Carbohydrate Metabolism Practice Questions

Q: Which glycogen storage disease also presents as a lysosomal storage disease?

Pompe's disease. ***Pompe's disease*** - Also known as **glycogen storage disease type II**, it is caused by a deficiency of **acid alpha-glucosidase (GAA)**, a *lysosomal enzyme*. - This deficiency leads to the accumulation of **glycogen in lysosomes**, particularly affecting muscle tissue, thereby earning its classification as both a glycogen storage disease and a lysosomal storage disease. *Von Gierke's disease* - This is **glycogen storage disease type I** and is due to a deficiency in **glucose-6-phosphatase**. - It primarily affects the **liver and kidneys**, causing severe **hypoglycemia** and **lactic acidosis**, but it is not classified as a lysosomal storage disease. *McArdle's disease* - This is **glycogen storage disease type V**, caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**. - It manifests as **exercise intolerance** and muscle pain, but it does not involve lysosomal enzyme defects or glycogen accumulation in lysosomes. *Andersen's disease* - This is **glycogen storage disease type IV**, caused by a deficiency in the **glycogen branching enzyme**. - It leads to the formation of **abnormal glycogen structures**, primarily affecting the liver and causing early liver failure, but it is not a lysosomal storage disorder.

Q: Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?

G6PD. ***G6PD*** - **Glucose-6-phosphate dehydrogenase (G6PD)** catalyzes the first committed step in the pentose phosphate pathway, converting **glucose-6-phosphate** to **6-phosphogluconolactone**. - This reaction involves the reduction of **NADP+ to NADPH**, making G6PD the primary enzyme for NADPH production in this pathway. *APDH* - **APDH (adenosine phosphosulfate reductase)** is involved in sulfur metabolism and has no direct role in the pentose phosphate pathway or NADPH production. - This enzyme primarily functions in prokaryotes for the **reduction of APS (adenosine 5'-phosphosulfate)**. *α-keto glutarate dehydrogenases* - **Alpha-ketoglutarate dehydrogenase** is a mitochondrial enzyme part of the **Krebs cycle**, converting **alpha-ketoglutarate to succinyl-CoA**. - This enzyme is crucial for ATP production and generates **NADH**, not NADPH, in its reaction. *None of the options* - This option is incorrect because **G6PD** is indeed the primary enzyme responsible for NADPH generation in the pentose phosphate pathway.

Q: Glucose is converted to glucuronate by which process?

Oxidation of the terminal alcohol group. ***Oxidation of the terminal alcohol group*** - **Glucuronate** is formed by the **oxidation of the C-6 carbon** (the terminal primary alcohol group) of glucose. - This process is crucial for the detoxification of various substances in the body, as glucuronate is a key component in **glucuronidation reactions**. *Oxidation of aldehyde group* - Oxidation of the **aldehyde group (C-1)** of glucose typically forms **gluconic acid**, not glucuronate. - Gluconate is derived from the oxidation of the first carbon, while glucuronate is derived from the oxidation of the last carbon. *Oxidation of both* - If both the aldehyde group (C-1) and the terminal alcohol group (C-6) of glucose were oxidized, it would result in the formation of **glucaric acid** (saccharic acid), not glucuronate. - Glucaric acid has two carboxyl groups, one at each end of the molecule. *None of the options* - This option is incorrect because the specific biochemical pathway for glucuronate formation involves the oxidation of the terminal alcohol group. - The conversion of glucose to glucuronate is a well-established metabolic process.

Q: ATP is consumed at which of the following steps of glycolysis?

Hexokinase. ***Hexokinase*** - This enzyme catalyzes the **first step of glycolysis**, the phosphorylation of glucose to **glucose-6-phosphate**, which requires the consumption of one molecule of **ATP**. - ATP is hydrolyzed to **ADP**, providing the necessary phosphate group and energy for this irreversible reaction. - Note: Hexokinase is one of **two ATP-consuming steps** in glycolysis (the other being phosphofructokinase in step 3). *Pyruvate kinase* - This enzyme catalyzes the **final step of glycolysis**, converting **phosphoenolpyruvate (PEP)** to pyruvate. - This reaction involves the **production of ATP** from ADP, not its consumption, as it's one of the substrate-level phosphorylation steps. *Isomerase* - Isomerase enzymes, like phosphoglucose isomerase, convert one isomer to another (e.g., glucose-6-phosphate to fructose-6-phosphate). - These reactions generally involve an **internal rearrangement of atoms** and do not directly consume or produce ATP. *Enolase* - Enolase catalyzes the reversible conversion of **2-phosphoglycerate to phosphoenolpyruvate (PEP)**, releasing a molecule of water. - This step occurs before the ATP-generating step catalyzed by pyruvate kinase and **does not consume or produce ATP**.

