Carbohydrate Metabolism Practice Questions

Q: When the insulin:glucagon ratio is decreased, which enzyme is active?

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - A decreased insulin:glucagon ratio indicates a **fasting state** or **catabolic state**, promoting glucose production and release rather than storage. - **Glucose-6-phosphatase** is the key enzyme that enables glucose release from the liver by removing the phosphate group from glucose-6-phosphate, producing free glucose that can exit hepatocytes. - This enzyme is active during both **gluconeogenesis** and **glycogenolysis** and is only present in liver, kidney, and intestinal cells. *Glucokinase* - **Glucokinase** is active in the **fed state** when insulin levels are high and the insulin:glucagon ratio is increased. - It phosphorylates glucose to trap it in hepatocytes for glycogen synthesis and metabolism, which is the opposite of what occurs during fasting. *Phosphofructokinase* - **Phosphofructokinase (PFK-1)** is the rate-limiting enzyme of **glycolysis**, active when glucose needs to be broken down for energy. - It is stimulated by high insulin:glucagon ratios and inhibited during fasting when gluconeogenesis (the reverse pathway) is active. *Hexokinase* - **Hexokinase** phosphorylates glucose in peripheral tissues for intracellular utilization. - During a low insulin:glucagon ratio, the priority is glucose **release** from the liver, not glucose **uptake** and phosphorylation in tissues.

Q: Iodine gives red color with:

Dextrin. ***Dextrin*** - **Dextrin** gives a characteristic **bright red to red-violet color** with iodine solution, which is the most distinctive "red" color among polysaccharides. - Dextrin is an intermediate product formed during **starch hydrolysis**, with a molecular structure between starch and simple sugars. - This color reaction is used as a **qualitative test** to identify dextrin and monitor starch breakdown during digestion. *Glycogen* - **Glycogen** produces a **red-brown to mahogany brown color** with iodine, which is more brown than red. - This brownish-red color is due to its **highly branched structure** with shorter chain lengths compared to starch. - While it has a reddish tinge, the predominant color is **brown**, making it distinct from the bright red of dextrin. *Starch* - **Starch** (particularly amylose) gives a distinctive **blue-black color** with iodine solution. - This occurs due to formation of a **starch-iodine complex** where iodine molecules fit into the helical structure of amylose. - Amylopectin (branched component) produces a **red-purple color**, but whole starch appears blue-black due to amylose dominance. *Inulin* - **Inulin**, a fructose polymer, shows **no color reaction** with iodine solution. - This absence of reaction is because inulin lacks the helical structure needed for iodine complex formation.

Q: Acid maltase deficiency occurs in

Pompe's disease. ***Pompe's disease*** - **Pompe's disease**, also known as Glycogen Storage Disease Type II, is caused by a deficiency of the lysosomal enzyme **alpha-1,4-glucosidase**, also called **acid maltase**. - This deficiency leads to the accumulation of **glycogen** in lysosomes within various tissues, particularly muscle, liver, and heart, causing muscle weakness and cardiomyopathy. *Von Gierke's disease* - **Von Gierke's disease** (Glycogen Storage Disease Type Ia) is caused by a deficiency of **glucose-6-phosphatase**, leading to accumulation of glycogen primarily in the liver and kidneys. - It presents with severe **hypoglycemia** and hepatomegaly, rather than acid maltase deficiency. *McArdle's disease* - **McArdle's disease** (Glycogen Storage Disease Type V) results from a deficiency of **muscle glycogen phosphorylase**. - This deficiency causes exercise intolerance, muscle pain, and cramps, and is not related to acid maltase. *Niemann-Pick disease* - **Niemann-Pick disease** is a group of lysosomal storage disorders caused by deficiencies in enzymes like **sphingomyelinase** (Type A and B) or mutations in the NPC1/NPC2 genes (Type C). - It leads to accumulation of **sphingomyelin** and cholesterol, affecting the liver, spleen, lungs, and brain, and is unrelated to glycogen metabolism or acid maltase.

Q: GLUT3 is seen in

Neurons. ***Neurons*** - **GLUT3** is the primary glucose transporter in **neurons** and is responsible for maintaining a constant supply of glucose to the brain, even at low blood glucose concentrations. - Its high affinity for glucose ensures that brain cells can take up glucose efficiently, which is crucial for their high metabolic demands. *Pancreas* - The pancreas primarily uses **GLUT2** on its beta cells, which has a low affinity and high capacity for glucose, allowing it to sense high blood glucose levels. - **GLUT1** is also found in pancreatic alpha cells. *Liver* - The liver predominantly expresses **GLUT2**, which facilitates both **glucose uptake** and **release** depending on the metabolic state. - Its low affinity for glucose allows the liver to act as a glucose sensor and regulate blood glucose homeostasis. *Spleen* - While various immune cells within the spleen express glucose transporters, no single GLUT isoform, such as **GLUT3**, is uniquely or predominantly associated with the general function of the spleen. - Most cells in the spleen express **GLUT1**, which provides basal glucose uptake.

