Carbohydrate Metabolism Practice Questions

Q: Pompe disease is seen due to deficiency of?

Lysosomal acid alpha glucosidase. ***Lysosomal acid alpha glucosidase*** - **Pompe disease**, also known as **glycogen storage disease type II**, is caused by the deficiency of **acid alpha-glucosidase (GAA)**, a lysosomal enzyme. - This deficiency leads to the accumulation of **glycogen** within lysosomes in various tissues, particularly muscle, liver, and heart, causing muscle weakness and cardiomegaly. *β-galactocerebrosidase* - Deficiency of **β-galactocerebrosidase** is associated with **Krabbe disease** (globoid cell leukodystrophy). - Krabbe disease involves the buildup of **galactocerebroside**, leading to progressive destruction of the myelin sheath in the central and peripheral nervous systems. *β-glucocerebrosidase* - Deficiency in **β-glucocerebrosidase** causes **Gaucher disease**, the most common lysosomal storage disorder. - This results in the accumulation of **glucocerebroside** in macrophages, leading to hepatosplenomegaly, bone pain, and neurologic symptoms in some forms. *β-hexosaminidase-A* - Deficiency of **β-hexosaminidase-A** is the underlying cause of **Tay-Sachs disease**. - Tay-Sachs disease is characterized by the accumulation of **GM2 gangliosides** in neurons, leading to progressive neurodegeneration, developmental delay, and a characteristic cherry-red spot in the retina.

Q: Which of the following is not a substrate for glucose formation?

Acetyl coenzyme A. ***Acetyl coenzyme A*** - **Acetyl CoA** cannot be converted to glucose because the two carbons from the acetyl group are lost as carbon dioxide in the **Krebs cycle**, making a net synthesis of glucose impossible. - The irreversible nature of the **pyruvate dehydrogenase complex** prevents the conversion of Acetyl CoA back to **pyruvate**, which is a crucial step for gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise and fasting, via the **Cori cycle**. - **Lactate dehydrogenase** converts lactate to **pyruvate**, which can then enter the gluconeogenic pathway. *Glycerol* - **Glycerol**, derived from triglyceride breakdown, enters gluconeogenesis by being converted to **glycerol-3-phosphate** and then to **dihydroxyacetone phosphate (DHAP)**. - DHAP is an intermediate in glycolysis and gluconeogenesis, allowing for its conversion to glucose. *Alanine* - **Alanine** is a **glucogenic amino acid** that can be transaminated to **pyruvate**. - **Pyruvate** can then proceed through the gluconeogenic pathway to synthesize glucose, especially during prolonged fasting.

Q: What is the primary mechanism for glucose uptake in neurons?

GLUT3. ***GLUT3*** - **GLUT3** is the primary glucose transporter in **neurons** and has a **high affinity** for glucose. - This high affinity ensures that neurons can continuously take up glucose, even when blood glucose levels are relatively low, to meet their significant energy demands. *GLUT1* - **GLUT1** is abundant in **red blood cells** and at the **blood-brain barrier**, where it provides basal glucose transport to many cell types. - While present in the brain, it is primarily responsible for glucose transport across the **blood-brain barrier** into the interstitial fluid, not directly into neurons as the main mechanism. *GLUT2* - **GLUT2** has a **low affinity** and **high capacity** for glucose, primarily found in the **liver, pancreatic beta cells, kidney, and intestine**. - Its role is to sense high glucose levels and transport large amounts of glucose accordingly, which is not characteristic of neuronal glucose uptake. *GLUT4* - **GLUT4** is the **insulin-sensitive** glucose transporter, predominantly found in **adipose tissue** and **skeletal muscle**. - Its translocation to the cell membrane is stimulated by insulin, a mechanism not central to neuronal glucose uptake.

Q: A 20-year-old male with no significant medical history comes to you with a urine positive for fructose. He does not have diabetes mellitus. Which enzyme is most likely to be deficient in this patient?

Fructokinase. ***Fructokinase*** - A urine positive for **fructose** without symptoms of diabetes mellitus (i.e., **benign fructosuria**) is characteristic of a **fructokinase deficiency**. - **Fructokinase** is the enzyme responsible for the first step in fructose metabolism, converting **fructose to fructose-1-phosphate**. *Pyruvate kinase* - Deficiency of **pyruvate kinase** primarily affects **glycolysis** in red blood cells and leads to **hemolytic anemia**, not fructosuria. - This enzyme converts **phosphoenolpyruvate to pyruvate**. *Lactase* - **Lactase** is an enzyme that digests **lactose** (milk sugar) into glucose and galactose. - A deficiency in lactase causes **lactose intolerance**, presenting with gastrointestinal symptoms like bloating and diarrhea after consuming dairy products, not fructose in the urine. *Aldolase B* - A deficiency in **aldolase B** leads to **hereditary fructose intolerance**, a severe condition where **fructose-1-phosphate accumulates** after fructose ingestion. - This typically presents with symptoms such as **hypoglycemia**, vomiting, jaundice, and liver damage, which are not described in this benign case of fructosuria.

