Carbohydrate Metabolism — MCQs
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Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?
Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
Which tissue cannot convert glucose 6-phosphate to free glucose due to lack of glucose-6-phosphatase?
Repetitive chains of glucosamine with uronic acid are seen in ?
What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?
Which of the following statements BEST describes the net ATP production in glycolysis?
Glucose is converted to glucuronate by which process?
Which of the following substances does not inhibit glycolysis?
Glycogen synthase is activated by?
Which one of the following statements concerning gluconeogenesis is correct?
Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs
Question 761: Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?
- A. Glucose-6-phosphate (Correct Answer)
- B. Uridine diphosphoglucose
- C. Fructose-6-phosphate
- D. Galactose-1-phosphate
Explanation: ***Glucose-6-phosphate*** - In **glycogenolysis**, **glycogen phosphorylase** breaks down glycogen into **glucose-1-phosphate**, which is then converted into **glucose-6-phosphate** by **phosphoglucomutase**. - In **glycolysis**, **glucose-6-phosphate** is isomerized to **fructose-6-phosphate** by **phosphoglucose isomerase**, committing it to the glycolytic pathway. - In **gluconeogenesis**, **glucose-6-phosphate** is the final product formed from other precursors; it can then be dephosphorylated to free glucose by **glucose-6-phosphatase**. *Galactose-1-phosphate* - This is an intermediate specifically in **galactose metabolism**, not directly involved in the central common pathways of glycogenolysis, glycolysis, or gluconeogenesis. - It is converted to **glucose-1-phosphate** via the **Leloir pathway** (involving **galactose-1-phosphate uridylyltransferase**), which can then enter glycogen metabolism. *Uridine diphosphoglucose* - **UDP-glucose** is crucial for **glycogen synthesis** (**glycogenesis**), serving as the activated glucose donor. - It is not directly a metabolite in the catabolic process of glycogenolysis, nor is it a direct intermediate in glycolysis or gluconeogenesis. *Fructose-6-phosphate* - **Fructose-6-phosphate** is a key intermediate in **glycolysis** and **gluconeogenesis**, specifically downstream from **glucose-6-phosphate**. - However, it is not directly produced from glycogenolysis; **glucose-6-phosphate** is the direct link between glycogenolysis and glycolysis.
Question 762: Which enzyme is primarily associated with the reduction of NADP+ to NADPH in the pentose phosphate pathway?
- A. G6PD (Correct Answer)
- B. APDH
- C. α-keto glutarate dehydrogenases
- D. None of the options
Explanation: ***G6PD*** - **Glucose-6-phosphate dehydrogenase (G6PD)** catalyzes the first committed step in the pentose phosphate pathway, converting **glucose-6-phosphate** to **6-phosphogluconolactone**. - This reaction involves the reduction of **NADP+ to NADPH**, making G6PD the primary enzyme for NADPH production in this pathway. *APDH* - **APDH (adenosine phosphosulfate reductase)** is involved in sulfur metabolism and has no direct role in the pentose phosphate pathway or NADPH production. - This enzyme primarily functions in prokaryotes for the **reduction of APS (adenosine 5'-phosphosulfate)**. *α-keto glutarate dehydrogenases* - **Alpha-ketoglutarate dehydrogenase** is a mitochondrial enzyme part of the **Krebs cycle**, converting **alpha-ketoglutarate to succinyl-CoA**. - This enzyme is crucial for ATP production and generates **NADH**, not NADPH, in its reaction. *None of the options* - This option is incorrect because **G6PD** is indeed the primary enzyme responsible for NADPH generation in the pentose phosphate pathway.
Question 763: Which tissue cannot convert glucose 6-phosphate to free glucose due to lack of glucose-6-phosphatase?
