Carbohydrate Metabolism Practice Questions

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: 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: 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: 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: 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: 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.

Question 1

Which of the following statements BEST describes the net ATP production in glycolysis?

Accepted Answer

Glycolysis produces a net gain of 2 ATP per glucose molecule

Answer

Glycolysis produces 2 molecules of pyruvate

Answer

Hexokinase consumes ATP during glycolysis

Answer

Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into two three-carbon molecules

Question 2

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 3

GLUT2 plays a functionally important role mainly in?

Accepted Answer

Pancreatic beta cells

Answer

Liver

Answer

Skeletal muscle tissue

Answer

Kidney

Question 4

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 5

Glycogen synthase is activated by?

Accepted Answer

Insulin

Answer

Glucagon

Answer

AMP

Answer

Epinephrine

Question 6

Enzymes of glycolysis are found in:

Accepted Answer

Cytosol

Answer

Cell membrane

Answer

Mitochondria

Answer

Ribosomes

Question 7

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 8

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 9

Which glycosaminoglycan is present in the cornea?

Accepted Answer

Keratan sulfate

Answer

Dermatan sulfate

Answer

Chondroitin sulfate

Answer

Hyaluronic acid

Question 10

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

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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