Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 791: In McArdle disease, muscle cannot make use of glycogen for energy because of a deficiency of?

A. Glucokinase
B. Phosphoglucomutase
C. G-6-phosphatase
D. Muscle phosphorylase (Correct Answer)

Explanation: ***Muscle phosphorylase*** - **Muscle phosphorylase (glycogen phosphorylase)** is responsible for breaking down glycogen into **glucose-1-phosphate** in muscle tissue, which is then used for energy production. - A deficiency in this enzyme causes **McArdle disease (Glycogen Storage Disease Type V)**, preventing muscles from accessing their stored glycogen, leading to exercise intolerance, muscle cramps, and myoglobinuria after exercise. - This is the rate-limiting step in muscle glycogenolysis. *Glucokinase* - **Glucokinase** is primarily found in the **liver and pancreatic beta cells**, where it phosphorylates glucose to glucose-6-phosphate. - It acts as a glucose sensor and plays a role in regulating blood glucose levels but is not involved in glycogen breakdown in muscle tissue. - Muscle uses **hexokinase** (not glucokinase) for glucose phosphorylation. *Phosphoglucomutase* - **Phosphoglucomutase** interconverts **glucose-1-phosphate** and **glucose-6-phosphate** in the glycogenolysis pathway. - While essential for utilizing the products of glycogen breakdown, its deficiency (Glycogen Storage Disease Type XIV - very rare) would affect the downstream pathway but not the initial breakdown of glycogen. - This enzyme functions after phosphorylase has already broken down glycogen. *G-6-phosphatase* - **Glucose-6-phosphatase** is primarily found in the **liver and kidneys** and is essential for releasing free glucose into the bloodstream. - This enzyme is **normally absent in muscle tissue**, which is why muscle cannot contribute to blood glucose maintenance. - Its deficiency causes Von Gierke disease (GSD Type I), affecting hepatic glucose release, but this is not relevant to muscle glycogen utilization for the muscle's own energy needs.

Question 792: Which of the following is true about the synthesis of glucose from pyruvate by gluconeogenesis?

A. Occurs exclusively in the cytosol.
B. Is inhibited by an elevated level of glucagon
C. Involves lactate as an intermediate
D. Requires the participation of biotin (Correct Answer)

Explanation: ***Requires the participation of biotin*** - **Biotin** is a required cofactor for **pyruvate carboxylase**, an enzyme that converts **pyruvate to oxaloacetate**, a crucial step in gluconeogenesis that bypasses the irreversible pyruvate kinase step. - This carboxylation reaction is the first committed step in overcoming the irreversible steps of glycolysis in gluconeogenesis. *Occurs exclusively in the cytosol.* - Gluconeogenesis is a complex process that occurs in **multiple cellular compartments**. - While many steps occur in the cytosol, the initial conversion of **pyruvate to oxaloacetate** by pyruvate carboxylase occurs in the **mitochondria**. *Is inhibited by an elevated level of glucagon* - **Glucagon** is a hormone that **stimulates gluconeogenesis**, not inhibits it. - High glucagon levels signal a need for increased glucose production, especially during fasting or hypoglycemia. *Involves lactate as an intermediate* - While **lactate can be a precursor for gluconeogenesis**, it is not an intermediate in the direct synthesis of glucose from pyruvate. - Lactate is converted to pyruvate, which then enters the gluconeogenic pathway.

Question 793: How many stereoisomers are possible for an aldohexose?

A. 32
B. 64
C. 16 (Correct Answer)
D. 8

Explanation: ***16*** - An aldohexose (like glucose) has **four chiral centers** (C2, C3, C4, and C5 in the open-chain form). - The number of possible stereoisomers for a molecule with 'n' chiral centers is given by the formula **2^n**. Therefore, 2^4 = **16 stereoisomers**. - These 16 stereoisomers include D-glucose, D-mannose, D-galactose, D-allose, and their corresponding L-forms. *32* - This number would be true if an aldohexose had **five chiral centers** (2^5 = 32), which it does not. - Aldohexoses are six-carbon sugars, but C1 (aldehyde carbon) and C6 (primary alcohol) are not chiral centers. *64* - This number would imply **six chiral centers** (2^6 = 64), which is incorrect for aldohexoses. - This would require all six carbons to be chiral centers, which is structurally impossible in an aldohexose. *8* - This number suggests **three chiral centers** (2^3 = 8), which is an underestimation. - Aldohexoses have **four chiral centers**, not three, resulting in 16 possible stereoisomers.

