Carbohydrate Metabolism Practice Questions

Q: What is the main enzyme involved in glycogen breakdown (glycogenolysis)?

Glycogen phosphorylase. ***Glycogen phosphorylase*** - This is the **rate-limiting and primary enzyme** for **glycogenolysis**, the breakdown of glycogen into glucose units. - It cleaves **α-1,4-glycosidic bonds** in glycogen, releasing **glucose-1-phosphate** units. - Regulated by both **allosteric mechanisms** and **hormonal control** (epinephrine, glucagon). - Works until it reaches 4 glucose residues from a branch point, where debranching enzyme takes over. *Glycogen synthase* - This is the main enzyme for **glycogenesis** (glycogen synthesis), not breakdown. - It catalyzes formation of α-1,4-glycosidic bonds to build glycogen chains. - This is the opposite direction of metabolism from what the question asks about. *Glucose-6-phosphatase* - This enzyme is involved in **gluconeogenesis** and the final step of converting **glucose-6-phosphate to free glucose**. - It is NOT directly involved in glycogen breakdown itself, but rather in the subsequent conversion pathway. - Found primarily in **liver and kidney** to release free glucose into blood. *Hexokinase* - This enzyme phosphorylates free glucose to **glucose-6-phosphate** (opposite direction). - It is involved in **glucose utilization**, not glycogen breakdown. - It traps glucose inside cells for metabolism or glycogen synthesis.

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: Increased uric acid levels are seen in which glycogen storage disease ?

Type I (Von Gierke's disease). ***Type I (Von Gierke's disease)*** - In **Von Gierke's disease**, the deficiency of **glucose-6-phosphatase** leads to accumulation of glucose-6-phosphate in hepatocytes. - **Hyperuricemia** occurs due to: 1. **Increased purine degradation** - Metabolic stress leads to accelerated ATP breakdown and increased uric acid production 2. **Decreased renal excretion** - Lactic acidosis (from G6P → pyruvate → lactate) competitively inhibits uric acid secretion in renal tubules 3. **Enhanced purine synthesis** - Increased availability of ribose-5-phosphate from pentose phosphate pathway - Classic triad: **Hepatomegaly, hypoglycemia, and lactic acidosis with hyperuricemia** *Type II (Pompe disease)* - Caused by a deficiency of **acid alpha-glucosidase** (acid maltase), leading to glycogen accumulation in **lysosomes**. - Primarily affects the **heart**, **muscles**, and **liver**, but does not cause hyperuricemia. *Type IV (Andersen disease)* - Results from a deficiency of **glycogen branching enzyme**, leading to the formation of abnormal glycogen with long, unbranched chains. - Primarily affects the **liver** and **spleen**, causing cirrhosis and hepatic failure, but not hyperuricemia. *Type III (Cori disease)* - Caused by a deficiency of **glycogen debranching enzyme** (amylo-1,6-glucosidase), leading to abnormal accumulation of glycogen with short outer branches. - Presents with hepatomegaly, hypoglycemia, and muscle weakness, but **hyperuricemia is not a characteristic feature**.

Q: At which step in glycolysis is NADH produced during the oxidation of glyceraldehyde-3-phosphate?

Glyceraldehyde-3-phosphate dehydrogenase. ***Glyceraldehyde-3-phosphate dehydrogenase*** - This enzyme catalyzes the oxidation and **phosphorylation** of glyceraldehyde-3-phosphate, producing **1,3-bisphosphoglycerate**. - During this reaction, **NAD+ is reduced to NADH**, which is a crucial step for energy production. *Pyruvate kinase* - This enzyme catalyzes the final step of glycolysis, transferring a phosphate group from **phosphoenolpyruvate** to ADP, forming ATP and pyruvate. - This step involves **substrate-level phosphorylation** for ATP production, not NADH. *Enolase* - This enzyme catalyzes the dehydration of **2-phosphoglycerate** to form **phosphoenolpyruvate (PEP)**. - This reaction involves the removal of a water molecule and does not produce NADH. *PFK-1* - **Phosphofructokinase-1 (PFK-1)** catalyzes the phosphorylation of fructose-6-phosphate to **fructose-1,6-bisphosphate**. - This is an ATP-consuming and a crucial regulatory step in glycolysis, but it does not involve NADH production.

