Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism Indian Medical PG Practice Questions and MCQs

Question 821: Which compound serves as a central metabolic intermediate that connects glycolysis with glycogenesis and glycogenolysis?

A. Glucose 1,6-bisphosphate
B. Glucose 1-phosphate
C. Glucose 6-phosphate (Correct Answer)
D. Fructose 1,6-bisphosphate

Explanation: ***Glucose 6-phosphate*** - **Glucose 6-phosphate** is the central metabolic hub connecting glycolysis, glycogenesis, and glycogenolysis - Can be **isomerized to fructose 6-phosphate** to enter glycolysis for energy production - Can be **converted to glucose 1-phosphate** via phosphoglucomutase for glycogen synthesis (glycogenesis) - During **glycogenolysis**, it is formed from glucose 1-phosphate and can either enter glycolysis or be dephosphorylated to free glucose (in liver) for release into bloodstream - This unique position makes it the **key branch point** connecting all three pathways *Glucose 1,6-bisphosphate* - Acts as a **cofactor for phosphoglucomutase enzyme**, facilitating the interconversion between glucose 1-phosphate and glucose 6-phosphate - Not a direct metabolic intermediate in the main pathways - Present in trace amounts and functions catalytically rather than as a pathway substrate *Glucose 1-phosphate* - Direct product of **glycogen breakdown** (glycogenolysis) via glycogen phosphorylase - Must be **converted to glucose 6-phosphate** by phosphoglucomutase before entering glycolysis - Converted to **UDP-glucose** for glycogen synthesis (glycogenesis) - Does not directly connect all three pathways as it requires conversion to G6P first *Fructose 1,6-bisphosphate* - Committed intermediate **exclusively in glycolysis**, formed by phosphofructokinase-1 (PFK-1) - Represents the **committed step** in glycolysis (irreversible under physiological conditions) - Does not participate in glycogenesis or glycogenolysis - Located downstream of the branch point, after pathway commitment

Question 822: Which of the following is a co-factor for phosphofructokinase?

A. Magnesium (Mg²⁺) (Correct Answer)
B. Manganese (Mn²⁺)
C. Iron (Fe²⁺)
D. Zinc (Zn)

Explanation: ***Magnesium (Mg²⁺)*** - **Magnesium** is a critical **cofactor for phosphofructokinase (PFK)**, an enzyme central to glycolysis. - It forms an **ATP-Mg²⁺ complex**, which is the actual substrate for PFK, and also stabilizes the enzyme's structure. *Manganese (Mn²⁺)* - While **manganese** can act as a cofactor for some enzymes, it is **not typically recognized** as a significant cofactor for phosphofructokinase. - It often plays roles in enzymes involved in **oxidative phosphorylation** and **antioxidant defense**. *Iron (Fe²⁺)* - **Iron** is essential for various enzymes, particularly those involved in **electron transport** and oxygen binding (e.g., in hemoglobin and cytochromes). - However, **iron is not a cofactor** for phosphofructokinase. *Zinc (Zn)* - **Zinc** is a critical cofactor for numerous enzymes, particularly those involved in genetic material replication and repair, and immune function. - It is **not a cofactor for phosphofructokinase**, which primarily requires magnesium.

Question 823: Muscles cannot release free glucose from glycogen stores because of a deficiency of:

A. Glucose-6-phosphatase (Correct Answer)
B. Hexokinase
C. Phosphoglucomutase
D. Glycogen phosphorylase

Explanation: ***Glucose-6-phosphatase*** - **Glucose-6-phosphatase** is the enzyme that dephosphorylates glucose-6-phosphate to free glucose, allowing its release into the bloodstream. - This enzyme is **physiologically absent in muscle tissue** (present only in liver and kidneys), meaning muscles can break down glycogen for their own energy needs but cannot release free glucose into circulation. - This ensures that muscle glycogen stores are reserved exclusively for muscle's own metabolic needs during contraction. *Glycogen phosphorylase* - **Glycogen phosphorylase** is present in muscle and catalyzes the breakdown of glycogen by cleaving α-1,4 glycosidic bonds to release glucose-1-phosphate. - Muscles have this enzyme and can normally break down glycogen for energy; deficiency causes **McArdle disease** (glycogen storage disease type V) with exercise intolerance. *Hexokinase* - **Hexokinase** is abundant in muscle tissue and phosphorylates free glucose to glucose-6-phosphate for entry into glycolysis. - This enzyme is necessary for utilizing both blood glucose and glycogen-derived glucose-6-phosphate. *Phosphoglucomutase* - **Phosphoglucomutase** is present in muscle and converts glucose-1-phosphate (from glycogen breakdown) to glucose-6-phosphate. - This enzyme is essential for channeling glycogen-derived glucose into glycolysis.

