Carbohydrate Metabolism Practice Questions

Q: Hepatomegaly with hypoglycemia occurs in deficiency of

Glucose-6-phosphatase. ***Acid maltase*** - Deficiency of acid maltase, also known as Pompe disease, leads to significant muscle and liver glycogen accumulation, which can cause an **AST/ALT ratio > 2** due to hepatocellular injury [1,2]. - It predominantly causes **myopathy** and hepatomegaly, making this enzyme deficiency clinically relevant in this context [1]. *Glucose-6-phosphotase* - This deficiency leads to **von Gierke disease**, characterized by severe hypoglycemia and increased lactate, but not specifically an AST/ALT ratio > 2 [1]. - It results in **glycogen accumulation in the liver**, affecting glucose metabolism but not primarily liver enzymes [1]. *Branching enzyme* - Branching enzyme deficiency results in **Andersen disease**, which is characterized by long unbranched glycogen chains, and predominantly causes **hepatic dysfunction** without a specific AST/ALT ratio > 2. - Clinical manifestations include **cirrhosis** and splenomegaly, but not elevation of liver transaminases in the described ratio. *Liver phosphorylase* - Liver phosphorylase deficiency results in **Cori disease**, which presents with hypoglycemic episodes and hepatomegaly, but not necessarily an AST/ALT ratio above 2 [1]. - The enzyme affects glycogen mobilization, and clinical features do not consistently include liver enzyme elevation as described. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 164-167. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 850-851.

Q: Cancer cells preferentially utilize glycolysis for energy production even in the presence of adequate oxygen. What is this phenomenon called?

Warburg effect. ***Warburg*** - The **Warburg effect** describes how cancer cells preferentially use glycolysis for energy production even in the presence of oxygen, allowing them to thrive in **hypoxic conditions** [1]. - This metabolic adaptation supports **cell proliferation** and survival in tumor microenvironments where oxygen is limited [1][3]. - Cancer cells upregulate glucose uptake and express specific metabolic enzymes like the M2 isoform of pyruvate kinase that facilitate this altered metabolism [2][4]. *Wanton* - This term typically refers to recklessness or extravagance and is not used in the context of cancer metabolism or hypoxia. - There are no associations with **cancer cell adaptation** under adverse environmental conditions. *Wormian* - **Wormian bones** are extra bone pieces within sutures of the skull, unrelated to cancer cell metabolism or survival mechanisms. - This term does not connect to **hypoxia** or metabolic adaptations in cancer biology. *Wolf* - "Wolf" has no recognized connection to cancer cell biology, particularly regarding metabolic adaptations under **hypoxic stress**. - It does not imply any concept associated with how cancer cells cope with adverse conditions. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 307-308. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 308-310. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 290-291. [4] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. With Illustrations By, pp. 26-27.

Q: Which of the following enzymes participates exclusively in glycolysis?

Pyruvate kinase. ***Pyruvate kinase*** - This enzyme catalyzes the **final step of glycolysis**, irreversibly converting **phosphoenolpyruvate (PEP)** to pyruvate, producing ATP. - **Exclusively participates in glycolysis** - it has no role in any other metabolic pathway, making it the most definitive answer. - All tissue-specific isoforms (M1, M2, L, R) perform the same glycolysis-exclusive function. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** catalyzes the committed step of glycolysis (Fructose-6-P → Fructose-1,6-BP) and is technically exclusive to the glycolytic pathway. - However, when the question refers to "phosphofructokinase" generically, it could include **PFK-2**, which produces fructose-2,6-bisphosphate (a regulatory molecule, not a glycolytic intermediate) and is part of the regulatory mechanism rather than the pathway itself. - **Pyruvate kinase is more unambiguously exclusive** to glycolysis as a metabolic enzyme. *Hexokinase* - While essential for the initial step of glycolysis, **hexokinase** phosphorylates multiple hexoses (glucose, mannose, fructose) and its product (G6P) can enter **multiple pathways**: glycolysis, pentose phosphate pathway, or glycogen synthesis. - **Not exclusive to glycolysis** - it serves as a branch point enzyme. *Glucose-6-phosphate dehydrogenase* - This enzyme is the rate-limiting step of the **pentose phosphate pathway (PPP)**, not glycolysis. - It catalyzes the oxidation of G6P to produce **NADPH** and ribose-5-phosphate for nucleotide synthesis, thereby diverting glucose-6-phosphate **away from glycolysis**.