Question 794: Glucose can be synthesized from all except:

A. Amino acids
B. Glycerol
C. Acetoacetate (Correct Answer)
D. Lactic acid

Explanation: ***Acetoacetate*** - **Acetoacetate** is a **ketone body** and is not a gluconeogenic precursor because its breakdown products, **acetyl-CoA**, cannot be converted to pyruvate in humans. - The carbons from **acetyl-CoA** are released as CO2 in the **TCA cycle** and therefore cannot be used for net glucose synthesis. *Amino acids* - Many **amino acids** (the **gluconeogenic amino acids**) can be converted to intermediates of the **TCA cycle** or pyruvate, allowing for subsequent glucose synthesis. - These include **alanine**, **glutamate**, **aspartate**, and others. *Glycerol* - **Glycerol**, derived from the breakdown of triglycerides, can be phosphorylated to **glycerol-3-phosphate** and then oxidized to **dihydroxyacetone phosphate (DHAP)**. - DHAP is an intermediate in glycolysis and gluconeogenesis, thus allowing for glucose synthesis. *Lactic acid* - **Lactate** is readily converted to **pyruvate** by the enzyme **lactate dehydrogenase**. - **Pyruvate** is a direct precursor for gluconeogenesis, especially important in the **Cori cycle**.

Question 795: Galactosemia is caused by a deficiency of which enzyme?

A. Galactose 1 phosphate uridyl transferase (Correct Answer)
B. Aldolase B
C. UDP galactose 4 epimerase
D. Fructokinase

Explanation: ***Galactose 1 phosphate uridyl transferase*** - **Classic galactosemia** is caused by a deficiency of the enzyme **galactose-1-phosphate uridyl transferase (GALT)**. - This enzyme is crucial for converting **galactose-1-phosphate** and UDP-glucose into UDP-galactose and glucose-1-phosphate in the Leloir pathway. *Aldolase B* - Deficiency of **aldolase B** is associated with **hereditary fructose intolerance**, not galactosemia. - This enzyme is responsible for cleaving **fructose-1-phosphate** into dihydroxyacetone phosphate and glyceraldehyde. *UDP galactose 4 epimerase* - A deficiency in **UDP-galactose 4-epimerase (GALE)** causes a milder and rarer form of galactosemia, known as **galactosemia type III**. - While related to galactose metabolism, the classic and most common form of galactosemia is due to GALT deficiency. *Fructokinase* - Deficiency of **fructokinase** causes **essential fructosuria**, a benign metabolic disorder where fructose accumulates in the urine. - This condition is typically asymptomatic and does not lead to severe clinical manifestations like galactosemia.

Question 796: Which of the following statements correctly describes the effect of insulin and glucagon on gluconeogenesis?

A. Glucagon decreases fructose 2,6-bisphosphate levels, stimulating gluconeogenesis. (Correct Answer)
B. Insulin increases the levels of fructose 2,6-bisphosphate, which inhibits gluconeogenesis.
C. Insulin acts through a kinase to promote glycolysis.
D. Fructose 2,6-bisphosphate is an activator of glycolysis.

Explanation: ***Glucagon decreases fructose 2,6-bisphosphate levels, stimulating gluconeogenesis.*** - **Glucagon** activates **cAMP-dependent protein kinase (PKA)**, which phosphorylates the bifunctional enzyme **PFK-2/FBPase-2**. - Phosphorylation activates the **fructose-2,6-bisphosphatase (FBPase-2)** activity, which breaks down **fructose 2,6-bisphosphate (F-2,6-BP)**. - Decreased **F-2,6-BP** removes the inhibition of **fructose-1,6-bisphosphatase**, a key regulatory enzyme in gluconeogenesis, thereby **stimulating gluconeogenesis**. - This is the primary mechanism by which glucagon promotes glucose production during fasting states. *Insulin increases the levels of fructose 2,6-bisphosphate, which inhibits gluconeogenesis.* - While this statement is biochemically accurate, **insulin's primary role is to inhibit gluconeogenesis**, not stimulate it. - Insulin activates the **kinase activity (PFK-2)** of the bifunctional enzyme, increasing **F-2,6-BP** levels. - Elevated **F-2,6-BP** inhibits **fructose-1,6-bisphosphatase**, thereby inhibiting gluconeogenesis. - However, the question asks about effects on gluconeogenesis, and **glucagon's stimulatory effect is more directly relevant** to understanding gluconeogenesis regulation. *Fructose 2,6-bisphosphate is an activator of glycolysis.* - This statement is true but incomplete in the context of the question. - **F-2,6-BP** is a potent allosteric activator of **phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis. - However, this option doesn't directly address the hormonal regulation of **gluconeogenesis** as requested in the question stem. *Insulin acts through a kinase to promote glycolysis.* - While insulin does activate various kinases (e.g., **Akt/PKB**) that promote glycolysis, this statement is too vague. - The question specifically asks about effects on **gluconeogenesis**, not glycolysis. - Insulin's effect on gluconeogenesis is through inhibition (via increased F-2,6-BP levels), which is not clearly stated in this option.

Question 797: The rate-limiting step in glycolysis is catalyzed by?

