Carbohydrate Metabolism Practice Questions

Q: Which enzyme catalyzes an important rate-limiting step in gluconeogenesis that bypasses the phosphofructokinase-1 reaction of glycolysis?

Fructose 1,6-bisphosphatase. ***Fructose 1,6-bisphosphatase*** - **Fructose 1,6-bisphosphatase** is a key regulatory enzyme in gluconeogenesis, catalyzing the irreversible hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate. - This step bypasses the irreversible phosphofructokinase-1 (PFK-1) reaction of glycolysis, making it a critical **rate-limiting and regulatory step** for glucose synthesis. - It is inhibited by AMP and fructose 2,6-bisphosphate, providing important metabolic regulation. *Glucokinase* - **Glucokinase** is an enzyme involved in glycolysis, phosphorylating glucose to glucose-6-phosphate, primarily in the liver and pancreatic beta cells. - Its high Km for glucose means it is active primarily when glucose concentrations are high, acting as a **glucose sensor** rather than a gluconeogenic enzyme. - In gluconeogenesis, **glucose-6-phosphatase** (not glucokinase) catalyzes the final step. *Fructokinase* - **Fructokinase** is an enzyme that phosphorylates fructose to fructose-1-phosphate, mainly in the liver. - It is involved in fructose metabolism, not a direct enzyme in the gluconeogenic pathway for glucose synthesis from non-carbohydrate precursors. *Hexokinase* - **Hexokinase** phosphorylates glucose to glucose-6-phosphate, the first step of glycolysis, in most tissues. - While it initiates glucose breakdown, it does not play a direct role in the synthesis of glucose during gluconeogenesis; rather, **glucose-6-phosphatase** is involved in the final step of gluconeogenesis.

Q: Which of the following is not a reducing sugar?

Sucrose. ***Sucrose*** - Sucrose is a **disaccharide** composed of **glucose and fructose** linked by an **α-1,2-glycosidic bond**. - This specific bond involves the **anomeric carbons** of both monosaccharides, rendering them unavailable to open into aldehyde or ketone groups required for reducing sugar activity. *Lactose* - Lactose is a **reducing disaccharide** made of glucose and galactose, having a **free anomeric carbon** on its glucose unit. - This allows it to open into an aldehyde form and act as a reducing agent. *Glucose* - Glucose is a **monosaccharide** and a **reducing sugar** due to its **free aldehyde group** in its open-chain form. - The aldehyde group can be oxidized, allowing glucose to reduce other compounds. *Fructose* - Fructose is a **monosaccharide** that, despite being a **ketose** (containing a ketone group), can isomerize to an **aldose** in alkaline solutions. - This isomerization via the **enediol intermediate** allows it to act as a reducing sugar.

Q: Fructose is transported by which transporter?

GLUT 5. ***GLUT 5*** - **GLUT 5** is the primary transporter responsible for **fructose absorption** in the intestine. - It facilitates **fructose diffusion** down its concentration gradient, independent of sodium. *GLUT 4* - **GLUT 4** is an **insulin-dependent** glucose transporter primarily found in **adipose tissue** and **skeletal muscle**. - Its translocation to the cell membrane is stimulated by insulin, allowing glucose uptake in response to fed states. *GLUT 3* - **GLUT 3** is a high-affinity glucose transporter found predominantly in **neurons** and the **brain**. - It ensures a constant supply of glucose to these vital tissues, even at low blood glucose concentrations. *GLUT 7* - **GLUT 7** is primarily expressed in the **endoplasmic reticulum** and is involved in **glucose-6-phosphate transport**. - It plays a crucial role in **gluconeogenesis** and glycogenolysis in the liver.

Q: All of the following are regulating enzymes of glycolysis, EXCEPT:

Enolase. ***Enolase*** - **Enolase** catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate, which is a reversible reaction that is not considered a primary regulatory step in glycolysis. - While it is an important enzyme in glycolysis, its activity is not a major **control point** for the overall pathway flux. *Hexokinase* - **Hexokinase** is a key regulatory enzyme in glycolysis, catalyzing the first irreversible step: the phosphorylation of glucose to glucose-6-phosphate. - It is inhibited by its product, **glucose-6-phosphate**, providing feedback regulation. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final irreversible step of glycolysis, converting phosphoenolpyruvate to pyruvate. - It is a crucial regulatory point, allosterically activated by **fructose-1,6-bisphosphate** and inhibited by ATP and alanine. *Phosphofructokinase I* - **Phosphofructokinase I (PFK-1)** is considered the most important regulatory enzyme in glycolysis, catalyzing the irreversible phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate. - Its activity is tightly controlled by allosteric activators like **AMP** and **fructose-2,6-bisphosphate**, and inhibitors like ATP and citrate.