Q: Mutation in GLUT-2 causes which syndrome?

Fanconi-Bickel syndrome. ***Fanconi-Bickel syndrome*** - This syndrome is caused by a **mutation in the GLUT-2 gene**, leading to dysfunctional glucose transport in the liver, kidneys, and intestines. - Key features include **hepatorenal glycogen accumulation**, **renal tubulopathy** (Fanconi syndrome), and **impaired glucose and galactose utilization**. *Dandy-Walker syndrome* - This is a **congenital brain malformation** involving the cerebellum and fourth ventricle. - It is often associated with hydrocephalus, but not directly linked to glucose transporter defects. *Beckwith-Wiedemann syndrome* - This is an **overgrowth disorder** characterized by a high risk of childhood cancer and congenital anomalies. - It is primarily caused by genetic abnormalities on **chromosome 11p15.5** and is unrelated to GLUT-2 mutations. *Menke's disease* - This is a rare X-linked recessive disorder of **copper metabolism**, leading to severe neurological degeneration. - It results from mutations in the **ATP7A gene**, which encodes a copper-transporting ATPase.

Q: Glycogen synthase is activated by?

Insulin. **Insulin** - Insulin activates **glycogen synthase** through a signaling cascade that dephosphorylates the enzyme, shifting it to its active form (glycogen synthase a). - This activation promotes **glycogen synthesis** in the liver and muscles, lowering blood glucose levels. *Glucagon* - **Glucagon** primarily acts to increase blood glucose levels by activating **glycogen phosphorylase** and inhibiting glycogen synthase. - It promotes the breakdown of glycogen (glycogenolysis) rather than its synthesis. *Epinephrine* - **Epinephrine** (adrenaline) also promotes **glycogenolysis** (glycogen breakdown) by activating glycogen phosphorylase. - Its main role is to provide rapid energy during stress, not to store glucose as glycogen. *AMP* - **AMP** (adenosine monophosphate) is a key allosteric activator of **AMP-activated protein kinase (AMPK)**, which phosphorylates and inactivates glycogen synthase. - High AMP levels signal a low energy state, prompting ATP-generating pathways like glycogenolysis, not glycogen synthesis.

Q: Which hormone is known to repress the biosynthesis of the enzyme pyruvate carboxylase?

Insulin. ***Insulin*** - **Insulin** is an anabolic hormone that promotes glucose utilization and opposes **gluconeogenesis**. - While insulin does inhibit hepatic glucose production, it primarily acts by **repressing PEPCK (phosphoenolpyruvate carboxykinase)**, the rate-limiting enzyme of gluconeogenesis, rather than directly repressing pyruvate carboxylase biosynthesis. - **Note:** Modern biochemistry emphasizes that insulin's main transcriptional target in gluconeogenesis is **PEPCK**, not pyruvate carboxylase. However, this was the expected answer for **NEET-2012**, reflecting the understanding at that time. - Insulin also promotes dephosphorylation and inactivation of gluconeogenic enzymes and enhances glucose uptake and glycolysis. *Glucagon* - **Glucagon** is a catabolic hormone that **activates** enzymes involved in **gluconeogenesis** and glycogenolysis to raise blood glucose levels. - It would **increase**, not repress, the biosynthesis and activity of gluconeogenic enzymes including **pyruvate carboxylase**. *Cortisol* - **Cortisol** is a glucocorticoid hormone that **stimulates gluconeogenesis** in the liver as part of the stress response. - It typically **upregulates** the synthesis and activity of gluconeogenic enzymes like **pyruvate carboxylase** and **PEPCK**. *Growth hormone* - **Growth hormone** generally **increases insulin resistance** and can have a **diabetogenic effect**, promoting glucose production rather than repressing gluconeogenic enzymes. - It does not directly repress gluconeogenic enzyme biosynthesis; its metabolic effects favor lipolysis and protein synthesis.

Q: Which of the following is an amino sugar formed from fructose-6-phosphate?