Q: Which of the following is present in skeletal muscle?

GLUT 4. ***GLUT 4*** - **GLUT 4** is the primary glucose transporter found in **skeletal muscle** and adipose tissue. - Its translocation to the cell membrane is **insulin-dependent** and also stimulated by muscle contraction, allowing increased glucose uptake. *GLUT 2* - **GLUT 2** is predominantly found in the liver, pancreas (beta cells), intestine, and kidney. - It has a **low affinity (high Km)** for glucose, allowing it to transport glucose efficiently only at high blood glucose concentrations. *GLUT 7* - **GLUT 7** is a glucose transporter located in the **endoplasmic reticulum** membrane of the liver and other gluconeogenic tissues. - It plays a role in the flux of glucose within the ER lumen, particularly in **hepatic glucose production**. *GLUT 5* - **GLUT 5** is primarily responsible for **fructose transport** in the small intestine, testes, and kidneys. - It does not transport glucose and has a specific affinity for fructose.

Q: Synovial fluid contains-

Hyaluronic acid. ***Hyaluronic acid*** - **Hyaluronic acid** is a major component of **synovial fluid**, providing **viscosity** and **lubrication** to joints, which is crucial for reducing friction between articular cartilages. - It's a **glycosaminoglycan** (GAG) responsible for the fluid's unique rheological properties, maintaining joint health and function. *Keratan sulphate* - **Keratan sulphate** is primarily found in **cartilage**, **cornea**, and **bone**, contributing to their structural integrity. - It is not a significant component of **synovial fluid** itself; rather, it is part of the extracellular matrix of surrounding tissues. *Dermatan sulphate* - **Dermatan sulphate** is typically found in **skin**, **blood vessels**, and **heart valves**, where it plays a role in tissue organization and repair. - It is not a characteristic or primary component of **synovial fluid**. *Chondroitin sulphate* - **Chondroitin sulphate** is a GAG abundant in **cartilage**, contributing to its **compressive strength** and elasticity. - While essential for **joint health**, it is found within the cartilage matrix, not freely in high concentrations within the **synovial fluid**.

Q: Gluconeogenesis is inhibited by?

Insulin. ***Insulin*** - **Insulin** is a key hormone released in response to high blood glucose, promoting glucose uptake and storage, and **inhibiting hepatic glucose production** through gluconeogenesis and glycogenolysis. - It achieves this by decreasing the transcription and activity of key gluconeogenic enzymes like **phosphoenolpyruvate carboxykinase (PEPCK)** and **glucose-6-phosphatase**. *Cholecystokinin* - **Cholecystokinin (CCK)** is a gastrointestinal hormone primarily involved in digestion, stimulating bile release and pancreatic enzyme secretion. - It does not directly regulate gluconeogenesis; its main role is related to **fat and protein digestion**. *5-alpha reductase* - **5-alpha reductase** is an enzyme involved in steroid metabolism, converting testosterone to the more potent androgen, dihydrotestosterone (DHT). - This enzyme has no direct role in the regulation of **gluconeogenesis**. *Glucagon* - **Glucagon** is a hormone that has the opposite effect of insulin, stimulating gluconeogenesis and glycogenolysis to increase blood glucose levels during fasting or hypoglycemia. - Its primary action is to **promote** hepatic glucose output, not inhibit it.

Q: Most Common enzyme deficient in galactosemics:

Galactose-1-phosphate uridyl transferase/GALT. ***Galactose-1-phosphate uridyl transferase/GALT*** - **GALT deficiency** is the most common cause of **classic galactosemia** (Type I), a severe inherited metabolic disorder. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the main pathway of galactose metabolism. - Accounts for approximately **95%** of all galactosemia cases. *Galactosidase* - **Galactosidase** enzymes are involved in the hydrolysis of galactose-containing oligosaccharides or glycoconjugates but are not the primary enzymes deficient in classic galactosemia. - This enzyme is not part of the Leloir pathway of galactose metabolism, which is the pathway affected in galactosemia. *UDP galactose epimerase* - Deficiency of **UDP galactose epimerase** (GALE) causes a milder form of galactosemia (Type III), but it is much less common than GALT deficiency. - GALE is involved in the interconversion of UDP-galactose and UDP-glucose. - This is the rarest form of galactosemia. *Galactokinase* - **Galactokinase deficiency** (GALK) causes a different, milder form of galactosemia (Type II), characterized by **cataracts** as the primary symptom. - It prevents the initial phosphorylation of galactose to galactose-1-phosphate. - This accounts for less than 5% of galactosemia cases.

