) with regular exercise and physical activities?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 10

Corneal Transparency is maintained by which of the following GAGs?

Accepted Answer

Keratan Sulphate

Answer

Dermatan Sulphate

Answer

Heparan Sulphate

Answer

Chondroitin Sulphate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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