Q: Which of the following processes is inhibited by glucagon?

Glycogenesis. ***Glycogenesis*** - **Glucagon** is a hormone that counteracts the effects of insulin, primarily to raise **blood glucose levels**. Therefore, it inhibits processes that store glucose, such as **glycogenesis** (the synthesis of glycogen from glucose). - High glucagon levels signal a need for glucose release, thus stopping processes that would remove glucose from the bloodstream. *Glycogenolysis* - **Glycogenolysis** is the breakdown of **glycogen** into **glucose**, which increases blood glucose levels. - **Glucagon** actually **stimulates**, rather than inhibits, glycogenolysis to release stored glucose from the liver. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of **glucose** from non-carbohydrate precursors (e.g., amino acids, glycerol). - **Glucagon** is a potent **stimulator** of gluconeogenesis, particularly during fasting states, to maintain blood glucose. *Lipolysis* - **Lipolysis** is the breakdown of **triglycerides** into **fatty acids** and **glycerol**, which can be used for energy or as substrates for gluconeogenesis. - **Glucagon** **stimulates** lipolysis in **adipose tissue** to provide alternative fuel sources and precursors for glucose production.

Q: What is the final product of anaerobic glycolysis?

Lactate. ***Lactate*** - Under **anaerobic conditions**, pyruvate is converted to **lactate** by **lactate dehydrogenase** to regenerate NAD+ for glycolysis continuation. - This process allows glycolysis to proceed in the absence of oxygen, providing a quick but limited supply of **ATP**. *Pyruvate* - **Pyruvate** is the *end product of aerobic glycolysis* but not the final product of *anaerobic* glycolysis. - In aerobic conditions, pyruvate enters the **Krebs cycle** after being converted to acetyl CoA. *Acetyl CoA* - **Acetyl CoA** is formed from pyruvate under **aerobic conditions** and enters the **Krebs cycle**. - It is not a product of *anaerobic glycolysis*; its formation requires oxygen. *Oxaloacetate* - **Oxaloacetate** is an intermediate of the **Krebs cycle** and plays a role in gluconeogenesis. - It is not directly produced in either aerobic or anaerobic glycolysis.

Q: Where is the GLUT1 transporter primarily located?

Brain. ***Brain*** - The **GLUT1 transporter** is primarily located in the **brain**, particularly in the **blood-brain barrier (BBB)** endothelial cells. - It is the major glucose transporter responsible for facilitating glucose transport from blood into the brain, which is essential since the brain depends almost exclusively on glucose for energy. - GLUT1 is constitutively expressed and provides continuous glucose supply to meet the brain's high metabolic demands. - This makes the brain the classic and primary site emphasized in medical textbooks for GLUT1 expression. *Placenta* - The **placenta** also has high expression of **GLUT1**, which facilitates glucose transport from maternal circulation to the fetus. - This is the second major site of GLUT1 expression and is critical for fetal development. - However, when asked about the "primary" location, brain (BBB) is the standard answer in medical education. *Heart* - While **GLUT1** is present in the heart, the heart muscle primarily uses **GLUT4**, which is insulin-dependent and responsible for most glucose uptake. - GLUT1 provides basal glucose uptake but is not the predominant transporter in cardiac tissue. *Liver* - The **liver** primarily uses **GLUT2**, a low-affinity, high-capacity bidirectional transporter that allows both glucose uptake and release depending on blood glucose levels. - GLUT2 is essential for the liver's role in glucose homeostasis, while GLUT1 is minimally expressed in hepatocytes.

Q: Which of the following pairs of compounds are enantiomers?

D-glucose and L-glucose. ***D-glucose and L-glucose*** - **Enantiomers** are stereoisomers that are **non-superimposable mirror images** of each other. - **D-glucose** and **L-glucose** fit this definition perfectly, as their configurations at all chiral centers are inverted, making them mirror images. *D-glucose and D-mannose* - These are **epimers**, specifically **C2 epimers**, meaning they differ in configuration at only one chiral center (the second carbon atom). - They are not mirror images of each other. *D-glucose and L-fructose* - These are **different types of sugar** (glucose is an aldohexose, fructose is a ketohexose) and are not structural isomers or stereoisomers in the sense of being enantiomers or epimers. - They also contain different functional groups and different carbon chain structures. *D-glucose and D-galactose* - These are also **epimers**, specifically **C4 epimers**, as they differ in configuration only at the fourth carbon atom. - Like epimers, they are considered diastereomers and are not mirror images.