Q: Which is branching enzyme?

Amylo-1, 4-1, 6-transglycolase. ***Amylo-1, 4-1, 6-transglycolase*** - This enzyme is also known as **glycogen branching enzyme**. - It catalyzes the formation of **α-1,6-glycosidic bonds** by transferring a segment of four to six glucosyl residues from the non-reducing end of a growing glycogen chain to another chain. *Glycogen synthase* - This enzyme is responsible for the **elongation of glycogen chains** by forming **α-1,4-glycosidic bonds**. - It adds glucose units to the non-reducing end of a pre-existing glycogen primer. *Glycogen Phosphorylase* - This enzyme is involved in **glycogen degradation**. - It catalyzes the **phosphorolytic cleavage** of α-1,4-glycosidic bonds, releasing glucose-1-phosphate. *Glucose-6 phosphatase* - This enzyme is primarily found in the **liver** and kidneys and is crucial for **gluconeogenesis** and **glycogenolysis**. - It dephosphorylates glucose-6-phosphate to **free glucose**, allowing its release into the bloodstream.

Q: Low glycemic index food is:

Has slower absorption. ***Has slower absorption*** - **Low glycemic index (GI)** foods are digested and absorbed more slowly, leading to a gradual rise in blood glucose and insulin levels. - This characteristic is beneficial for managing **blood sugar** and providing sustained energy. *Easily digestible* - **Easily digestible** foods often have a **high glycemic index** because their carbohydrates are rapidly broken down and absorbed. - Low GI foods, by contrast, contain more complex carbohydrates and fiber, making them slower to digest. *Increase glycogen deposits* - While all carbohydrates are eventually converted to **glucose** and can contribute to **glycogen synthesis**, low GI foods do not uniquely or preferentially increase glycogen deposits compared to high GI foods. - Glycogen synthesis is primarily influenced by insulin levels and the total amount of carbohydrates consumed, irrespective of GI. *Increases plasma glucose* - All carbohydrate-containing foods will eventually increase **plasma glucose**, but low GI foods cause a **slower and smaller rise** in blood glucose compared to high GI foods. - They prevent the sharp spikes in blood sugar that are associated with high GI foods.

Question 1

Pompe disease is seen due to deficiency of?

Accepted Answer

Lysosomal acid alpha glucosidase

Answer

β-galactocerebrosidase

Answer

β-glucocerebrosidase

Answer

β-hexosaminidase-A

Question 2

Which of the following is not a substrate for glucose formation?

Accepted Answer

Acetyl coenzyme A

Answer

Lactate

Answer

Glycerol

Answer

Alanine

Question 3

What is the primary mechanism for glucose uptake in neurons?

Accepted Answer

GLUT3

Answer

GLUT1

Answer

GLUT2

Answer

GLUT4

Question 4

A 20-year-old male with no significant medical history comes to you with a urine positive for fructose. He does not have diabetes mellitus. Which enzyme is most likely to be deficient in this patient?

Accepted Answer

Fructokinase

Answer

Pyruvate kinase

Answer

Lactase

Answer

Aldolase B

Question 5

Which of the following is present in skeletal muscle?

Accepted Answer

GLUT 4

Answer

GLUT 2

Answer

GLUT 7

Answer

GLUT 5

Question 6

Synovial fluid contains-

Accepted Answer

Hyaluronic acid

Answer

Keratan sulphate

Answer

Dermatan sulphate

Answer

Chondroitin sulphate

Question 7

Gluconeogenesis is inhibited by?

Accepted Answer

Insulin

Answer

Cholecystokinin

Answer

5-alpha reductase

Answer

Glucagon

Question 8

Most Common enzyme deficient in galactosemics:

Accepted Answer

Galactose-1-phosphate uridyl transferase/GALT

Answer

Galactosidase

Answer

UDP galactose epimerase

Answer

Galactokinase

Question 9

Which is branching enzyme?

Accepted Answer

Amylo-1, 4-1, 6-transglycolase

Answer

Glycogen synthase

Answer

Glycogen Phosphorylase

Answer

Glucose-6 phosphatase

Question 10

Low glycemic index food is:

Accepted Answer

Has slower absorption

Answer

Easily digestible

Answer

Increase glycogen deposits

Answer

Increases plasma glucose

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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