Q: Low insulin to glucagon ratio leads to increase in the activity of

Glucose-6-phosphatase. ***Glucose-6-phosphatase*** - A low **insulin to glucagon ratio** signals a state of **low blood glucose**, leading to increased **glucagon** secretion. - Glucagon activates **gluconeogenesis** and **glycogenolysis** in the liver, and **glucose-6-phosphatase** is a key enzyme in the final step of both pathways, releasing free glucose into the bloodstream. *Hexokinase* - This enzyme is responsible for the **phosphorylation of glucose in most tissues** to trap it within the cell for glycolysis. - Its activity is generally high during periods of **high glucose and insulin levels** to promote glucose utilization. *Glucokinase* - This is an isoform of hexokinase found in the **liver and pancreatic beta cells**, with a higher Km for glucose, meaning it is active primarily at **high glucose concentrations**. - Its activity is increased by **insulin**, promoting glucose uptake and utilization in times of plenty. *Pyruvate kinase* - This enzyme catalyzes the final step of **glycolysis**, converting phosphoenolpyruvate to pyruvate. - Its activity is stimulated by **insulin** and inhibited by **glucagon**, reflecting its role in glucose breakdown, not production.

Q: Glutathione is maintained in reduced state by the help of ?

HMP shunt. ***HMP shunt*** - The **hexose monophosphate (HMP) shunt** produces **NADPH**, which is crucial for reducing **oxidized glutathione** back to its reduced form via **glutathione reductase**. - **Reduced glutathione** protects cells from **oxidative damage** by detoxifying harmful **reactive oxygen species.** *Transamination* - **Transamination** is a process involving the transfer of an **amino group** from an amino acid to a keto acid. - This pathway is primarily involved in **amino acid metabolism** and the synthesis of **non-essential amino acids**, not directly in glutathione reduction. *Uronic acid pathway* - The **uronic acid pathway** is involved in the synthesis of **glycolipids**, **sugars**, and **vitamin C** (in some animals). - It does not directly produce **NADPH** or enzymes necessary for maintaining **glutathione** in its reduced state. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen** from **glucose** for storage, typically occurring in the liver and muscles. - This pathway is involved in **glucose storage** and **energy regulation**, not in the **redox state of glutathione**.

Q: McArdle's disease is caused by deficiency of which enzyme?

Muscle phosphorylase. ***Muscle phosphorylase (Myophosphorylase)*** - McArdle's disease is **Glycogen Storage Disease Type V** caused by deficiency of **muscle phosphorylase** (myophosphorylase) - This enzyme is essential for **glycogenolysis in skeletal muscle**, breaking down glycogen to glucose-1-phosphate - Patients experience **exercise intolerance, muscle cramps, and myoglobinuria** after intense exercise - Characteristic **second-wind phenomenon** occurs when alternative energy sources (fatty acids, blood glucose) become available - Diagnosed by **ischemic forearm exercise test** showing no rise in venous lactate *Glucose-6-phosphatase* - Deficiency causes **Type I Glycogen Storage Disease (von Gierke disease)** - Affects **liver and kidneys**, not primarily skeletal muscle - Presents with **hepatomegaly, hypoglycemia, lactic acidosis** *Branching enzyme* - Deficiency causes **Type IV Glycogen Storage Disease (Andersen disease)** - Results in abnormal glycogen with **fewer branch points** - Presents with **progressive cirrhosis and hepatosplenomegaly** *Debranching enzyme* - Deficiency causes **Type III Glycogen Storage Disease (Cori disease)** - Affects both **liver and muscle** metabolism - Presents with **hepatomegaly, hypoglycemia, and mild myopathy**

Q: History of dislike for sweet food items is typically present in:

Hereditary fructose intolerance. ***Hereditary fructose intolerance*** - Patients with hereditary fructose intolerance develop severe symptoms like **nausea, vomiting, abdominal pain, and hypoglycemia** after ingesting fructose, leading to an aversive response and **dislike for sweet food items**. - This aversion is a protective mechanism, as avoiding fructose-containing foods (including many sweets) prevents the accumulation of toxic metabolites due to a deficiency in **hepatic aldolase B**. *Glycogen storage disease* - While glycogen storage diseases can cause hypoglycemia, they typically do not lead to a specific **aversion to sweet foods**. - The primary defect is in **glycogen synthesis or breakdown**, leading to symptoms like hepatomegaly, muscle weakness, and exercise intolerance. *Diabetes mellitus* - Patients with diabetes mellitus often have a **craving for sweet foods** due to uncontrolled blood glucose levels and insulin resistance, rather than a dislike. - The condition is characterized by **hyperglycemia** and may involve polydipsia, polyuria, and polyphagia. *Galactosemia* - Galactosemia involves an inability to metabolize galactose, leading to symptoms such as **vomiting, lethargy, and jaundice** upon milk ingestion [1]. - While patients will avoid milk, their aversion is not generally to all sweet foods, as sweet foods do not always contain galactose [1].