Q: An L-isomer of a monosaccharide formed in the human body is

L-Xylulose. ***L-Xylulose*** - **L-xylulose** is a naturally occurring L-isomer of a monosaccharide that plays a role in the **uronic acid pathway** (glucuronic acid pathway) and is an intermediate in **essential pentosuria**. - Its presence in the human body is part of normal metabolic processes, unlike most other monosaccharides which are primarily found in their D-configurations. - L-xylulose is formed from D-glucuronic acid and is the only significant L-sugar found in human metabolism. *L-Fructose* - **Fructose** primarily exists as a **D-isomer** in nature and is a common dietary sugar; the L-isomer is generally not found or synthesized in significant amounts in the human body. - While D-fructose is metabolized, L-fructose would require specific enzymes not typically present, preventing its significant formation or use. *L-Erythrose* - **Erythrose** is a **tetrose sugar** that, like most monosaccharides, primarily occurs as a **D-isomer** in biological systems. - L-erythrose is not known to be naturally formed or to have any significant metabolic role in the human body. *L-Xylose* - **Xylose** is a common sugar, particularly in plant matter, and exists predominantly as **D-xylose** in biological contexts. - The L-isomer of xylose is not typically synthesized or found in significant amounts within the human body.

Q: What are the immediate products formed when fructose 1,6-bisphosphate is cleaved by aldolase in glycolysis?

Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. ***Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate*** - **Fructose 1,6-bisphosphate** is cleaved by the enzyme **aldolase** into these two 3-carbon isomers. - This is the fourth step in **glycolysis**, a key pathway for energy production. *Glyceraldehyde-3-phosphate and 3-phosphoglycerate* - **3-phosphoglycerate** is formed later in glycolysis from 1,3-bisphosphoglycerate, not directly from fructose 1,6-bisphosphate. - The immediate products of fructose 1,6-bisphosphate cleavage are its two 3-carbon isomers. *1,3-bisphosphoglycerate and 3-phosphoglycerate* - **1,3-bisphosphoglycerate** is an intermediate formed during the oxidation of glyceraldehyde-3-phosphate, further down the glycolytic pathway. - Neither of these are direct products of the initial cleavage of fructose 1,6-bisphosphate. *Dihydroxyacetone phosphate and 3-phosphoglycerate* - While **dihydroxyacetone phosphate** is an initial product, **3-phosphoglycerate** is not. - The other initial product is **glyceraldehyde-3-phosphate**, which is an isomer of dihydroxyacetone phosphate.

Question 1

Which enzyme catalyzes an important rate-limiting step in gluconeogenesis that bypasses the phosphofructokinase-1 reaction of glycolysis?

Accepted Answer

Fructose 1,6-bisphosphatase

Answer

Glucokinase

Answer

Fructokinase

Answer

Hexokinase

Question 2

Which of the following is not a reducing sugar?

Accepted Answer

Sucrose

Answer

Lactose

Answer

Glucose

Answer

Fructose

Question 3

Fructose is transported by which transporter?

Accepted Answer

GLUT 5

Answer

GLUT 4

Answer

GLUT 3

Answer

GLUT 7

Question 4

All of the following are regulating enzymes of glycolysis, EXCEPT:

Accepted Answer

Enolase

Answer

Pyruvate kinase

Answer

Hexokinase

Answer

Phosphofructokinase I

Question 5

Which of the following processes is inhibited by glucagon?

Accepted Answer

Glycogenesis

Answer

Gluconeogenesis

Answer

Lipolysis

Answer

Glycogenolysis

Question 6

What is the final product of anaerobic glycolysis?

Accepted Answer

Lactate

Answer

Pyruvate

Answer

Acetyl CoA

Answer

Oxaloacetate

Question 7

Where is the GLUT1 transporter primarily located?

Accepted Answer

Brain

Answer

Liver

Answer

Placenta

Answer

Heart

Question 8

Which of the following pairs of compounds are enantiomers?

Accepted Answer

D-glucose and L-glucose

Answer

D-glucose and D-mannose

Answer

D-glucose and L-fructose

Answer

D-glucose and D-galactose

Question 9

An L-isomer of a monosaccharide formed in the human body is

Accepted Answer

L-Xylulose

Answer

L-Fructose

Answer

L-Erythrose

Answer

L-Xylose

Question 10

What are the immediate products formed when fructose 1,6-bisphosphate is cleaved by aldolase in glycolysis?

Accepted Answer

Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate

Answer

1,3-bisphosphoglycerate and 3-phosphoglycerate

Answer

Glyceraldehyde-3-phosphate and 3-phosphoglycerate

Answer

Dihydroxyacetone phosphate and 3-phosphoglycerate

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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