Question 824: Which enzyme is deficient in Galactosemia?

A. Hexosaminidase B
B. Hexosaminidase A
C. Galactose 1-phosphate uridyltransferase (Correct Answer)
D. Glucocerebrosidase

Explanation: ***Galactose 1-phosphate uridyltransferase*** - **Classical galactosemia** (Type I) is caused by a deficiency in this enzyme, which converts **galactose-1-phosphate** and UDP-glucose into UDP-galactose and glucose-1-phosphate. - This deficiency leads to the accumulation of toxic galactose metabolites, such as **galactitol** and galactose-1-phosphate. *Hexosaminidase B* - Deficiency of this enzyme is seen in **Sandhoff disease**, a lysosomal storage disorder, which leads to the accumulation of **GM2 gangliosides** in neurons. - This enzyme is less commonly associated with the primary defect in **Tay-Sachs disease**, which is predominantly due to hexosaminidase A deficiency. *Hexosaminidase A* - A deficiency in **hexosaminidase A** causes **Tay-Sachs disease**, another lysosomal storage disorder, leading to the accumulation of **GM2 gangliosides** primarily in nerve cells. - This enzyme is not involved in the metabolism of galactose. *Glucocerebrosidase* - A deficiency in **glucocerebrosidase** causes **Gaucher disease**, which leads to the accumulation of **glucocerebroside** in macrophages and other cells. - This enzyme is not involved in the metabolic pathway of galactose.

Question 825: Pyruvate is converted to which substance to start gluconeogenesis?

A. Oxaloacetate (Correct Answer)
B. Cis-aconitate
C. Succinate
D. Phosphoenol pyruvate

Explanation: ***Oxaloacetate*** - Pyruvate is converted to **oxaloacetate** via **pyruvate carboxylase** in the mitochondria, which is the first committed step of gluconeogenesis. - This step is an anaplerotic reaction that replenishes intermediates of the **TCA cycle** while also initiating glucose synthesis. *Phosphoenol pyruvate* - **Phosphoenol pyruvate (PEP)** is formed directly from oxaloacetate by **PEP carboxykinase** in the cytosol or mitochondria, not directly from pyruvate as the initial step for gluconeogenesis. - While PEP is a later intermediate in gluconeogenesis, it is not the substance into which pyruvate is first converted. *Cis-aconitate* - **Cis-aconitate** is an intermediate in the **TCA cycle**, formed from citrate by aconitase. - It is not directly involved in the initial steps of gluconeogenesis from pyruvate. *Succinate* - **Succinate** is also an intermediate in the **TCA cycle**, formed from succinyl-CoA. - It is not involved in the conversion of pyruvate to initiate gluconeogenesis.

Question 826: Glycogen storage disorder due to muscle phosphorylase deficiency is which type of disease?

A. Pompe's disease
B. Andersen's disease
C. Tarui's disease
D. McArdle's disease (Correct Answer)

Explanation: ***McArdle's disease*** - This condition is also known as **Glycogen Storage Disease Type V**, which is specifically caused by a deficiency in **muscle phosphorylase** (myophosphorylase). - Patients typically present with exercise intolerance, muscle pain, and cramping due to the inability to break down muscle glycogen for energy. *Pompe's disease* - This is **Glycogen Storage Disease Type II**, caused by a deficiency in **acid alpha-glucosidase** (acid maltase). - It primarily affects the heart and skeletal muscles, leading to cardiomegaly and hypotonia, and is not a muscle phosphorylase deficiency. *Andersen's disease* - Also known as **Glycogen Storage Disease Type IV**, this results from a deficiency in **glycogen-branching enzyme**. - It leads to the accumulation of abnormal glycogen structures in the liver and muscles, causing liver cirrhosis and muscle weakness. *Tarui's disease* - This is **Glycogen Storage Disease Type VII**, caused by a deficiency in **phosphofructokinase-1 (PFK-1)**, an enzyme involved in glycolysis. - Like McArdle's, it presents with exercise intolerance and muscle pain, but the enzymatic defect is distinct from muscle phosphorylase.

Question 827: A medical student consumes a meal containing 60g carbohydrates, 20g protein, and 15g fat after an 8-hour fast. Which one of the following is the MOST prominent effect of this meal on the student's metabolic state?

