Carbohydrate Metabolism Practice Questions

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: Which tissue cannot convert glucose 6-phosphate to free glucose due to lack of glucose-6-phosphatase?

Muscle. ***Muscle*** - Muscle tissue lacks the enzyme **glucose-6-phosphatase**, which is essential for hydrolyzing glucose 6-phosphate back to **free glucose**. - Therefore, glucose 6-phosphate in muscle is primarily used for **glycolysis** (energy production) or stored as glycogen for local use. *Liver* - The liver contains **glucose-6-phosphatase**, allowing it to convert **glucose 6-phosphate** to **free glucose**. - This capability is crucial for maintaining **blood glucose homeostasis** and releasing glucose into circulation. *Adipose tissue* - Adipose tissue, like muscle, **lacks glucose-6-phosphatase** and cannot convert glucose 6-phosphate back to free glucose. - Glucose 6-phosphate in adipose tissue is primarily channeled into **fatty acid synthesis** and storage. *Kidney* - The kidney, particularly the renal cortex, possesses **glucose-6-phosphatase** and can convert glucose 6-phosphate to **free glucose**. - This contributes to **gluconeogenesis** and release of glucose into the blood, especially during fasting.

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: 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.

Q: Gluconeogenesis in the fasting state is indicated by:

Pyruvate Carboxylase activation by Acetyl CoA. ***Pyruvate Carboxylase activation by Acetyl CoA*** - In the **fasting state**, Acetyl CoA activates **pyruvate carboxylase**, shunting pyruvate away from glycolysis towards gluconeogenesis. - This activation is crucial for the synthesis of **oxaloacetate** from pyruvate, a key step in glucose production. *Citrate activation by acetyl-CoA carboxylase* - **Citrate** is an allosteric activator of **acetyl-CoA carboxylase**, which is involved in **fatty acid synthesis**, not gluconeogenesis. - This enzyme's activity is favored during states of **energy surplus**, not fasting. *Fructose 1,6 bisphosphate activates Pyruvate Kinase* - **Fructose 1,6-bisphosphate** activates **pyruvate kinase** as part of **glycolysis**, promoting glucose breakdown. - This regulatory mechanism operates during **fed states** to increase glucose utilization, which is opposite to the fasting state. *Fructose 2,6 bisphosphate activates PFK-1* - **Fructose 2,6-bisphosphate** activates **phosphofructokinase-1 (PFK-1)**, a key regulatory enzyme in **glycolysis**. - High levels of fructose 2,6-bisphosphate promote **glucose breakdown**, which is inhibited during the fasting state to conserve glucose.

Q: Which glycosaminoglycan is present in the cornea?

Keratan sulfate. ***Keratan sulfate*** - **Keratan sulfate** is a prominent **glycosaminoglycan (GAG)** found in the corneal stroma, playing a crucial role in maintaining corneal transparency. - Its unique structure and interaction with collagen fibrils contribute to the precise spacing and organization of the **corneal lamellae**. *Dermatan sulfate* - **Dermatan sulfate** is primarily found in the skin, blood vessels, and heart valves, but it is not a major GAG component of the cornea. - It is involved in regulating collagen fibril assembly and stability in these specific tissues. *Chondroitin sulfate* - **Chondroitin sulfate** is abundant in cartilage, bone, and connective tissues, providing resistance to compression. - While trace amounts might be present, it is not the primary GAG responsible for the unique properties of the cornea. *Hyaluronic acid* - **Hyaluronic acid**, a non-sulfated GAG, is found in the vitreous humor and synovial fluid, contributing to lubrication and hydration. - It is generally absent or present in very small quantities in the adult cornea under normal physiological conditions.

Q: Facilitative glucose transporters (GLUTs) comprise a large family. Which of the following is a glucose transporter in myocytes?

GLUT 4. ***GLUT 4*** - **GLUT 4** is the primary **insulin-dependent glucose transporter** found in **adipose tissue** and **striated muscle (skeletal and cardiac muscle)**, including myocytes. - Its translocation to the cell surface is stimulated by **insulin** and muscle contraction, facilitating glucose uptake into these cells. *GLUT 1* - **GLUT 1** is ubiquitously expressed in nearly all mammalian cells, providing a **basal level of glucose uptake**. - It is particularly abundant in **red blood cells** and the **blood-brain barrier**. *GLUT 2* - **GLUT 2** is a **low-affinity, high-capacity glucose transporter** found in the **liver, pancreatic beta cells, kidney, and small intestine**. - It plays a crucial role in **glucose sensing** and the release of insulin. *GLUT 3* - **GLUT 3** is primarily found in **neurons** and the **placenta**, where it is responsible for **high-affinity glucose uptake**. - It ensures a constant supply of glucose to these tissues, even at low blood glucose concentrations.

