Carbohydrate Metabolism Practice Questions

Q: Enzymes of glycolysis are found in:

Cytosol. ***Cytosol*** - Glycolysis is a metabolic pathway that occurs in the **cytosol** of cells. - All the enzymes required for the conversion of glucose to pyruvate are freely dissolved in the **cytoplasm**. *Cell membrane* - The cell membrane is primarily involved in **regulating the passage of substances** into and out of the cell, as well as cell signaling. - Glycolytic enzymes are not associated with the cell membrane. *Mitochondria* - Mitochondria are the primary site of **oxidative phosphorylation** and the **citric acid cycle**, not glycolysis. - While pyruvate (the end product of glycolysis) moves into the mitochondria for further metabolism, the initial glycolytic steps do not occur there. *Ribosomes* - Ribosomes are responsible for **protein synthesis** (translation). - They do not contain enzymes for metabolic pathways like glycolysis.

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 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: 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: GLUT2 plays a functionally important role mainly in?

Pancreatic beta cells. ***Pancreatic beta cells*** - **GLUT2** acts as a **glucose sensor** in pancreatic beta cells, which is its **most functionally critical role** in the body. - Its high Km (~15-20 mM, low affinity) ensures that glucose uptake is **proportional to blood glucose concentration**, enabling the beta cells to accurately sense glucose levels and secrete insulin accordingly. - This glucose-sensing mechanism is **essential for maintaining glycemic homeostasis** and makes GLUT2's role in beta cells uniquely important compared to its presence in other tissues. - Without functional GLUT2 in beta cells, the body cannot properly regulate insulin secretion in response to changing glucose levels. *Liver* - While **GLUT2** is abundantly expressed in hepatocytes and allows for bidirectional glucose transport (both uptake and release), its role here is **facilitative rather than regulatory**. - The liver has multiple other glucose-regulating mechanisms (glucokinase, glucose-6-phosphatase, glycogen metabolism). - GLUT2's function in the liver is important but not as uniquely critical as its glucose-sensing role in beta cells. *Skeletal muscle tissue* - **Skeletal muscle** primarily utilizes **GLUT4** (not GLUT2) for insulin-dependent glucose uptake. - **GLUT2** is not significantly expressed in skeletal muscle tissue. - This makes GLUT2 functionally unimportant in skeletal muscle. *Kidney* - The **kidney** expresses **GLUT2** in proximal tubule cells where it participates in glucose reabsorption from the glomerular filtrate. - However, this role is **secondary to SGLT2** (sodium-glucose cotransporter 2), which performs the primary active reabsorption. - GLUT2's function here is important but not the **"mainly"** critical role compared to its glucose-sensing function in beta cells.

Q: Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?

Glucose-6-phosphate. ***Glucose-6-phosphate*** - In **glycogenolysis**, **glycogen phosphorylase** breaks down glycogen into **glucose-1-phosphate**, which is then converted into **glucose-6-phosphate** by **phosphoglucomutase**. - In **glycolysis**, **glucose-6-phosphate** is isomerized to **fructose-6-phosphate** by **phosphoglucose isomerase**, committing it to the glycolytic pathway. - In **gluconeogenesis**, **glucose-6-phosphate** is the final product formed from other precursors; it can then be dephosphorylated to free glucose by **glucose-6-phosphatase**. *Galactose-1-phosphate* - This is an intermediate specifically in **galactose metabolism**, not directly involved in the central common pathways of glycogenolysis, glycolysis, or gluconeogenesis. - It is converted to **glucose-1-phosphate** via the **Leloir pathway** (involving **galactose-1-phosphate uridylyltransferase**), which can then enter glycogen metabolism. *Uridine diphosphoglucose* - **UDP-glucose** is crucial for **glycogen synthesis** (**glycogenesis**), serving as the activated glucose donor. - It is not directly a metabolite in the catabolic process of glycogenolysis, nor is it a direct intermediate in glycolysis or gluconeogenesis. *Fructose-6-phosphate* - **Fructose-6-phosphate** is a key intermediate in **glycolysis** and **gluconeogenesis**, specifically downstream from **glucose-6-phosphate**. - However, it is not directly produced from glycogenolysis; **glucose-6-phosphate** is the direct link between glycogenolysis and glycolysis.

