Carbohydrate Metabolism Practice Questions

Q: Both the liver and muscle contain glycogen, yet, unlike the liver, muscle is not capable of contributing glucose to the circulation. What is the reason for this?

Does not have the enzyme glucose-6-phosphatase. ***Does not have the enzyme glucose-6-phosphatase*** - **Glucose-6-phosphatase** is the enzyme responsible for dephosphorylating **glucose-6-phosphate** to glucose, allowing it to exit the cell and enter the bloodstream. - Since muscle cells lack this enzyme, the glucose-6-phosphate produced from glycogenolysis is trapped within the muscle cell and used for its own energy needs. *Glycolytic activity consumes all of the glucose it generates, preventing release into circulation.* - While muscle does utilize the glucose it generates for its own energy via glycolysis, the fundamental reason for trapping glucose within the cell is the absence of **glucose-6-phosphatase**, not just the consumption itself. - If **glucose-6-phosphatase** were present, the muscle could still release glucose even if some was used for glycolysis, especially under conditions of high glycogenolysis. *Does not have the enzyme glucose-1-phosphatase.* - **Glucose-1-phosphatase** is not a commonly recognized enzyme in glucose metabolism; the conversion between glucose-1-phosphate and glucose-6-phosphate is catalyzed by **phosphoglucomutase**. - Therefore, the absence of an enzyme with this specific name is not the reason muscle cannot release glucose into circulation. *Does not have the enzyme glycogen phosphorylase.* - Muscle tissue readily expresses **glycogen phosphorylase**, which is the enzyme responsible for breaking down glycogen into **glucose-1-phosphate** during glycogenolysis. - If muscle lacked **glycogen phosphorylase**, it would not be able to break down glycogen at all, which is contrary to its role as an energy reserve.

Q: What type of carbohydrate is inulin classified as?

Fructosan. ***Fructosan*** - **Inulin** is a naturally occurring **polysaccharide** composed primarily of **fructose** units. - As such, it is classified as a **fructosan**, a type of **fructan**, meaning its main monosaccharide component is fructose. *Glucosan* - A **glucosan** is a polysaccharide primarily made up of **glucose** units, such as **starch** or **glycogen**. - Inulin's monomeric units are predominantly fructose, not glucose. *Galactosan* - A **galactosan** is a polysaccharide primarily composed of **galactose** units. - Inulin does not primarily consist of galactose units. *Mannosan* - A **mannosan** is a polysaccharide primarily composed of **mannose** units. - Inulin's structure is based on fructose, not mannose.

Q: Which of the following metabolic pathways in carbohydrate metabolism is required for the synthesis of nucleic acids?

HMP shunt. ***HMP shunt*** - The **hexose monophosphate (HMP) shunt**, also known as the **pentose phosphate pathway**, is crucial for producing **ribose-5-phosphate**. - **Ribose-5-phosphate** is a direct precursor for the synthesis of **nucleotides** and subsequently **nucleic acids** (DNA and RNA). *Gluconeogenesis* - This pathway is responsible for the synthesis of **glucose from non-carbohydrate precursors**, primarily to maintain blood glucose levels during fasting. - It does not directly produce components needed for nucleic acid synthesis. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down **glucose into pyruvate**, generating ATP. - While it produces intermediates, it is not directly involved in creating the specific pentose sugars required for nucleic acids. *Glycogenesis* - **Glycogenesis** is the process of synthesizing **glycogen from glucose** for storage in the liver and muscles. - This pathway is focused on glucose storage and has no direct role in nucleic acid synthesis.

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: 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: 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: 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**.

Question 1

Both the liver and muscle contain glycogen, yet, unlike the liver, muscle is not capable of contributing glucose to the circulation. What is the reason for this?

Accepted Answer

Does not have the enzyme glucose-6-phosphatase

Answer

Glycolytic activity consumes all of the glucose it generates, preventing release into circulation.

Answer

Does not have the enzyme glucose-1-phosphatase.

Answer

Does not have the enzyme glycogen phosphorylase.

Question 2

What type of carbohydrate is inulin classified as?

Accepted Answer

Fructosan

Answer

Glucosan

Answer

Galactosan

Answer

Mannosan

Question 3

Which of the following metabolic pathways in carbohydrate metabolism is required for the synthesis of nucleic acids?

Accepted Answer

HMP shunt

Answer

Gluconeogenesis

Answer

Glycolysis

Answer

Glycogenesis

Question 4

Which of the following statements is true regarding the pentose phosphate pathway?

Accepted Answer

The pentose phosphate pathway is a direct oxidative pathway of glucose metabolism

Answer

Glucose is the only substrate that can enter this pathway

Answer

The pathway has only monophosphates as intermediates

Answer

None of the options

Question 5

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 6

How many stereoisomers are possible for an aldohexose?

Accepted Answer

16

Answer

32

Answer

64

Answer

8

Question 7

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 8

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 9

Glucose can be synthesized from all except:

Accepted Answer

Acetoacetate

Answer

Amino acids

Answer

Glycerol

Answer

Lactic acid

Question 10

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.

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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