Carbohydrate Metabolism Practice Questions

Q: Which enzyme deficiency is associated with increased susceptibility to infections, including E. coli?

Glucose-6-phosphate dehydrogenase (G6PD) deficiency. ***Glucose-6-phosphate dehydrogenase (G6PD) deficiency*** - G6PD deficiency impairs the **pentose phosphate pathway**, reducing the production of **NADPH** in phagocytes. - Reduced **NADPH** leads to an inability to generate a robust **respiratory burst** required to kill pathogens like *E. coli*, thereby increasing susceptibility to infections. *Lactase deficiency* - This deficiency affects the digestion of **lactose**, leading to gastrointestinal symptoms such as bloating and diarrhea. - It does not directly impact the **immune system's ability to combat bacterial infections**. *Pepsin deficiency* - Pepsin is a gastric enzyme essential for **protein digestion** in the stomach. - While it can lead to impaired protein breakdown, it is not directly linked to an increased susceptibility to **bacterial infections** like *E. coli*. *Trypsin deficiency* - Trypsin is a pancreatic enzyme crucial for **protein digestion** in the small intestine. - Its deficiency affects nutrient absorption but does not directly compromise the **innate immune response** against bacteria.

Q: Neonatal hypoglycemia that does not respond to treatment with counter-regulatory hormones is diagnostic of which condition?

Von Gierke's disease. ***Von Gierke's disease*** - This condition (Glycogen Storage Disease Type I) results from a **deficiency of glucose-6-phosphatase**, essential for releasing glucose from the liver. - The inability to produce free glucose from glycogen or gluconeogenesis leads to severe hypoglycemia that **does not respond to counter-regulatory hormones** like glucagon, as the enzyme needed for glucose release is non-functional. *Hereditary fructose intolerance* - This condition involves a deficiency in **aldolase B**, leading to the accumulation of fructose-1-phosphate after fructose ingestion. - While it can cause hypoglycemia, it generally occurs after **fructose exposure** and is not characterized by hypoglycemia refractory to counter-regulatory hormones in the neonatal period without such exposure. *Cori's disease (Glycogen storage disease type III)* - Caused by a deficiency in the **glycogen debranching enzyme**, leading to the accumulation of abnormal glycogen. - Patients can present with hypoglycemia, but often respond to glucagon administration, as the remaining glycogen structure can still be partially broken down, unlike in Von Gierke's. *Anderson's disease (Glycogen storage disease type IV)* - Result of a deficiency in the **glycogen branching enzyme**, leading to the formation of abnormally structured glycogen with long, unbranched chains. - This disease primarily affects the liver and muscles, causing **cirrhosis** and muscle weakness, and typically does not present with severe, refractory neonatal hypoglycemia as the primary or most characteristic symptom.

Q: Gas released from oligosaccharide metabolism by intestinal bacteria is

Carbon dioxide. ***Carbon dioxide*** - **Carbon dioxide (CO₂)** is the **most universally produced gas** from oligosaccharide and carbohydrate fermentation by intestinal bacteria in the colon. - Nearly all colonic bacteria produce CO₂ during the fermentation of undigested carbohydrates and oligosaccharides. - Along with **hydrogen (H₂)**, CO₂ forms the bulk of intestinal gas from bacterial metabolism. - This is part of normal gut flora activity contributing to **flatulence**. *Methane* - While **methane (CH₄)** is produced during oligosaccharide fermentation, it is only generated by individuals harboring **methanogenic archaea** (approximately 30-50% of the population). - Methane production is not universal, unlike CO₂, making it less representative as "THE gas" from oligosaccharide metabolism. - Methanogens use H₂ and CO₂ to produce methane as a secondary process. *Sulfur dioxide* - **Sulfur dioxide (SO₂)** is primarily associated with industrial pollution and is not a product of normal intestinal bacterial metabolism. - Hydrogen sulfide (H₂S) may be produced from sulfur-containing compounds, but not sulfur dioxide. *Nitric oxide* - **Nitric oxide (NO)** is a signaling molecule involved in vasodilation and immune responses. - It is not a major gas produced from bacterial fermentation of oligosaccharides in the intestines.

