Carbohydrate Metabolism Practice Questions

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

Muscle phosphorylase. ***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.

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

Requires the participation of biotin. ***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.

Q: How many stereoisomers are possible for an aldohexose?

16. ***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.

Q: Glucose can be synthesized from all except:

Acetoacetate. ***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**.

Q: Galactosemia is caused by a deficiency of which enzyme?

Galactose 1 phosphate uridyl transferase. ***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.

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

Cornea. ***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.

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

Phosphofructokinase. ***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.

Q: Which mucopolysaccharide does not contain uronic acid?

Keratan sulfate. ***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**.

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

β-1,4. ***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.

Question 1

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

Accepted Answer

Muscle phosphorylase

Answer

Glucokinase

Answer

Phosphoglucomutase

Answer

G-6-phosphatase

Question 2

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

Accepted Answer

Requires the participation of biotin

Answer

Occurs exclusively in the cytosol.

Answer

Is inhibited by an elevated level of glucagon

Answer

Involves lactate as an intermediate

Question 3

How many stereoisomers are possible for an aldohexose?

Accepted Answer

16

Answer

32

Answer

64

Answer

8

Question 4

Glucose can be synthesized from all except:

Accepted Answer

Acetoacetate

Answer

Amino acids

Answer

Glycerol

Answer

Lactic acid

Question 5

Galactosemia is caused by a deficiency of which enzyme?

Accepted Answer

Galactose 1 phosphate uridyl transferase

Answer

Aldolase B

Answer

UDP galactose 4 epimerase

Answer

Fructokinase

Question 6

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

Accepted Answer

Glucagon decreases fructose 2,6-bisphosphate levels, stimulating gluconeogenesis.

Answer

Insulin increases the levels of fructose 2,6-bisphosphate, which inhibits gluconeogenesis.

Answer

Insulin acts through a kinase to promote glycolysis.

Answer

Fructose 2,6-bisphosphate is an activator of glycolysis.

Question 7

In which anatomical structure is keratan sulfate I primarily located?

Accepted Answer

Cornea

Answer

Skin

Answer

Bone

Answer

Lung

Question 8

The rate-limiting step in glycolysis is catalyzed by?

Accepted Answer

Phosphofructokinase

Answer

Enolase

Answer

Glucokinase

Answer

Pyruvate kinase

Question 9

Which mucopolysaccharide does not contain uronic acid?

Accepted Answer

Keratan sulfate

Answer

Hyaluronic acid

Answer

Heparin

Answer

Dermatan sulfate

Question 10

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

Accepted Answer

β-1,4

Answer

α-1,4

Answer

α-1,6

Answer

β-1,6

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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