Carbohydrate Metabolism Practice Questions

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**.

Q: What is the number of ATP produced by red blood cells (RBC) during glycolysis when utilizing the Rapoport-Luebering pathway?

1. ***Correct Option: 1*** - The Rapoport-Luebering pathway (2,3-BPG shunt) bypasses the **phosphoglycerate kinase** step in glycolysis - This bypass sacrifices **1 ATP molecule** that would normally be generated at this step - Normal glycolysis produces a net of 2 ATP per glucose (4 ATP produced - 2 ATP consumed) - With the Rapoport-Luebering shunt active, one of the two 1,3-bisphosphoglycerate molecules is diverted through the shunt - This reduces the total ATP yield from 4 to 3 in the payoff phase - **Net yield: 3 ATP (payoff) - 2 ATP (investment) = 1 ATP per glucose** *Incorrect Option: 2* - This represents the net ATP yield from **normal glycolysis without** the Rapoport-Luebering pathway - When the 2,3-BPG shunt is active, ATP production is reduced by one molecule - This option would be correct if the question asked about standard glycolysis *Incorrect Option: 6* - This value vastly exceeds the actual ATP production in RBC glycolysis - Even normal glycolysis (without any shunt) only yields a net of 2 ATP, not 6 - This may represent a confusion with ATP production from other metabolic pathways *Incorrect Option: 8* - This number is significantly higher than any ATP yield from glycolysis alone - The maximum theoretical net ATP from glycolysis is only 2 ATP under normal conditions - This option has no basis in the biochemistry of RBC glucose metabolism *Incorrect Option: 0* - While the Rapoport-Luebering pathway reduces ATP production, it does not eliminate it completely - The **pyruvate kinase** step still generates 2 ATP molecules (one from each of the two phosphoenolpyruvate molecules) - Even with one ATP lost to the shunt, the net yield remains positive at 1 ATP

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: 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: 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: A 3-year-old child was brought to the emergency room with complaints of hematuria after accidental ingestion of naphthalene balls, which indicates:

G6PD deficiency. ***G6PD deficiency*** - Ingestion of naphthalene balls (mothballs) contains **naphthalene**, a potent oxidant known to trigger **hemolysis** in individuals with G6PD deficiency. - This hemolysis leads to the rapid destruction of red blood cells, causing **hemoglobinuria** which manifests as gross hematuria or **dark urine**. *Purpura* - Purpura refers to **purple skin lesions** resulting from bleeding under the skin, often associated with platelet disorders or vasculitis. - While naphthalene exposure can cause hemolysis, purpura is **not the primary or direct symptom** of naphthalene-induced hemolytic anemia. *Black water fever* - Black water fever is a severe complication of **malaria** (especially *Plasmodium falciparum*) characterized by widespread intravascular hemolysis and **dark brown/black urine** due to hemoglobinuria. - It is **caused by malarial infection**, not naphthalene ingestion, although both can present with dark urine due to hemolysis. *Hereditary spherocytosis* - This is an **inherited disorder** of red blood cell membranes, leading to fragile, spherical red blood cells that are prematurely destroyed in the spleen. - While it causes chronic hemolytic anemia, it is a **genetic condition** and not triggered by acute ingestion of oxidants like naphthalene.

Q: Which of the following inhibits pyruvate kinase?

ATP. ***ATP*** - **ATP** acts as an **allosteric inhibitor** of pyruvate kinase, signaling a high-energy state within the cell. - When ATP levels are elevated, feedback inhibition slows down glycolysis, conserving glucose for other pathways. - This is a classic example of negative feedback regulation in metabolic pathways. *Insulin* - **Insulin** generally **activates** pyruvate kinase by inducing its synthesis and promoting dephosphorylation (active form). - This enhances glucose utilization and storage in response to high blood glucose levels. - Insulin favors the fed state and promotes glycolysis. *Fructose-1,6-bisphosphate* - **Fructose-1,6-bisphosphate** is a potent **activator** of pyruvate kinase through feed-forward stimulation. - Its accumulation signals high flux through early glycolysis, prompting the pathway to proceed to completion. - This ensures efficient conversion of glucose to pyruvate when glycolysis is active. *None of the options* - This option is incorrect because **ATP** clearly functions as an allosteric inhibitor of pyruvate kinase. - Pyruvate kinase regulation is crucial for controlling glycolysis rate in response to cellular energy demands.

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

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 2

Glucose can be synthesized from all except:

Accepted Answer

Acetoacetate

Answer

Amino acids

Answer

Glycerol

Answer

Lactic acid

Question 3

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 4

What is the number of ATP produced by red blood cells (RBC) during glycolysis when utilizing the Rapoport-Luebering pathway?

Accepted Answer

1

Answer

6

Answer

8

Answer

0

Question 5

The rate-limiting step in glycolysis is catalyzed by?

Accepted Answer

Phosphofructokinase

Answer

Enolase

Answer

Glucokinase

Answer

Pyruvate kinase

Question 6

In which anatomical structure is keratan sulfate I primarily located?

Accepted Answer

Cornea

Answer

Skin

Answer

Bone

Answer

Lung

Question 7

Which mucopolysaccharide does not contain uronic acid?

Accepted Answer

Keratan sulfate

Answer

Hyaluronic acid

Answer

Heparin

Answer

Dermatan sulfate

Question 8

A 3-year-old child was brought to the emergency room with complaints of hematuria after accidental ingestion of naphthalene balls, which indicates:

Accepted Answer

G6PD deficiency

Answer

Purpura

Answer

Black water fever

Answer

Hereditary spherocytosis

Question 9

Which of the following inhibits pyruvate kinase?

Accepted Answer

ATP

Answer

Insulin

Answer

Fructose -1,6 bisphosphate

Answer

None of the options

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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