Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 811: Fructose is transported by which transporter?

A. GLUT 5 (Correct Answer)
B. GLUT 4
C. GLUT 3
D. GLUT 7

Explanation: ***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.

Question 812: All of the following are regulating enzymes of glycolysis, EXCEPT:

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

Explanation: ***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.

Question 813: Low insulin to glucagon ratio is associated with all of the following processes except:

A. Glycogen breakdown
B. Glycogen synthesis (Correct Answer)
C. Gluconeogenesis
D. Ketogenesis

Explanation: ***Glycogen synthesis*** - A low **insulin-to-glucagon ratio** signifies a state of **fasting** or **catabolism**, where the body aims to raise blood glucose. - **Glycogen synthesis** (glycogenesis) is an anabolic process that stores glucose, which would be inhibited by a low insulin-to-glucagon ratio, making it the exception. *Glycogen breakdown* - A low **insulin-to-glucagon ratio** activates **glycogenolysis** in the liver to release stored glucose, thereby increasing blood glucose levels. - **Glucagon** is the primary hormone stimulating this process during periods of low blood sugar. *Gluconeogenesis* - A low **insulin-to-glucagon ratio** strongly promotes **gluconeogenesis** in the liver and kidneys to synthesize new glucose from non-carbohydrate precursors. - This is a critical process for maintaining blood glucose during prolonged fasting. *Ketogenesis* - In a state of low **insulin-to-glucagon ratio**, the body shifts to utilizing **fatty acids** for energy, leading to increased **ketogenesis** in the liver. - This provides an alternative fuel source (ketone bodies) for tissues, especially the brain, during glucose scarcity.

Question 814: What is the final product of anaerobic glycolysis?

A. Pyruvate
B. Acetyl CoA
C. Lactate (Correct Answer)
D. Oxaloacetate

Explanation: ***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.

Question 815: Where is the GLUT1 transporter primarily located?

A. Liver
B. Brain (Correct Answer)
C. Placenta
D. Heart

Explanation: ***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.

Question 816: Which of the following pairs of compounds are enantiomers?

A. D-glucose and L-glucose (Correct Answer)
B. D-glucose and D-mannose
C. D-glucose and L-fructose
D. D-glucose and D-galactose

Explanation: ***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.

Question 817: An L-isomer of a monosaccharide formed in the human body is

A. L-Fructose
B. L-Erythrose
C. L-Xylulose (Correct Answer)
D. L-Xylose

Explanation: ***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.

Question 818: What are the immediate products formed when fructose 1,6-bisphosphate is cleaved by aldolase in glycolysis?

A. Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (Correct Answer)
B. 1,3-bisphosphoglycerate and 3-phosphoglycerate
C. Glyceraldehyde-3-phosphate and 3-phosphoglycerate
D. Dihydroxyacetone phosphate and 3-phosphoglycerate

Explanation: ***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 819: Which enzyme is responsible for converting glucose-6-phosphate to glucose, allowing for the mobilization of glucose into the bloodstream?

A. Glucose-6-phosphate dehydrogenase
B. Glucose-6-phosphatase (Correct Answer)
C. Glucokinase
D. Phosphoglucomutase

Explanation: ***Glucose-6-phosphatase*** - This enzyme is crucial for the final step of **gluconeogenesis** and **glycogenolysis**, removing the phosphate group from **glucose-6-phosphate** to release free glucose into the bloodstream. - Its presence is primarily found in the liver and kidneys, allowing these organs to supply glucose for systemic use. *Glucose-6-phosphate dehydrogenase* - This enzyme is involved in the **pentose phosphate pathway**, which generates NADPH and precursors for nucleotide biosynthesis. - It does not directly convert glucose-6-phosphate to free glucose. *Phosphoglucomutase* - This enzyme catalyzes the interconversion of **glucose-1-phosphate** and **glucose-6-phosphate**. - Its role is in glycogen synthesis and breakdown, but not in releasing free glucose from glucose-6-phosphate. *Glucokinase* - This enzyme phosphorylates glucose to **glucose-6-phosphate**, primarily in the liver and pancreatic beta cells. - It performs the opposite function of what is required to release glucose into the bloodstream.

Question 820: Which enzyme involved in glucose metabolism is most significantly influenced by insulin?

A. Glucokinase (Correct Answer)
B. Hexokinase
C. Glucose-6-phosphatase
D. Adenylate kinase

Explanation: ***Glucokinase*** - **Insulin** significantly **induces** the synthesis of **glucokinase** in the liver, leading to increased glucose phosphorylation and uptake. - Glucokinase has a **high Km** for glucose, meaning it is only active when glucose levels are high, effectively clearing excess glucose from the blood after a meal. *Hexokinase* - **Hexokinase** is saturated at normal glucose levels and is found in most tissues, but its activity is primarily inhibited by its product, **glucose-6-phosphate**, not directly regulated by insulin in a major way. - Unlike glucokinase, hexokinase has a **low Km** for glucose, allowing it to efficiently phosphorylate glucose even at low concentrations. *Glucose-6-phosphatase* - **Glucose-6-phosphatase** is primarily involved in **gluconeogenesis** and glycogenolysis, releasing free glucose into the bloodstream, and its activity is *decreased* by insulin (insulin suppresses glucose production). - It is not an enzyme of **glycolysis** (the breakdown of glucose) but rather of glucose synthesis or release. *Adenylate kinase* - **Adenylate kinase** interconverts adenine nucleotides (ATP, ADP, AMP) and is not directly involved in either glycolysis or its regulation by insulin. - Its main role is to maintain **cellular energy homeostasis** by rapidly balancing the levels of these nucleotides.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

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Gluconeogenesis: Reactions and Regulation

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Glycogen Metabolism: Synthesis and Breakdown

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Glycogen Storage Diseases

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Pentose Phosphate Pathway

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Metabolism of Fructose and Galactose

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Disorders of Fructose and Galactose Metabolism

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Blood Glucose Regulation

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Diabetes Mellitus: Biochemical Aspects

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Glycosylation and Glycoproteins

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Lactose Intolerance and Galactosemia

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