Carbohydrate Metabolism Practice Questions

Q: Name the antigen marked as X determining blood group A.

N-Acetyl-Galactosamine. ***N-Acetyl-Galactosamine*** - Blood group A antigens are formed by the addition of **N-acetylgalactosamine** to the H antigen precursor molecule on the surface of red blood cells. - This sugar modification is catalyzed by the **A transferase enzyme**, which is specific for N-acetylgalactosamine. *N-Acetyl-Glucosamine* - While N-acetylglucosamine is a component of many glycans, it is not the terminal sugar that defines the **blood group A antigen**. - **N-acetylglucosamine** is a key building block for the H antigen and other blood group precursors, but not the specific modifying sugar for A. *Dermatan sulphate* - **Dermatan sulfate** is a **glycosaminoglycan** primarily found in connective tissues, skin, and blood vessels. - It plays a role in wound healing and coagulation, but is not involved in **ABO blood group determination**. *Keratan sulfate* - **Keratan sulfate** is another **glycosaminoglycan** found in cartilage, cornea, and bone. - It contributes to tissue hydration and structural integrity, but it is not part of the **ABO blood group antigens**.

Q: The enzyme catalyzing this pathway is absent in?

Muscle. The image depicts the conversion of **Glucose-6-phosphate** to **Glucose**, catalyzed by the enzyme **Glucose-6-phosphatase**. The question asks where this enzyme is *absent*. ***Correct: Muscle*** - **Muscle tissue lacks glucose-6-phosphatase**, ensuring that glucose-6-phosphate formed within muscle cells is committed to intracellular use (glycolysis or glycogen synthesis) - This prevents muscle from releasing free glucose into the bloodstream, which is physiologically appropriate since muscle primarily uses glucose for its own energy needs - The absence of this enzyme in muscle is a key concept in understanding **tissue-specific glucose metabolism** and why muscle glycogen cannot directly contribute to blood glucose levels *Incorrect: Liver* - Liver is one of the **primary sites where glucose-6-phosphatase is highly expressed** - This enzyme is essential for hepatic glucose production during **gluconeogenesis** and **glycogenolysis** - Allows the liver to release free glucose into the bloodstream to maintain blood glucose homeostasis *Incorrect: Kidney* - Kidney also **contains glucose-6-phosphatase**, particularly in the renal cortex - The kidney contributes to glucose homeostasis through **renal gluconeogenesis**, especially during prolonged fasting - Can release free glucose into the circulation *Incorrect: Intestine* - Small intestine (particularly during fed state) **expresses glucose-6-phosphatase** - Plays a role in glucose production, though to a lesser extent than liver and kidney - Can contribute to postprandial glucose homeostasis **Clinical Relevance:** Deficiency of glucose-6-phosphatase causes **Von Gierke disease (Glycogen Storage Disease Type I)**, characterized by severe hypoglycemia, hepatomegaly, and lactic acidosis.

Q: A 50-year-old male with family history for type 2 diabetes underwent a urine test for reducing substances. The test tube containing 0.5 ml of urine and 5 ml Benedict's reagent was put in a water bath for 2 minutes and change in colour of tube was noticed. Which is the correct statement about the concentration of sugar in the test tube?

2 % sugar. ***2 % sugar*** - A red or brick-red precipitate in Benedict's test indicates a **very high concentration of reducing sugars**, typically **2% or greater** - This represents the most intense positive reaction with **complete reduction of cupric ions (Cu²⁺) to cuprous oxide (Cu₂O)**, producing the characteristic brick-red color - In clinical context, such high urinary glucose indicates **severe hyperglycemia** requiring immediate evaluation *0.5 % sugar* - A 0.5% sugar concentration produces a **green or yellowish-green precipitate**, indicating a trace to moderate amount of reducing sugar - This represents **partial reduction** of the Benedict's reagent with less intense color change - Clinically significant but suggests better glycemic control than higher concentrations *1 % sugar* - A 1% sugar concentration produces an **orange or yellow-orange precipitate**, indicating significant glycosuria - This intermediate color reflects **moderate reduction** of cupric ions - While this indicates poor glycemic control, it is less severe than the 2% concentration *1.5 % sugar* - A 1.5% sugar concentration produces a **reddish-orange precipitate**, approaching but not reaching brick-red intensity - This represents **substantial but incomplete maximal reduction** of the reagent - The distinction from 2% lies in the color intensity - reddish-orange versus pure brick-red

Q: Which transporter helps in the improvement of insulin resistance in type 2 diabetes mellitus (DM2) with regular exercise and physical activities?