Q: Repetitive chains of glucosamine with uronic acid are seen in ?

Heparan sulphate. ***Heparan sulphate*** - Heparan sulfate is a **linear polysaccharide** composed of repeating disaccharide units of either **glucosamine** and **glucuronic acid** or **glucosamine** and **iduronic acid** (a uronic acid). - It plays crucial roles in various biological processes, including cell adhesion, cell growth, and coagulation, primarily due to its ability to bind to a wide range of proteins. *N-acetylneuraminic acid* - **N-acetylneuraminic acid** is a type of **sialic acid**, a nine-carbon sugar acid, and is not a repeating chain of glucosamine and uronic acid. - It is often found at the **non-reducing ends of glycan chains** on cell surfaces and in secreted glycoproteins, contributing to cell recognition and signaling. *Galactosaminoglycan* - This term is a general descriptor for a group of glycosaminoglycans that contain **N-acetylgalactosamine** as one of their repeating units, such as **chondroitin sulfate** and **dermatan sulfate**. - While they are repeating disaccharide units, they feature **galactosamine** (or its N-acetylated form) instead of glucosamine and are also linked to uronic acids. *None of the options* - This option is incorrect because **heparan sulfate** accurately describes a polysaccharide with repeating units of glucosamine and uronic acid.

Q: Which carbohydrate is primarily metabolized by Aldolase-B?

Fructose. ***Fructose*** - **Aldolase B** is a key enzyme in the liver responsible for the metabolism of **fructose**, specifically cleaving **fructose-1-phosphate** into **dihydroxyacetone phosphate** and **glyceraldehyde**. - A deficiency in **Aldolase B** leads to **hereditary fructose intolerance**, causing an accumulation of **fructose-1-phosphate** after fructose ingestion. *Galactose* - **Galactose** is primarily metabolized by enzymes in the **Leloir pathway**, including **galactokinase** and **galactose-1-phosphate uridylyltransferase**. - **Aldolase B** plays no significant role in the metabolism of galactose. *Sucrose* - **Sucrose** is a disaccharide composed of **glucose** and **fructose**. - It is first broken down by **sucrase** in the small intestine into its constituent monosaccharides before they are metabolized further. *None of the options* - This option is incorrect because **fructose** is indeed a carbohydrate primarily metabolized by Aldolase-B. - The enzyme's specific role in fructose metabolism is well-established.

Question 1

Enzymes of glycolysis are found in:

Accepted Answer

Cytosol

Answer

Cell membrane

Answer

Mitochondria

Answer

Ribosomes

Question 2

Glycogen synthase is activated by?

Accepted Answer

Insulin

Answer

Glucagon

Answer

AMP

Answer

Epinephrine

Question 3

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

Accepted Answer

Insulin

Answer

Cortisol

Answer

Glucagon

Answer

Growth hormone

Question 4

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 5

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 6

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

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Isomerase

Answer

Enolase

Question 7

GLUT2 plays a functionally important role mainly in?

Accepted Answer

Pancreatic beta cells

Answer

Liver

Answer

Skeletal muscle tissue

Answer

Kidney

Question 8

Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?

Accepted Answer

Glucose-6-phosphate

Answer

Uridine diphosphoglucose

Answer

Fructose-6-phosphate

Answer

Galactose-1-phosphate

Question 9

Repetitive chains of glucosamine with uronic acid are seen in ?

Accepted Answer

Heparan sulphate

Answer

None of the options

Answer

N-acetylneuraminic acid

Answer

Galactosaminoglycan

Question 10

Which carbohydrate is primarily metabolized by Aldolase-B?

Accepted Answer

Fructose

Answer

Galactose

Answer

Sucrose

Answer

None of the options

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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