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: In glycogen synthesis the active form of glucose used is-

UDP glucose. **UDP glucose** - **UDP-glucose** (uridine diphosphate glucose) is the activated form of glucose that donates glucose units for the elongation of the **glycogen chain** during glycogen synthesis. - The formation of UDP-glucose from **glucose-1-phosphate** and **UTP** (uridine triphosphate) is catalyzed by UDP-glucose pyrophosphorylase, making glucose-1-phosphate a precursor, not the active form. *Glucose 6 phosphate* - **Glucose 6-phosphate** is an important intermediate in glycolysis and gluconeogenesis, and it can be isomerized to glucose 1-phosphate, but it is not the direct substrate for glycogen synthase. - Its formation is the first committed step in glucose metabolism within the cell, trapping glucose inside. *Glucose I phosphate* - **Glucose 1-phosphate** is a precursor to UDP-glucose, formed from glucose 6-phosphate by **phosphoglucomutase**. - While essential for glycogen synthesis, it is not the directly active form that donates glucose to the glycogen chain itself. *GTP glucose* - **GTP glucose** is not a known active form of glucose involved in glycogen synthesis. - **GTP** (guanosine triphosphate) is primarily involved in other metabolic processes, such as protein synthesis and signal transduction.

Q: All of the following are true about lactate utilization in the liver except -

Total net number of ATP formed because of Cori's cycle is 4. ***Total net number of ATP formed because of cori's cycle is 4*** - This statement is incorrect. The **Cori cycle (lactic acid cycle)** is an energy-consuming process overall, as **6 ATP** molecules are consumed in the liver for gluconeogenesis to resynthesize glucose from lactate, while only a total of **2 ATP** are gained from glycolysis in the muscle. - The primary purpose of the Cori cycle is not net ATP production, but rather to shift the metabolic burden and regenerate glucose for tissues that rely on glycolysis (e.g., muscle, red blood cells). *Cori's cycle shifts the metabolic burden from muscle to liver* - This is true because **lactate produced in muscle** (during anaerobic conditions) is transported to the liver, where it is converted back to glucose. - The liver then bears the metabolic cost of **gluconeogenesis**, allowing the muscle to continue glycolysis and ATP production. *Cori's cycle can not be sustained indefinitely because it is energetically unfavourable* - This is true because the cycle involves a net consumption of ATP. **Six ATP equivalents** are used in gluconeogenesis in the liver to convert two molecules of lactate to one molecule of glucose. - In contrast, the glycolysis that produces the two lactate molecules in muscle yields only **two net ATP**. This energy deficit makes prolonged reliance on the Cori cycle unsustainable. *Cori's cycle is linked to glycogen synthesis in muscle* - This is true because the **glucose produced by the liver** via gluconeogenesis (from lactate) is released into the bloodstream. - This glucose can then be taken up by muscles and other tissues to **replenish glycogen stores** or be used for energy.

Q: Which among the following glucose transporters is present in beta cells?

GLUT2. ***GLUT2*** - **GLUT2** is a **low-affinity** glucose transporter predominantly found in pancreatic **beta cells**, liver, kidneys, and intestines. - Its low affinity allows beta cells to accurately sense high blood glucose levels, triggering **insulin release**. *GLUT1* - **GLUT1** is a widely distributed glucose transporter found in nearly all mammalian cells, including **red blood cells** and cells of the **blood-brain barrier**. - It exhibits **high affinity** for glucose, responsible for basal glucose uptake. *GLUT3* - **GLUT3** is a high-affinity glucose transporter primarily found in **neurons** and the **placenta**. - Its high affinity ensures a constant glucose supply to these metabolically demanding tissues, even at low blood glucose concentrations. *GLUT4* - **GLUT4** is an **insulin-sensitive** glucose transporter found in **adipose tissue** and **striated muscle** (skeletal and cardiac). - Its translocation to the cell surface from intracellular vesicles is stimulated by insulin, promoting glucose uptake into these tissues.