Glucosamine-6-phosphate. ***Glucosamine-6-phosphate*** - This amino sugar is directly synthesized from **fructose-6-phosphate** via a transamidation reaction, where an amino group replaces a hydroxyl group. - It is a key intermediate in the biosynthesis of other **amino sugars** and **glycosaminoglycans**. *N-acetylglucosamine-6-phosphate* - This is formed from **glucosamine-6-phosphate** by the addition of an **acetyl group**, making it a subsequent product, not the initial amino sugar from fructose-6-phosphate. - The N-acetylation step is crucial for its role in cellular signaling and structural components. *Galactosamine-6-phosphate* - While an amino sugar, **galactosamine-6-phosphate** is derived from UDP-N-acetylglucosamine, not directly from fructose-6-phosphate. - Its formation involves an **epimerization** step of an existing N-acetylglucosamine structure. *UDP-N-acetylglucosamine* - This is an **activated form** of N-acetylglucosamine, formed by the addition of UTP to N-acetylglucosamine-1-phosphate. - It serves as a precursor for the synthesis of complex **carbohydrates** and glycoproteins, far downstream from fructose-6-phosphate.

Q: What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?

2 ATP, 2 NADH. **2 ATP, 2 NADH** - Glycolysis has a net yield of **2 molecules of ATP** because 4 ATP molecules are produced, but 2 ATP molecules are consumed during the initial energy investment phase. - **2 molecules of NADH** are also produced during the energy generation phase when glyceraldehyde-3-phosphate is oxidized. *4 ATP, 2 NADH* - While 4 ATP molecules are indeed produced during glycolysis, this option does not account for the **2 ATP molecules consumed** in the initial steps, leading to an incorrect net value. - The production of **2 NADH** is correct, but the ATP count is the gross rather than the net. *4 ATP, 4 NADH* - This option overstates the production of both ATP and NADH. While **4 ATP are produced (gross)**, the net is 2 ATP. - Only **2 NADH** molecules are formed per glucose molecule in glycolysis, not 4. *2 ATP, 4 NADH* - This option accurately reflects the **net ATP yield of 2 molecules**. - However, it exaggerates the production of NADH, as only **2 molecules of NADH** are formed during glycolysis, not 4.

Question 1

Which glycogen storage disease also presents as a lysosomal storage disease?

Accepted Answer

Pompe's disease

Answer

Von Gierke's disease

Answer

McArdle's disease

Answer

Andersen's disease

Question 2

Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?

Accepted Answer

G6PD

Answer

APDH

Answer

α-keto glutarate dehydrogenases

Answer

None of the options

Question 3

Glucose is converted to glucuronate by which process?

Accepted Answer

Oxidation of the terminal alcohol group

Answer

Oxidation of aldehyde group

Answer

Oxidation of both

Answer

None of the options

Question 4

ATP is consumed at which of the following steps of glycolysis?

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Isomerase

Answer

Enolase

Question 5

Mutation in GLUT-2 causes which syndrome?

Accepted Answer

Fanconi-Bickel syndrome

Answer

Dandy walker syndrome

Answer

Beckwith-Wiedemann syndrome

Answer

Menke's disease

Question 6

Glycogen synthase is activated by?

Accepted Answer

Insulin

Answer

Glucagon

Answer

AMP

Answer

Epinephrine

Question 7

Which hormone is known to repress the biosynthesis of the enzyme pyruvate carboxylase?

Accepted Answer

Insulin

Answer

Cortisol

Answer

Glucagon

Answer

Growth hormone

Question 8

Which one of the following statements concerning gluconeogenesis is correct?

Accepted Answer

It is important in maintaining blood glucose during the normal overnight fast.

Answer

It occurs primarily in the liver.

Answer

It is stimulated by elevated levels of acetyl CoA.

Answer

It is primarily inhibited by insulin.

Question 9

Which of the following is an amino sugar formed from fructose-6-phosphate?

Accepted Answer

Glucosamine-6-phosphate

Answer

N-acetylglucosamine-6-phosphate

Answer

Galactosamine-6-phosphate

Answer

UDP-N-acetylglucosamine

Question 10

What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?

Accepted Answer

2 ATP, 2 NADH

Answer

4 ATP, 2 NADH

Answer

4 ATP, 4 NADH

Answer

2 ATP, 4 NADH

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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