- A. Liver
- B. Kidney
- C. Adipose tissue
- D. Muscle (Correct Answer)
Explanation: ***Muscle*** - Muscle tissue lacks the enzyme **glucose-6-phosphatase**, which is essential for hydrolyzing glucose 6-phosphate back to **free glucose**. - Therefore, glucose 6-phosphate in muscle is primarily used for **glycolysis** (energy production) or stored as glycogen for local use. *Liver* - The liver contains **glucose-6-phosphatase**, allowing it to convert **glucose 6-phosphate** to **free glucose**. - This capability is crucial for maintaining **blood glucose homeostasis** and releasing glucose into circulation. *Adipose tissue* - Adipose tissue, like muscle, **lacks glucose-6-phosphatase** and cannot convert glucose 6-phosphate back to free glucose. - Glucose 6-phosphate in adipose tissue is primarily channeled into **fatty acid synthesis** and storage. *Kidney* - The kidney, particularly the renal cortex, possesses **glucose-6-phosphatase** and can convert glucose 6-phosphate to **free glucose**. - This contributes to **gluconeogenesis** and release of glucose into the blood, especially during fasting.
Question 764: Repetitive chains of glucosamine with uronic acid are seen in ?
- A. None of the options
- B. Heparan sulphate (Correct Answer)
- C. N-acetylneuraminic acid
- D. Galactosaminoglycan
Explanation: ***Heparan sulphate*** - Heparan sulfate is a **linear polysaccharide** composed of repeating disaccharide units of either **glucosamine** and **glucuronic acid** or **glucosamine** and **iduronic acid** (a uronic acid). - It plays crucial roles in various biological processes, including cell adhesion, cell growth, and coagulation, primarily due to its ability to bind to a wide range of proteins. *N-acetylneuraminic acid* - **N-acetylneuraminic acid** is a type of **sialic acid**, a nine-carbon sugar acid, and is not a repeating chain of glucosamine and uronic acid. - It is often found at the **non-reducing ends of glycan chains** on cell surfaces and in secreted glycoproteins, contributing to cell recognition and signaling. *Galactosaminoglycan* - This term is a general descriptor for a group of glycosaminoglycans that contain **N-acetylgalactosamine** as one of their repeating units, such as **chondroitin sulfate** and **dermatan sulfate**. - While they are repeating disaccharide units, they feature **galactosamine** (or its N-acetylated form) instead of glucosamine and are also linked to uronic acids. *None of the options* - This option is incorrect because **heparan sulfate** accurately describes a polysaccharide with repeating units of glucosamine and uronic acid.
Question 765: What is the net number of ATP molecules and NADH formed in glycolysis per glucose molecule?
- A. 4 ATP, 2 NADH
- B. 4 ATP, 4 NADH
- C. 2 ATP, 4 NADH
- D. 2 ATP, 2 NADH (Correct Answer)
Explanation: **2 ATP, 2 NADH** - Glycolysis has a net yield of **2 molecules of ATP** because 4 ATP molecules are produced, but 2 ATP molecules are consumed during the initial energy investment phase. - **2 molecules of NADH** are also produced during the energy generation phase when glyceraldehyde-3-phosphate is oxidized. *4 ATP, 2 NADH* - While 4 ATP molecules are indeed produced during glycolysis, this option does not account for the **2 ATP molecules consumed** in the initial steps, leading to an incorrect net value. - The production of **2 NADH** is correct, but the ATP count is the gross rather than the net. *4 ATP, 4 NADH* - This option overstates the production of both ATP and NADH. While **4 ATP are produced (gross)**, the net is 2 ATP. - Only **2 NADH** molecules are formed per glucose molecule in glycolysis, not 4. *2 ATP, 4 NADH* - This option accurately reflects the **net ATP yield of 2 molecules**. - However, it exaggerates the production of NADH, as only **2 molecules of NADH** are formed during glycolysis, not 4.
Question 766: Which of the following statements BEST describes the net ATP production in glycolysis?