Q: All are Glucogenic hormones except?

ADH. ***ADH*** - **Antidiuretic hormone (ADH)** primarily regulates **water balance** by increasing water reabsorption in the kidneys, and does not directly promote glucose production. - While stress can increase ADH levels and indirectly affect glucose, ADH itself is not considered a primary **glucogenic hormone**. *Glucagon* - **Glucagon** is a key **glucogenic hormone** that raises blood glucose levels by stimulating **glycogenolysis** and **gluconeogenesis** in the liver. - It is released from pancreatic alpha cells in response to low blood glucose. *Glucocorticoids* - **Glucocorticoids** (e.g., cortisol) are potent **glucogenic hormones** that promote **gluconeogenesis** in the liver and reduce glucose utilization by peripheral tissues. - They help maintain blood glucose levels during stress and fasting. *Thyroxine* - **Thyroxine (T4)**, a thyroid hormone, increases **metabolic rate**, which includes enhancing glucose absorption from the gut, promoting **glycogenolysis**, and sometimes **gluconeogenesis**. - While not acting as directly on glucose as glucagon, it does contribute to **glucose homeostasis** through its metabolic effects.

Q: Which carbon of glucose is oxidized to form glucuronic acid?

Oxidation of the terminal alcohol group only. ***Oxidation of the terminal alcohol group only*** - Glucuronic acid is formed by the **oxidation of the C6 carbon (terminal alcohol group)** of glucose, while the aldehyde group (C1) remains intact. - This specific oxidation converts glucose into a **uronic acid**, essential for detoxification and connective tissue synthesis. *Oxidation of the aldehyde group only* - The oxidation of the **aldehyde group (C1)** of glucose would yield **gluconic acid**, not glucuronic acid. - This reaction typically occurs during the conversion of glucose to gluconolactone, a step in the pentose phosphate pathway for example. *No oxidation occurs* - The formation of glucuronic acid is explicitly an **oxidative process**, as a hydroxyl group is converted to a carboxyl group. - If no oxidation occurred, glucose would remain glucose, or undergo other non-oxidative transformations. *Oxidation of both the aldehyde and terminal alcohol groups* - Oxidation of **both the aldehyde (C1) and terminal alcohol (C6)** groups of glucose would lead to the formation of **glucaric acid (saccharic acid)**. - Glucaric acid has carboxyl groups at both ends, making it different from glucuronic acid, which only has a carboxyl group at C6.

Question 1

When the insulin:glucagon ratio is decreased, which enzyme is active?

Accepted Answer

Glucose-6-phosphatase

Answer

Glucokinase

Answer

Phosphofructokinase

Answer

Hexokinase

Question 2

Iodine gives red color with:

Accepted Answer

Dextrin

Answer

Glycogen

Answer

Starch

Answer

Inulin

Question 3

Acid maltase deficiency occurs in

Accepted Answer

Pompe's disease

Answer

Von Gierke's disease

Answer

McArdle's disease

Answer

Niemann-Pick disease

Question 4

GLUT3 is seen in

Accepted Answer

Neurons

Answer

Pancreas

Answer

Liver

Answer

Spleen

Question 5

Low insulin to glucagon ratio leads to increase in the activity of

Accepted Answer

Glucose-6-phosphatase

Answer

Hexokinase

Answer

Glucokinase

Answer

Pyruvate kinase

Question 6

Glutathione is maintained in reduced state by the help of ?

Accepted Answer

HMP shunt

Answer

Transamination

Answer

Uronic acid pathway

Answer

Glycogenesis

Question 7

McArdle's disease is caused by deficiency of which enzyme?

Accepted Answer

Muscle phosphorylase

Answer

Glucose-6-phosphatase

Answer

Branching enzyme

Answer

Debranching enzyme

Question 8

History of dislike for sweet food items is typically present in:

Accepted Answer

Hereditary fructose intolerance

Answer

Glycogen storage disease

Answer

Diabetes mellitus

Answer

Galactosemia

Question 9

All are Glucogenic hormones except?

Accepted Answer

ADH

Answer

Glucagon

Answer

Glucocorticoids

Answer

Thyroxine

Question 10

Which carbon of glucose is oxidized to form glucuronic acid?

Accepted Answer

Oxidation of the terminal alcohol group only

Answer

Oxidation of the aldehyde group only

Answer

No oxidation occurs

Answer

Oxidation of both the aldehyde and terminal alcohol groups

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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