Carbohydrate Metabolism Practice Questions

Q: Which glucose transporter is primarily affected in diabetes mellitus?

GLUT-4. ***GLUT-4*** - **GLUT-4** is the primary glucose transporter in **insulin-sensitive** tissues such as muscle and adipose tissue. - In **diabetes mellitus**, impaired insulin signaling leads to reduced translocation of GLUT-4 to the cell membrane, resulting in decreased glucose uptake by these tissues and subsequently **hyperglycemia**. *GLUT-2* - **GLUT-2** is found in the **liver**, **pancreatic beta cells**, kidneys, and small intestine. - It has a low affinity for glucose and is primarily involved in **high-capacity glucose transport**, serving as a glucose sensor in beta cells and allowing efficient glucose uptake/release in the liver. *GLUT-5* - **GLUT-5** is a fructose transporter predominantly found in the **small intestine** and testes. - It is responsible for the absorption of **fructose** from the diet and is not directly involved in glucose regulation relevant to diabetes mellitus. *SGLT-2* - **SGLT-2** (Sodium-Glucose Co-transporter 2) is found in the **proximal tubules of the kidneys**. - It is responsible for reabsorbing approximately 90% of the **filtered glucose** from the renal filtrate back into the bloodstream, and its inhibition is a therapeutic target in diabetes.

Q: What is the biochemical structure of cellulose?

β (1,4) glucose. ***β (1,4) glucose*** - Cellulose is a linear polysaccharide made of repeating **glucose units** joined by **β-1,4 glycosidic bonds**. - This specific linkage allows for strong hydrogen bonding between adjacent cellulose chains, contributing to its structural rigidity in plant cell walls. *α (1,4) glucose* - This linkage is characteristic of starch (amylose) and glycogen, forming helical structures that are readily digestible by humans. - Unlike cellulose, these **α-1,4 linkages** result in a coiled, rather than linear, polysaccharide structure. *β (1,6) glucose* - While beta linkages are present in some polysaccharides, the **β-1,6 linkage** is not the primary linkage for the main chain of cellulose. - This linkage is primarily found at branch points in certain complex carbohydrates. *α (1,6) glucose* - This linkage forms branch points in branched polysaccharides like amylopectin (a component of starch) and glycogen. - It allows for a more compact and easily accessible energy storage molecule, very different from the structural role of cellulose.

Q: Cataract in cases of diabetes mellitus is due to accumulation of ?

Glycated crystallins. ***Glycated crystallins*** - In diabetes, high glucose levels lead to **non-enzymatic glycation** of lens proteins, primarily **crystallins**. - Glycation causes aggregation and insolubilization of crystallins, leading to increased **light scattering** and cataract formation. - Additionally, the **polyol pathway** (glucose → sorbitol via aldose reductase) causes sorbitol accumulation in the lens, contributing to osmotic stress and protein damage. - Both glycation and sorbitol accumulation are key mechanisms in diabetic cataractogenesis. *Calcified crystallins* - While calcification can occur in some forms of cataracts (e.g., age-related, traumatic), it is **not the primary mechanism** for diabetic cataracts. - Diabetic cataracts are predominantly linked to **metabolic changes** (hyperglycemia-induced glycation and polyol pathway) rather than mineral deposition. *Glycated fibrillins* - **Fibrillin** is an extracellular matrix glycoprotein important for elastic fibers in connective tissues, but it is **not a major structural protein of the lens**. - The lens is composed primarily of **crystallins** (α, β, γ), not fibrillins. - Therefore, fibrillin glycation would not directly cause lens opacification. *Calcified fibrillins* - This option incorrectly identifies both the protein type and the mechanism. - **Fibrillin calcification** is not a known pathway for diabetic cataractogenesis. - Diabetic cataracts result from crystallin modifications, not fibrillin changes.

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: 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: Which of the following is an aldose sugar?

Glyceraldehyde. ***Glyceraldehyde*** - **Glyceraldehyde** is the simplest **aldose**, a monosaccharide with an **aldehyde group** (CHO) at one end of its carbon chain. - Its chemical structure is a three-carbon chain with the aldehyde group on the first carbon, making it an **aldo sugar**. *Ribulose* - **Ribulose** is a **ketose**, specifically a **ketopentose**, meaning it is a five-carbon sugar with a **ketone group** (C=O) in its structure. - The ketone group in ribulose is typically located on the second carbon, distinguishing it from aldoses. *Fructose* - **Fructose** is another example of a **ketose**, specifically a **ketohexose**, as it is a six-carbon sugar containing a **ketone group**. - Its ketone group is usually found on the second carbon atom, which differentiates ketoses from aldoses structurally. *None of the options* - This option is incorrect because **glyceraldehyde** is indeed an aldose sugar, fitting the definition of a monosaccharide with an aldehyde functional group. - As **glyceraldehyde** is correctly identified as an aldose, this choice would contradict the chemical classification of sugars.

Q: What is the cause of lactose intolerance?

Deficiency of Lactase. ***Deficiency of Lactase*** - Lactose intolerance results from the insufficient production of the enzyme **lactase**, which is responsible for breaking down **lactose** (a disaccharide found in milk and dairy products) into glucose and galactose. - When lactase is deficient, undigested lactose passes into the colon, where it is fermented by bacteria, leading to symptoms like **bloating**, **gas**, **diarrhea**, and **abdominal pain**. *Deficiency of Galactokinase* - A deficiency in **galactokinase** causes **Type II galactosemia**, a disorder involving the inability to metabolize galactose. - This condition primarily leads to **cataracts** and does not directly cause the digestive symptoms associated with lactose intolerance. *Deficiency of Uridyl transferase* - A deficiency in **uridyl transferase** causes **classic galactosemia (Type I)**, the most severe form of galactosemia. - This condition results in a buildup of toxic galactose metabolites, leading to **liver damage**, **renal failure**, and **developmental delay**, not lactose intolerance. *Deficiency of Enteropeptidase* - **Enteropeptidase** (also known as enterokinase) is an enzyme in the small intestine that activates trypsinogen to trypsin, which then activates other pancreatic proteases. - A deficiency leads to **protein malabsorption** and failure to thrive, not the fermentation of lactose by gut bacteria.

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**.

Question 1

Which glucose transporter is primarily affected in diabetes mellitus?

Accepted Answer

GLUT-4

Answer

GLUT-2

Answer

GLUT-5

Answer

SGLT-2

Question 2

What is the biochemical structure of cellulose?

Accepted Answer

β (1,4) glucose

Answer

β (1,6) glucose

Answer

α (1,6) glucose

Answer

α (1,4) glucose

Question 3

Cataract in cases of diabetes mellitus is due to accumulation of ?

Accepted Answer

Glycated crystallins

Answer

Calcified crystallins

Answer

Glycated fibrillins

Answer

Calcified fibrillins

Question 4

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 5

Hepatomegaly with hypoglycemia occurs in deficiency of

Accepted Answer

Glucose-6-phosphatase

Answer

Branching enzyme

Answer

Debranching enzyme

Answer

Hepatic phosphorylase

Question 6

Which of the following is an aldose sugar?

Accepted Answer

Glyceraldehyde

Answer

Ribulose

Answer

Fructose

Answer

None of the options

Question 7

What is the cause of lactose intolerance?

Accepted Answer

Deficiency of Lactase

Answer

Deficiency of Galactokinase

Answer

Deficiency of Uridyl transferase

Answer

Deficiency of Enteropeptidase

Question 8

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 9

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 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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