Q: Which of the following steps is specific for gluconeogenesis?

Oxaloacetate to PEP. ***Oxaloacetate to PEP*** - This step, catalyzed by **PEP carboxykinase (PEPCK)**, is a bypass reaction necessary to overcome the irreversible pyruvate kinase step in glycolysis. - It is a key regulatory point in **gluconeogenesis**, allowing the synthesis of glucose from non-carbohydrate precursors. *Oxaloacetate to citrate* - This reaction is part of the **Krebs cycle (citric acid cycle)**, where oxaloacetate combines with acetyl-CoA to form citrate. - It does not directly lead to **glucose synthesis** and is not unique to gluconeogenesis. *Oxaloacetate to glucose* - This is an **overly broad statement** and not a direct, single enzymatic step in gluconeogenesis. - While oxaloacetate is an intermediate in the gluconeogenic pathway, it must first be converted to **PEP** and then proceed through several more steps to become glucose. *Pyruvate to acetyl CoA* - This reaction is catalyzed by the **pyruvate dehydrogenase complex** and represents a committed step into oxidative metabolism, primarily the Krebs cycle. - This step is **irreversible** in mammals and prevents the direct conversion of acetyl-CoA back to pyruvate or glucose, making it not relevant for gluconeogenesis.

Q: Which of the following enzymes do not catalyze an irreversible step in glycolysis?

Phosphoglycerate kinase. ***Phosphoglycerate kinase*** - This enzyme catalyzes the conversion of **1,3-bisphosphoglycerate** to **3-phosphoglycerate**, generating ATP. - This reaction is considered reversible because the free energy change is close to zero under physiological conditions, allowing for both forward (glycolysis) and reverse (gluconeogenesis) flux. *Hexokinase* - This enzyme catalyzes the **irreversible phosphorylation** of glucose to glucose-6-phosphate, trapping glucose within the cell. - It is one of the key regulatory enzymes in glycolysis, and its irreversibility ensures that glucose uptake and phosphorylation proceed in one direction. *Pyruvate kinase* - This enzyme catalyzes the final, **irreversible step** of glycolysis, converting phosphoenolpyruvate to pyruvate and generating ATP. - This reaction is a major control point for glycolysis due to its large negative free energy change. *Phosphofructokinase* - This enzyme catalyzes the **irreversible phosphorylation** of fructose-6-phosphate to fructose-1,6-bisphosphate. - It is considered the **rate-limiting step** and a primary control point in glycolysis, making it highly regulated and unidirectional.

Q: In which of the following pathways is UDP glucose not utilized?

HMP shunt. ***HMP shunt*** - The **Hexose Monophosphate Shunt (HMP shunt)**, also known as the **pentose phosphate pathway**, primarily uses **glucose-6-phosphate** as its substrate. - Its main products are **NADPH** and **ribose-5-phosphate**, and it does not involve **UDP-glucose**. *Uronic acid pathway* - The **uronic acid pathway** converts **glucose** to **glucuronic acid**, **L-xylulose**, and **ascorbic acid (in some animals)**, utilizing **UDP-glucose** as an intermediate. - Specifically, **UDP-glucose dehydrogenase** oxidizes UDP-glucose to **UDP-glucuronate**. *Glycogen synthesis* - In **glycogen synthesis (glycogenesis)**, **UDP-glucose** is the direct precursor for adding glucose units to the growing **glycogen chain**. - The enzyme **glycogen synthase** catalyzes the transfer of glucose from UDP-glucose to the non-reducing end of glycogen. *Galactose metabolism* - In **galactose metabolism**, **UDP-glucose** plays a crucial role in the conversion of **galactose-1-phosphate** to **glucose-1-phosphate**. - This occurs via the enzyme **galactose-1-phosphate uridyltransferase**, which exchanges UDP from UDP-glucose with the phosphate from galactose-1-phosphate, forming **UDP-galactose** and **glucose-1-phosphate**.

