Carbohydrate Metabolism Practice Questions

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: 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.

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 of the following substances does not inhibit glycolysis?

Fluoroacetate. ***Fluoroacetate*** - **Fluoroacetate** is not a direct inhibitor of glycolysis. Instead, it is metabolized to **fluorocitrate**, which then acts as an inhibitor of **aconitase** in the **Krebs cycle (TCA cycle)**, thereby affecting cellular respiration at a later stage. - Its primary role in metabolic inhibition is within the **mitochondria**, impacting energy production via the TCA cycle rather than the glycolytic pathway. *Fluoride* - **Fluoride** is a known inhibitor of **enolase**, an enzyme in the penultimate step of glycolysis. - It forms a complex with **magnesium** and **phosphate** to block the active site of enolase, preventing the conversion of 2-phosphoglycerate to phosphoenolpyruvate. *Arsenite* - **Arsenite** inhibits glycolysis by targeting enzymes containing **sulfhydryl (–SH) groups**, particularly **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**, a critical enzyme in the glycolytic pathway. - It also inhibits the **pyruvate dehydrogenase complex** (linking glycolysis to the TCA cycle) and TCA cycle enzymes like **α-ketoglutarate dehydrogenase**, thereby affecting multiple stages of cellular respiration. *Iodoacetate* - **Iodoacetate** is a potent inhibitor of the enzyme **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**. - It specifically alkylates the **cysteine residue** at the active site of GAPDH, preventing the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, thereby blocking glycolysis.

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: 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 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: 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: 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: Which enzyme is active when the insulin:glucagon ratio is low?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - A low **insulin:glucagon ratio** occurs during **fasting, starvation, or stress** (catabolic state). - In this metabolic state, **glucose-6-phosphatase** is **highly active** in the **liver** (and kidneys), catalyzing the final step of both **gluconeogenesis** and **glycogenolysis**. - It converts **glucose-6-phosphate** to **free glucose**, which is released into the bloodstream to maintain **blood glucose levels**. - This enzyme is stimulated by **glucagon** and inhibited by **insulin**. *Fructokinase* - This enzyme phosphorylates **fructose** to **fructose-1-phosphate** in the liver. - It is regulated by **fructose availability**, not by the **insulin:glucagon ratio**. - During a low insulin:glucagon state, **gluconeogenic** and **glycogenolytic** pathways are favored, not fructose metabolism. *Lactate dehydrogenase* - **LDH** interconverts **pyruvate** and **lactate** and functions in both **glycolysis** and **gluconeogenesis**. - While it plays a role in various metabolic states, it is **not specifically activated** by a low insulin:glucagon ratio. - It operates constitutively based on substrate availability rather than hormonal regulation. *Acetyl-CoA carboxylase* - This is the **rate-limiting enzyme** of **fatty acid synthesis** (lipogenesis). - It is **activated by insulin** (fed state) and **inhibited by glucagon** (fasted state). - When the insulin:glucagon ratio is **low**, this enzyme is **phosphorylated and INACTIVE**, shutting down fatty acid synthesis.

Question 1

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 2

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

Question 3

Glycogen synthase is activated by?

Accepted Answer

Insulin

Answer

Glucagon

Answer

AMP

Answer

Epinephrine

Question 4

Which of the following substances does not inhibit glycolysis?

Accepted Answer

Fluoroacetate

Answer

Fluoride

Answer

Arsenite

Answer

Iodoacetate

Question 5

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 6

GLUT2 plays a functionally important role mainly in?

Accepted Answer

Pancreatic beta cells

Answer

Liver

Answer

Skeletal muscle tissue

Answer

Kidney

Question 7

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 8

Enzymes of glycolysis are found in:

Accepted Answer

Cytosol

Answer

Cell membrane

Answer

Mitochondria

Answer

Ribosomes

Question 9

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

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Isomerase

Answer

Enolase

Question 10

Which enzyme is active when the insulin:glucagon ratio is low?

Accepted Answer

Glucose-6-phosphatase

Answer

Fructokinase

Answer

Lactate dehydrogenase

Answer

Acetyl-CoA carboxylase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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