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: 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: What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?

NADP. ***NADP*** - **NADP+** (nicotinamide adenine dinucleotide phosphate) acts as the **electron acceptor** in the **glucose-6-phosphate dehydrogenase (G6PD)** reaction, becoming **NADPH**. - **NADPH** is crucial for maintaining the **redox balance** in cells, particularly in red blood cells, by reducing **oxidative stress**. *NAD* - **NAD+** (nicotinamide adenine dinucleotide) is a primary cofactor for many **dehydrogenase reactions** in catabolic pathways like **glycolysis** and the **Krebs cycle**. - It primarily functions as an electron acceptor in pathways that generate **ATP**, distinct from the role of **NADPH** in reductive biosynthesis and antioxidant defense. *FAD* - **FAD** (flavin adenine dinucleotide) is a coenzyme derived from **riboflavin (vitamin B2)** that is involved in various redox reactions, often in the form of **flavoproteins**. - Enzymes like **succinate dehydrogenase** in the electron transport chain utilize **FAD** as an electron acceptor, which is not the case for G6PD. *FMN* - **FMN** (flavin mononucleotide) is another coenzyme derived from **riboflavin**, structurally similar to FAD but lacking the additional adenosine monophosphate. - It participates in electron transfer reactions, particularly within **complex I** of the **electron transport chain**, but is not a cofactor for G6PD.

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: Which of the following GAG is not sulfated?

Hyaluronic acid. ***Hyaluronic acid*** - **Hyaluronic acid** is unique among glycosaminoglycans (GAGs) because it is the only one that is **not sulfated**. - It also distinguishes itself by being the only GAG that does **not form proteoglycans** and is not synthesized in the Golgi apparatus. *Chondroitin sulfate* - **Chondroitin sulfate** is a sulfated glycosaminoglycan that is a major component of the **extracellular matrix**, particularly in cartilage. - Its sulfate groups contribute to its **negative charge**, allowing it to attract water and provide resistance to compression. *Dermatan sulfate* - **Dermatan sulfate** is another sulfated GAG, found predominantly in the skin, blood vessels, and heart valves. - It contains **sulfate groups**, which are crucial for its interactions with various proteins and its role in tissue structure. *Keratan sulfate* - **Keratan sulfate** is a sulfated GAG found in the cornea, cartilage, and bone. - It is distinct from other GAGs due to its **lack of uronic acid** and the presence of sulfate groups.

Question 1

What is the main enzyme involved in glycogen breakdown (glycogenolysis)?

Accepted Answer

Glycogen phosphorylase

Answer

Glycogen synthase

Answer

Glucose-6-phosphatase

Answer

Hexokinase

Question 2

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 3

Increased uric acid levels are seen in which glycogen storage disease ?

Accepted Answer

Type I (Von Gierke's disease)

Answer

Type II (Pompe disease)

Answer

Type IV (Andersen disease)

Answer

Type III (Cori disease)

Question 4

At which step in glycolysis is NADH produced during the oxidation of glyceraldehyde-3-phosphate?

Accepted Answer

Glyceraldehyde-3-phosphate dehydrogenase

Answer

Pyruvate kinase

Answer

Enolase

Answer

PFK-1

Question 5

Which of the following substances does not inhibit glycolysis?

Accepted Answer

Fluoroacetate

Answer

Fluoride

Answer

Arsenite

Answer

Iodoacetate

Question 6

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 7

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 8

What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?

Accepted Answer

NADP

Answer

NAD

Answer

FAD

Answer

FMN

Question 9

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 10

Which of the following GAG is not sulfated?

Accepted Answer

Hyaluronic acid

Answer

Keratan sulfate

Answer

Dermatan sulfate

Answer

Chondroitin sulfate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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