Q: Which of the following is not a step in gluconeogenesis?

Conversion of pyruvate to acetyl-CoA. ***Conversion of pyruvate to acetyl-CoA*** - This step is a key irreversible reaction catalyzed by the **pyruvate dehydrogenase complex** that commits pyruvate to oxidative metabolism via the **Krebs cycle** or to fatty acid synthesis. - It is **not a part of gluconeogenesis**, as acetyl-CoA cannot be converted back to pyruvate or glucose in mammals. - This reaction is irreversible and represents a point of no return for glucose synthesis. *Conversion of glucose-6-phosphate to glucose* - This is the **final step in gluconeogenesis**, catalyzed by **glucose-6-phosphatase** in the liver and kidney. - This enzyme allows free glucose to be released into the bloodstream. - It is an essential gluconeogenic step that bypasses the irreversible hexokinase/glucokinase reaction of glycolysis. *Conversion of oxaloacetate to phosphoenolpyruvate* - This is a **key bypass step in gluconeogenesis** that overcomes the irreversible pyruvate kinase reaction in glycolysis. - It is catalyzed by **phosphoenolpyruvate carboxykinase (PEPCK)** and requires GTP. - This is crucial for synthesizing glucose from non-carbohydrate precursors like amino acids and lactate. *Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate* - This is an important **bypass step in gluconeogenesis**, catalyzed by **fructose-1,6-bisphosphatase**. - This irreversible reaction bypasses the phosphofructokinase-1 step of glycolysis. - It is one of the three key regulatory steps unique to gluconeogenesis.

Q: In the oxidative phase of pentose phosphate pathway, NADPH is produced in?

Cytosol. ***Cytosol*** - The **oxidative phase** of the **pentose phosphate pathway (PPP)**, which produces **NADPH**, occurs exclusively in the **cytosol**. - Two key enzymes generate NADPH: **glucose-6-phosphate dehydrogenase (G6PD)** and **6-phosphogluconate dehydrogenase**. - **NADPH** is crucial for **reductive biosynthesis** (e.g., fatty acid synthesis, cholesterol synthesis) and for maintaining **redox balance** (e.g., reducing glutathione to protect against oxidative stress). *Mitochondria* - While mitochondria are central to **oxidative phosphorylation** and the **Krebs cycle**, they primarily produce **NADH** and **FADH2** for ATP generation. - The pentose phosphate pathway does not occur in mitochondria. *Ribosome* - **Ribosomes** are responsible for **protein synthesis** (translation) and are not involved in metabolic pathways or NADPH production. - They are cellular machinery for translation, not metabolic compartments. *Peroxisomes* - **Peroxisomes** are involved in **fatty acid β-oxidation** and **detoxification** of hydrogen peroxide. - While peroxisomes have some oxidative enzymes, they are not the site of the pentose phosphate pathway or its NADPH production.

Q: Insulin resistance down-regulates the translocation of which glucose transporter to the cell membrane?

GLUT-4. ***GLUT-4*** - **Insulin resistance** primarily affects cells that express **GLUT-4**, such as muscle and adipose tissue, by impairing its translocation from intracellular vesicles to the cell membrane. - This reduced translocation leads to decreased glucose uptake in response to insulin, a hallmark of **type 2 diabetes**. *GLUT-1* - **GLUT-1** is responsible for basal glucose uptake in nearly all cells, including **erythrocytes** and endothelial cells of the blood-brain barrier. - Its activity is largely **insulin-independent** and not significantly affected by insulin resistance in the same way as GLUT-4. *GLUT-2* - **GLUT-2** is found primarily in **pancreatic β-cells**, hepatocytes, renal tubular cells, and enterocytes. - It has a low affinity but high capacity for glucose transport, playing a key role in **glucose sensing** and facilitating glucose flux in and out of these cells, independent of insulin translocation. *GLUT-3* - **GLUT-3** is predominantly expressed in **neurons** and the placenta, where it facilitates high-affinity glucose uptake. - It is crucial for maintaining the brain's glucose supply and its activity is also **insulin-independent**.