- A. Glycolysis produces 2 molecules of pyruvate
- B. Glycolysis produces a net gain of 2 ATP per glucose molecule (Correct Answer)
- C. Hexokinase consumes ATP during glycolysis
- D. Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into two three-carbon molecules
Explanation: ***Glycolysis produces a net gain of 2 ATP per glucose molecule*** - In the initial "investment" phase of glycolysis, **2 ATP molecules are consumed** to phosphorylate glucose. - In the subsequent "payoff" phase, **4 ATP molecules are produced** through substrate-level phosphorylation, resulting in a net gain of 2 ATP. *Glycolysis produces 2 molecules of pyruvate* - While glycolysis does produce **2 molecules of pyruvate** from one glucose molecule, this statement describes the end product of the pathway, not the net ATP production. - Pyruvate is a crucial product that can be further metabolized in aerobic or anaerobic conditions, but it does not directly represent the energy yield in terms of ATP. *Hexokinase consumes ATP during glycolysis* - **Hexokinase** is indeed the enzyme that catalyzes the first ATP-consuming step in glycolysis, phosphorylating glucose to glucose-6-phosphate. - However, this statement describes only one aspect of ATP utilization within the pathway and does not account for the total ATP produced or the overall net gain. *Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into two three-carbon molecules* - **Aldolase** is a key enzyme in glycolysis responsible for cleaving **fructose-1,6-bisphosphate** into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. - This step is part of the preparatory phase of glycolysis but does not directly describe the net ATP production.
Question 767: Glucose is converted to glucuronate by which process?
- A. Oxidation of aldehyde group
- B. Oxidation of both
- C. Oxidation of the terminal alcohol group (Correct Answer)
- D. None of the options
Explanation: ***Oxidation of the terminal alcohol group*** - **Glucuronate** is formed by the **oxidation of the C-6 carbon** (the terminal primary alcohol group) of glucose. - This process is crucial for the detoxification of various substances in the body, as glucuronate is a key component in **glucuronidation reactions**. *Oxidation of aldehyde group* - Oxidation of the **aldehyde group (C-1)** of glucose typically forms **gluconic acid**, not glucuronate. - Gluconate is derived from the oxidation of the first carbon, while glucuronate is derived from the oxidation of the last carbon. *Oxidation of both* - If both the aldehyde group (C-1) and the terminal alcohol group (C-6) of glucose were oxidized, it would result in the formation of **glucaric acid** (saccharic acid), not glucuronate. - Glucaric acid has two carboxyl groups, one at each end of the molecule. *None of the options* - This option is incorrect because the specific biochemical pathway for glucuronate formation involves the oxidation of the terminal alcohol group. - The conversion of glucose to glucuronate is a well-established metabolic process.
Question 768: Which of the following substances does not inhibit glycolysis?
- A. Fluoride
- B. Arsenite
- C. Iodoacetate
- D. Fluoroacetate (Correct Answer)
Explanation: ***Fluoroacetate*** - **Fluoroacetate** is not a direct inhibitor of glycolysis. Instead, it is metabolized to **fluorocitrate**, which then acts as an inhibitor of **aconitase** in the **Krebs cycle (TCA cycle)**, thereby affecting cellular respiration at a later stage. - Its primary role in metabolic inhibition is within the **mitochondria**, impacting energy production via the TCA cycle rather than the glycolytic pathway. *Fluoride* - **Fluoride** is a known inhibitor of **enolase**, an enzyme in the penultimate step of glycolysis. - It forms a complex with **magnesium** and **phosphate** to block the active site of enolase, preventing the conversion of 2-phosphoglycerate to phosphoenolpyruvate. *Arsenite* - **Arsenite** inhibits glycolysis by targeting enzymes containing **sulfhydryl (–SH) groups**, particularly **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**, a critical enzyme in the glycolytic pathway. - It also inhibits the **pyruvate dehydrogenase complex** (linking glycolysis to the TCA cycle) and TCA cycle enzymes like **α-ketoglutarate dehydrogenase**, thereby affecting multiple stages of cellular respiration. *Iodoacetate* - **Iodoacetate** is a potent inhibitor of the enzyme **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**. - It specifically alkylates the **cysteine residue** at the active site of GAPDH, preventing the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, thereby blocking glycolysis.
