Carbohydrate Metabolism Practice Questions

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.

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.

Question 1

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 2

Which of the following is present in skeletal muscle?

Accepted Answer

GLUT 4

Answer

GLUT 2

Answer

GLUT 7

Answer

GLUT 5

Question 3

Synovial fluid contains-

Accepted Answer

Hyaluronic acid

Answer

Keratan sulphate

Answer

Dermatan sulphate

Answer

Chondroitin sulphate

Question 4

Gluconeogenesis is inhibited by?

Accepted Answer

Insulin

Answer

Cholecystokinin

Answer

5-alpha reductase

Answer

Glucagon

Question 5

Most Common enzyme deficient in galactosemics:

Accepted Answer

Galactose-1-phosphate uridyl transferase/GALT

Answer

Galactosidase

Answer

UDP galactose epimerase

Answer

Galactokinase

Question 6

Which is branching enzyme?

Accepted Answer

Amylo-1, 4-1, 6-transglycolase

Answer

Glycogen synthase

Answer

Glycogen Phosphorylase

Answer

Glucose-6 phosphatase

Question 7

Low glycemic index food is:

Accepted Answer

Has slower absorption

Answer

Easily digestible

Answer

Increase glycogen deposits

Answer

Increases plasma glucose

Question 8

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

Accepted Answer

Glucose-6-phosphatase

Answer

Glucokinase

Answer

Phosphofructokinase

Answer

Hexokinase

Question 9

Iodine gives red color with:

Accepted Answer

Dextrin

Answer

Glycogen

Answer

Starch

Answer

Inulin

Question 10

Acid maltase deficiency occurs in

Accepted Answer

Pompe's disease

Answer

Von Gierke's disease

Answer

McArdle's disease

Answer

Niemann-Pick disease

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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