Carbohydrate Metabolism Practice Questions

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: 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: 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: 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: 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: 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: 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.

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: 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 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.

Question 1

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 2

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

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

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

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 5

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 6

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 7

Which enzyme converts glucose to sorbitol?

Accepted Answer

Aldose reductase

Answer

Sorbitol dehydrogenase

Answer

Aldolase B

Answer

Glucose-6-phosphate dehydrogenase

Question 8

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 9

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 10

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

Accepted Answer

Phosphoglycerate kinase

Answer

Pyruvate kinase

Answer

Phosphofructokinase

Answer

Hexokinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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