Q: Source of ATP in RBCs is

Glucose. ***Glucose*** - Red blood cells (RBCs) lack mitochondria, so they cannot perform **aerobic respiration**, **fatty acid oxidation**, or utilize **ketone bodies** for ATP production. - Their sole source of ATP is through **anaerobic glycolysis**, which metabolizes glucose to produce a small amount of ATP. *Fatty acid* - Fatty acid oxidation, or **beta-oxidation**, occurs in the mitochondria. - Since mature **RBCs lack mitochondria**, they cannot metabolize fatty acids to produce ATP. *Amino acid* - Amino acid metabolism for energy primarily involves the **Krebs cycle** and **oxidative phosphorylation**, which also take place in the mitochondria. - **RBCs lack the necessary enzymatic machinery** and organelles for this process. *Ketone body* - Ketone bodies are metabolized for energy in mitochondria, particularly in tissues like the brain and muscle, through the **Krebs cycle**. - **RBCs do not have mitochondria** and thus cannot utilize ketone bodies as an energy source.

Q: In glycolysis, which of the following enzymes is not involved?

Pyruvate dehydrogenase. ***Pyruvate dehydrogenase*** - **Pyruvate dehydrogenase** is a mitochondrial enzyme complex that converts **pyruvate** to **acetyl-CoA** in the link reaction, which occurs after glycolysis and prepares for the citric acid cycle. - It is not directly involved in the ten-step glycolytic pathway itself, which converts glucose to pyruvate. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is a key regulatory enzyme in glycolysis, catalyzing the phosphorylation of **fructose-6-phosphate** to **fructose-1,6-bisphosphate**. - This step is often considered the **rate-limiting step** of glycolysis. *Glucokinase* - **Glucokinase**, located primarily in the liver and pancreatic beta cells, phosphorylates glucose to **glucose-6-phosphate** in the first step of glycolysis. - It has a high **Km** (low affinity) for glucose, allowing it to respond to high glucose concentrations. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, transferring a phosphate group from **phosphoenolpyruvate (PEP)** to ADP to form **ATP** and **pyruvate**. - This is one of the **irreversible** steps in glycolysis and a point of regulation.

Question 1

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 2

Which tissue cannot convert glucose 6-phosphate to free glucose due to lack of glucose-6-phosphatase?

Accepted Answer

Muscle

Answer

Liver

Answer

Kidney

Answer

Adipose tissue

Question 3

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 4

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

Question 5

Gluconeogenesis in the fasting state is indicated by:

Accepted Answer

Pyruvate Carboxylase activation by Acetyl CoA

Answer

Citrate activation by acetyl-CoA carboxylase

Answer

Fructose 1,6 bisphosphate activates Pyruvate Kinase

Answer

Fructose 2,6 bisphosphate activates PFK-1

Question 6

What happens to glucose consumed in a soft drink by a 19-year-old athlete during a track event?

Accepted Answer

Muscles will primarily use glucose for immediate energy production.

Answer

Hexokinase helps convert glucose to energy quickly during exercise.

Answer

The glucose from the soda will be stored as glycogen in the liver.

Answer

Glucose will be used by both muscles and liver.

Question 7

Which glycosaminoglycan is present in the cornea?

Accepted Answer

Keratan sulfate

Answer

Dermatan sulfate

Answer

Chondroitin sulfate

Answer

Hyaluronic acid

Question 8

Facilitative glucose transporters (GLUTs) comprise a large family. Which of the following is a glucose transporter in myocytes?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 9

Source of ATP in RBCs is

Accepted Answer

Glucose

Answer

Fatty acid

Answer

Amino acid

Answer

Ketone body

Question 10

In glycolysis, which of the following enzymes is not involved?

Accepted Answer

Pyruvate dehydrogenase

Answer

Phosphofructokinase

Answer

Glucokinase

Answer

Pyruvate kinase

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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