Question 769: Glycogen synthase is activated by?
- A. Insulin (Correct Answer)
- B. Glucagon
- C. AMP
- D. Epinephrine
Explanation: **Insulin** - Insulin activates **glycogen synthase** through a signaling cascade that dephosphorylates the enzyme, shifting it to its active form (glycogen synthase a). - This activation promotes **glycogen synthesis** in the liver and muscles, lowering blood glucose levels. *Glucagon* - **Glucagon** primarily acts to increase blood glucose levels by activating **glycogen phosphorylase** and inhibiting glycogen synthase. - It promotes the breakdown of glycogen (glycogenolysis) rather than its synthesis. *Epinephrine* - **Epinephrine** (adrenaline) also promotes **glycogenolysis** (glycogen breakdown) by activating glycogen phosphorylase. - Its main role is to provide rapid energy during stress, not to store glucose as glycogen. *AMP* - **AMP** (adenosine monophosphate) is a key allosteric activator of **AMP-activated protein kinase (AMPK)**, which phosphorylates and inactivates glycogen synthase. - High AMP levels signal a low energy state, prompting ATP-generating pathways like glycogenolysis, not glycogen synthesis.
Question 770: Which one of the following statements concerning gluconeogenesis is correct?
- A. It occurs primarily in the liver.
- B. It is stimulated by elevated levels of acetyl CoA.
- C. It is important in maintaining blood glucose during the normal overnight fast. (Correct Answer)
- D. It is primarily inhibited by insulin.
Explanation: ***It is important in maintaining blood glucose during the normal overnight fast.*** - **This is the BEST answer** as it emphasizes the **primary physiological role** of gluconeogenesis in human metabolism. - During the **overnight fast** (8-12 hours), hepatic glycogen stores become depleted, making gluconeogenesis the **critical mechanism** to maintain blood glucose for glucose-dependent tissues like the **brain** (requires ~120g glucose/day) and **red blood cells**. - Without gluconeogenesis, blood glucose would drop dangerously during fasting, leading to hypoglycemia and neurological dysfunction. *It occurs primarily in the liver.* - This statement is **technically correct** - the liver accounts for approximately **90%** of total gluconeogenesis under normal conditions. - However, the **kidney cortex** also contributes significantly (10% normally, up to 40% during prolonged fasting), and the **intestine** plays a minor role. - While true, this is more of a **anatomical fact** rather than highlighting the critical physiological importance of the pathway, making it a less comprehensive answer than Option 1. *It is stimulated by elevated levels of acetyl CoA.* - This statement is **biochemically correct** - **Acetyl-CoA** is an important **allosteric activator** of **pyruvate carboxylase**, the first committed enzyme of gluconeogenesis. - However, this represents just **one regulatory mechanism** at the enzymatic level, not the overall physiological significance. - Primary regulation occurs through **hormones** (glucagon, cortisol, epinephrine) that coordinate the entire pathway, making this a narrower answer than Option 1. *It is primarily inhibited by insulin.* - This statement is also **correct** - **Insulin** is the primary hormonal **inhibitor** of gluconeogenesis. - Insulin suppresses gluconeogenesis by inhibiting key enzymes (PEPCK, glucose-6-phosphatase) and decreasing transcription of gluconeogenic genes. - However, this describes **inhibition** rather than the positive physiological role, making it less representative of gluconeogenesis's essential function than Option 1. **Note:** All four statements are technically correct, but Option 1 best captures the **essential physiological importance** of gluconeogenesis in human metabolism, which is why it is the preferred answer for this question.
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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