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: Which of the following is NOT a key enzyme of gluconeogenesis?

Pyruvate kinase. ***Pyruvate kinase*** - **Pyruvate kinase** is a key regulatory enzyme in **glycolysis**, catalyzing the irreversible conversion of phosphoenolpyruvate (PEP) to pyruvate. - Since gluconeogenesis is essentially the reversal of glycolysis, pyruvate kinase is **NOT involved in gluconeogenesis**. Instead, this glycolytic step must be bypassed by different enzymes. - This is the correct answer as it is NOT a key enzyme of gluconeogenesis. *Pyruvate carboxylase* - This **IS a key enzyme of gluconeogenesis**, converting **pyruvate to oxaloacetate** in the mitochondria, thereby bypassing the pyruvate kinase step of glycolysis. - It uses **biotin** as a coenzyme and requires ATP. - This is one of the four key regulatory enzymes unique to gluconeogenesis. *PEP carboxykinase* - This **IS a key enzyme of gluconeogenesis**, converting **oxaloacetate to phosphoenolpyruvate (PEP)**, bypassing the irreversible pyruvate kinase step of glycolysis. - This enzyme is located in both the cytoplasm and mitochondria, depending on the species (cytoplasmic in humans). - This is one of the four key regulatory enzymes unique to gluconeogenesis. *Glucose-6-phosphatase* - This **IS a key enzyme of gluconeogenesis**, catalyzing the final step by dephosphorylating **glucose-6-phosphate to free glucose**, enabling its release from the liver into the bloodstream. - This enzyme bypasses the irreversible hexokinase/glucokinase step of glycolysis. - Located in the endoplasmic reticulum, it is exclusively found in gluconeogenic tissues like the liver and kidney.

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.

Question 1

Which enzyme deficiency is associated with increased susceptibility to infections, including E. coli?

Accepted Answer

Glucose-6-phosphate dehydrogenase (G6PD) deficiency

Answer

Lactase deficiency

Answer

Pepsin deficiency

Answer

Trypsin deficiency

Question 2

Neonatal hypoglycemia that does not respond to treatment with counter-regulatory hormones is diagnostic of which condition?

Accepted Answer

Von Gierke's disease

Answer

Hereditary fructose intolerance

Answer

Cori's disease (Glycogen storage disease type III)

Answer

Anderson's disease (Glycogen storage disease type IV)

Question 3

Gas released from oligosaccharide metabolism by intestinal bacteria is

Accepted Answer

Carbon dioxide

Answer

Sulfur dioxide

Answer

Nitric oxide

Answer

Methane

Question 4

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 5

In glycogen synthesis the active form of glucose used is-

Accepted Answer

UDP glucose

Answer

Glucose 6 phosphate

Answer

GTP glucose

Answer

Glucose 1 phosphate

Question 6

All of the following are true about lactate utilization in the liver except -

Accepted Answer

Total net number of ATP formed because of Cori's cycle is 4

Answer

Cori's cycle shifts the metabolic burden from muscle to liver

Answer

Cori's cycle can not be sustained indefinitely because it is energetically unfavourable

Answer

Cori's cycle is linked to glycogen synthesis in muscle

Question 7

Which among the following glucose transporters is present in beta cells?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 8

Which glucose transporter is primarily affected in diabetes mellitus?

Accepted Answer

GLUT-4

Answer

GLUT-2

Answer

GLUT-5

Answer

SGLT-2

Question 9

Which of the following is NOT a key enzyme of gluconeogenesis?

Accepted Answer

Pyruvate kinase

Answer

Pyruvate carboxylase

Answer

PEP carboxykinase

Answer

Glucose-6-phosphatase

Question 10

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

Accepted Answer

Glycated crystallins

Answer

Calcified crystallins

Answer

Glycated fibrillins

Answer

Calcified fibrillins

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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