A. Phosphofructokinase (Correct Answer)
B. Enolase
C. Glucokinase
D. Pyruvate kinase

Explanation: ***Phosphofructokinase*** - **Phosphofructokinase-1 (PFK-1)** is the primary regulatory enzyme and **rate-limiting step** in glycolysis. - It catalyzes the irreversible phosphorylation of **fructose-6-phosphate to fructose-1,6-bisphosphate**, a crucial commitment step. *Enolase* - **Enolase** catalyzes the conversion of **2-phosphoglycerate to phosphoenolpyruvate** in glycolysis. - While essential for glycolysis, it is not the rate-limiting step. *Glucokinase* - **Glucokinase** catalyzes the phosphorylation of glucose to **glucose-6-phosphate** in the liver and pancreatic beta cells. - This is the first step in glycolysis but is not the rate-limiting step for the entire pathway once glucose has entered the cell. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of glycolysis, converting **phosphoenolpyruvate to pyruvate**. - Although it is a regulated enzyme, it is not the primary rate-limiting step that controls the overall flux through the glycolytic pathway.

Question 798: In which anatomical structure is keratan sulfate I primarily located?

A. Skin
B. Bone
C. Cornea (Correct Answer)
D. Lung

Explanation: ***Cornea*** - **Keratan sulfate I** is a major component of the **corneal stroma**, contributing significantly to its transparency and hydration. - Its unique structure and interaction with **collagen fibrils** are crucial for maintaining the precise spacing required for optical clarity. *Skin* - The primary glycosaminoglycans found in the skin are **hyaluronic acid** and **dermatan sulfate**, which provide hydration and tensile strength. - While other **proteoglycans** are present, keratan sulfate is not a predominant component of the dermal extracellular matrix. *Bone* - Bone matrix is rich in **chondroitin sulfate** and **hyaluronic acid**, which play roles in bone formation, mineralization, and structural integrity. - **Keratan sulfate** is present in relatively small amounts in bone, with a more significant role in cartilage. *Lung* - The lung extracellular matrix contains a variety of **glycosaminoglycans** including **hyaluronic acid**, **chondroitin sulfate**, and **heparan sulfate**, important for tissue elasticity and gas exchange. - **Keratan sulfate** is not a primary or abundant proteoglycan found in the normal lung parenchyma.

Question 799: Which mucopolysaccharide does not contain uronic acid?

A. Keratan sulfate (Correct Answer)
B. Hyaluronic acid
C. Heparin
D. Dermatan sulfate

Explanation: ***Keratan sulphate*** - **Keratan sulfate** is unique among the common **glycosaminoglycans** as it contains **galactose** instead of a **uronic acid** residue. - Its repeating unit is usually **N-acetylglucosamine-6-sulfate** and **galactose**. *Heparin* - This **mucopolysaccharide** contains **iduronic acid** (a type of uronic acid) as part of its repeating disaccharide unit. - Its primary role is as an **anticoagulant**, achieved through its interaction with antithrombin. *Hyaluronic acid* - **Hyaluronic acid** is composed of repeating units of **D-glucuronic acid** and **N-acetylglucosamine**. - It is a crucial component of **extracellular matrix** and **synovial fluid**, providing lubrication and shock absorption. *Dermatan sulfate* - **Dermatan sulfate** contains **L-iduronic acid** as its principal uronic acid, with some glucuronic acid also present. - It plays a significant role in **skin structure**, **blood vessel elasticity**, and **tissue repair**.

Question 800: Which type of bonds in cellulose make it resistant to digestion?

A. α-1,4
B. β-1,4 (Correct Answer)
C. α-1,6
D. β-1,6

Explanation: ***β-1,4*** - The **β-1,4 glycosidic bonds** found in cellulose create long, unbranched chains that are very stable. - Mammalian digestive enzymes, such as **amylase**, are unable to hydrolyze these specific bonds, making cellulose indigestible for humans. *α-1,6* - **α-1,6 glycosidic bonds** are characteristic of branching points in polysaccharides like **glycogen** and **amylopectin**. - These bonds are readily broken down by human digestive enzymes, unlike the bonds in cellulose. *β-1,6* - Although **β-glycosidic bonds** are typically more resistant to digestion than α-bonds, **β-1,6 linkages** are not the primary structural bond responsible for cellulose's indigestibility. - This type of bond is generally less common in major dietary polysaccharides compared to β-1,4. *α-1,4* - **α-1,4 glycosidic bonds** are the primary linkages found in starch (amylose and amylopectin), which is readily digestible by human **amylase**. - These bonds allow for coiled structures that are easily accessed and broken down by digestive enzymes.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

Disorders of Fructose and Galactose Metabolism

Practice Questions

Blood Glucose Regulation

Practice Questions

Diabetes Mellitus: Biochemical Aspects

Practice Questions

Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

Practice Questions

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