Q: Which enzyme converts glucose to sorbitol?

Aldose reductase. ***Aldose reductase*** - This enzyme is crucial in the **polyol pathway**, reducing **glucose to sorbitol** by using **NADPH** as a cofactor. - In conditions of high glucose (e.g., uncontrolled diabetes), increased activity of **aldose reductase** leads to sorbitol accumulation, contributing to **osmotic damage** in certain tissues like the lens, nerves, and kidneys. *Sorbitol dehydrogenase* - This enzyme is responsible for the subsequent step in the polyol pathway, **oxidizing sorbitol to fructose** using NAD+ as a cofactor. - While related to sorbitol metabolism, it does not convert glucose to sorbitol; instead, it metabolizes sorbitol further. *Aldolase B* - This enzyme is involved in **fructose metabolism**, specifically cleaving **fructose-1-phosphate** into **dihydroxyacetone phosphate** and **glyceraldehyde**. - It plays no direct role in the conversion of glucose to sorbitol. *Glucose-6-phosphate dehydrogenase* - This is the rate-limiting enzyme of the **pentose phosphate pathway**, catalyzing the oxidation of glucose-6-phosphate to 6-phosphoglucono-δ-lactone. - While it also uses NADPH (producing it rather than consuming it), it is not involved in the polyol pathway or sorbitol synthesis.

Question 1

Hepatomegaly with hypoglycemia occurs in deficiency of

Accepted Answer

Glucose-6-phosphatase

Answer

Branching enzyme

Answer

Debranching enzyme

Answer

Hepatic phosphorylase

Question 2

Cancer cells preferentially utilize glycolysis for energy production even in the presence of adequate oxygen. What is this phenomenon called?

Accepted Answer

Warburg effect

Answer

Hypoxic adaptation

Answer

Anoxic survival

Answer

Oxygen-independent metabolism

Question 3

Which of the following enzymes participates exclusively in glycolysis?

Accepted Answer

Pyruvate kinase

Answer

Phosphofructokinase

Answer

Glucose-6-phosphate dehydrogenase

Answer

Hexokinase

Question 4

Which of the following steps is specific for gluconeogenesis?

Accepted Answer

Oxaloacetate to PEP

Answer

Oxaloacetate to citrate

Answer

Oxaloacetate to glucose

Answer

Pyruvate to acetyl CoA

Question 5

Which of the following enzymes do not catalyze an irreversible step in glycolysis?

Accepted Answer

Phosphoglycerate kinase

Answer

Pyruvate kinase

Answer

Phosphofructokinase

Answer

Hexokinase

Question 6

In which of the following pathways is UDP glucose not utilized?

Accepted Answer

HMP shunt

Answer

Uronic acid pathway

Answer

Glycogen synthesis

Answer

Galactose metabolism

Question 7

Which of the following is not a step in gluconeogenesis?

Accepted Answer

Conversion of pyruvate to acetyl-CoA

Answer

Conversion of glucose-6-phosphate to glucose

Answer

Conversion of oxaloacetate to phosphoenolpyruvate

Answer

Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate

Question 8

In the oxidative phase of pentose phosphate pathway, NADPH is produced in?

Accepted Answer

Cytosol

Answer

Mitochondria

Answer

Ribosome

Answer

Peroxisomes

Question 9

Insulin resistance down-regulates the translocation of which glucose transporter to the cell membrane?

Accepted Answer

GLUT-4

Answer

GLUT-1

Answer

GLUT-2

Answer

GLUT-3

Question 10

Which enzyme converts glucose to sorbitol?

Accepted Answer

Aldose reductase

Answer

Sorbitol dehydrogenase

Answer

Aldolase B

Answer

Glucose-6-phosphate dehydrogenase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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