GLUT 4. ***GLUT 4*** - **GLUT 4** is the **insulin-sensitive glucose transporter** predominantly found in **skeletal muscle** and **adipose tissue**. Regular exercise and physical activity increase the expression and translocation of GLUT 4 to the cell membrane, enhancing glucose uptake independent of insulin, thereby improving **insulin sensitivity** and reducing insulin resistance in DM2. - Exercise also stimulates **AMPK (AMP-activated protein kinase)**, which promotes GLUT 4 translocation to the cell surface, facilitating glucose uptake and utilization in active muscles. *GLUT 1* - **GLUT 1** is a basal glucose transporter found in nearly all cells, responsible for **basal glucose uptake** to meet basic metabolic needs, especially in **red blood cells** and the **blood-brain barrier**. - Its activity is largely **insulin-independent** and does not significantly contribute to the exercise-induced improvement in insulin sensitivity in skeletal muscle. *GLUT 2* - **GLUT 2** is primarily found in the **liver, pancreatic beta cells, kidneys, and small intestine**. It has a **low affinity** but **high capacity** for glucose transport, serving as a glucose sensor in beta cells and allowing efficient glucose uptake/release in the liver. - It plays a role in glucose homeostasis but is **not directly involved in the exercise-mediated improvement of insulin resistance** in peripheral tissues like muscle. *GLUT 3* - **GLUT 3** is a **high-affinity glucose transporter** primarily expressed in **neurons** and the **placenta**. It is crucial for providing a constant supply of glucose to the brain, even at low glucose concentrations. - Like GLUT 1 and GLUT 2, its activity is largely **insulin-independent** and does not play a significant role in improving insulin resistance through exercise in DM2.

Q: Corneal Transparency is maintained by which of the following GAGs?

Keratan Sulphate. ***Keratan Sulphate*** - **Keratan sulfate** is a major glycosaminoglycan (GAG) found in the **cornea**, where its specific highly hydrated structure and arrangement help maintain corneal transparency. - The uniform spacing of collagen fibrils, maintained by keratan sulfate, is crucial for minimizing light scattering and allowing light to pass through the cornea. *Dermatan Sulphate* - **Dermatan sulfate** is primarily found in **skin, blood vessels, and heart valves**, contributing to tissue strength and elasticity. - It plays a significant role in wound healing and cardiovascular function, but not directly in maintaining corneal transparency. *Heparan Sulphate* - **Heparan sulfate** is ubiquitously found on **cell surfaces and in the extracellular matrix**, particularly in the basement membranes. - It is involved in cell adhesion, growth factor binding, and anticoagulant activity, but is not the primary GAG responsible for corneal transparency. *Chondroitin Sulphate* - **Chondroitin sulfate** is abundant in **cartilage, bone, and connective tissues**, providing compressive strength and elasticity. - While present in some ocular tissues, it is not the dominant GAG responsible for the unique transparent properties of the cornea.

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**.

Question 1

Name the antigen marked as X determining blood group A.

Accepted Answer

N-Acetyl-Galactosamine

Answer

N-Acetyl-Glucosamine

Answer

Dermatan sulphate

Answer

Keratan sulfate

Question 2

The enzyme catalyzing this pathway is absent in?

Accepted Answer

Muscle

Answer

Liver

Answer

Kidney

Answer

Intestine

Question 3

A 50-year-old male with family history for type 2 diabetes underwent a urine test for reducing substances. The test tube containing 0.5 ml of urine and 5 ml Benedict's reagent was put in a water bath for 2 minutes and change in colour of tube was noticed. Which is the correct statement about the concentration of sugar in the test tube?

Accepted Answer

2 % sugar

Answer

0.5 % sugar

Answer

1 % sugar

Answer

1.5 % sugar

Question 4

Which transporter helps in the improvement of insulin resistance in type 2 diabetes mellitus (DM2) with regular exercise and physical activities?

Accepted Answer

GLUT 4

Answer

GLUT 1

Answer

GLUT 2

Answer

GLUT 3

Question 5

Corneal Transparency is maintained by which of the following GAGs?

Accepted Answer

Keratan Sulphate

Answer

Dermatan Sulphate

Answer

Heparan Sulphate

Answer

Chondroitin Sulphate

Question 6

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

Accepted Answer

Acetyl coenzyme A

Answer

Lactate

Answer

Glycerol

Answer

Alanine

Question 7

What is the primary mechanism for glucose uptake in neurons?

Accepted Answer

GLUT3

Answer

GLUT1

Answer

GLUT2

Answer

GLUT4

Question 8

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 9

Which of the following is present in skeletal muscle?

Accepted Answer

GLUT 4

Answer

GLUT 2

Answer

GLUT 7

Answer

GLUT 5

Question 10

Synovial fluid contains-

Accepted Answer

Hyaluronic acid

Answer

Keratan sulphate

Answer

Dermatan sulphate

Answer

Chondroitin sulphate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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