A. Liver glycogen stores will be replenished. (Correct Answer)
B. The rate at which fatty acids are converted to adipose triacylglycerols will be increased.
C. Blood glucagon levels will decrease.
D. The rate of gluconeogenesis will be increased.

Explanation: ***Liver glycogen stores will be replenished.*** - After an 8-hour fast, **liver glycogen stores** are significantly depleted (reduced by ~50-70%), as the liver uses glycogen to maintain **blood glucose homeostasis**. - A meal rich in carbohydrates (60g in this case) provides sufficient glucose for the liver to actively replenish its glycogen reserves through **glycogenesis**. - This is the **most prominent and immediate metabolic priority** after fasting, as restoring hepatic glycogen is essential for maintaining glucose homeostasis between meals. *The rate at which fatty acids are converted to adipose triacylglycerols will be increased.* - While this is true—dietary fats will be stored as **triacylglycerols** and insulin promotes lipogenesis—this process is secondary to glucose homeostasis. - The conversion and storage of fat occurs but is **not the most prominent effect** compared to immediate glycogen replenishment. *Blood glucagon levels will decrease.* - This statement is **factually correct**—glucagon levels do decrease after a carbohydrate-rich meal as insulin rises. - However, the decrease in glucagon is a **hormonal regulatory response**, not the primary metabolic outcome. - The question asks for the most prominent **effect on metabolic state**, which refers to the major substrate flux changes (glycogen synthesis), not the hormonal signal itself. *The rate of gluconeogenesis will be increased.* - This is **incorrect**. After an 8-hour fast, gluconeogenesis is active to maintain blood glucose. - Following a carbohydrate-rich meal, dietary glucose becomes available, and rising **insulin levels** suppress gluconeogenesis. - The rate of gluconeogenesis will **decrease**, not increase.

Question 828: What is the primary reason for decreased energy production in thiamine deficiency?

A. It is involved in the metabolism of branched-chain amino acids.
B. It is essential for the conversion of pyruvate to acetyl-CoA. (Correct Answer)
C. It is a co-factor in the citric acid cycle.
D. It is required for the process of glycolysis.

Explanation: ***It is essential for the conversion of pyruvate to acetyl-CoA.*** - Thiamine, in its active form **thiamine pyrophosphate (TPP)**, is a crucial coenzyme for the **pyruvate dehydrogenase complex**. - Without thiamine, pyruvate cannot be converted to **acetyl-CoA**, thereby blocking its entry into the **citric acid cycle** for energy generation. - This represents the **primary and most significant block** in energy production, as it prevents glucose-derived pyruvate from entering oxidative metabolism. *It is a co-factor in the citric acid cycle.* - Thiamine pyrophosphate (TPP) **is indeed a direct cofactor** for **α-ketoglutarate dehydrogenase** within the citric acid cycle itself. - However, the **primary reason** for decreased energy production in thiamine deficiency is the earlier blockage at pyruvate dehydrogenase, which prevents substrate entry into the cycle. - Even with α-ketoglutarate dehydrogenase affected, the more critical bottleneck occurs upstream at pyruvate conversion. *It is involved in the metabolism of branched-chain amino acids.* - Thiamine is indeed a coenzyme for the **branched-chain alpha-keto acid dehydrogenase complex**, which is involved in branched-chain amino acid metabolism. - However, the **primary impact** on energy production in thiamine deficiency stems from its role in glucose metabolism rather than amino acid metabolism. - Glucose metabolism is the body's primary energy source, making the pyruvate dehydrogenase block more significant. *It is required for the process of glycolysis.* - **Glycolysis** is the metabolic pathway that breaks down glucose into pyruvate, and it does **not require thiamine** as a coenzyme. - Thiamine's role in glucose metabolism begins *after* glycolysis, at the step where pyruvate is converted to acetyl-CoA. - Glycolysis can proceed normally in thiamine deficiency, but the products cannot enter oxidative metabolism efficiently.

Question 829: What does the glycemic index of a starchy food measure?

A. The caloric content of the food
B. The blood glucose response to carbohydrates (Correct Answer)
C. The nutritional value of the food
D. The fiber content of the food

Explanation: ***The blood glucose response to carbohydrates*** - The **glycemic index (GI)** specifically quantifies how quickly and how much a food's **carbohydrates** raise **blood glucose levels** after consumption. - Foods with a high GI are rapidly digested and absorbed, causing a **sharp rise in blood sugar**, while low GI foods lead to a more gradual increase. - GI is measured by comparing the blood glucose response to 50g of carbohydrate from the test food versus 50g of glucose or white bread as a reference standard. *The caloric content of the food* - **Caloric content** measures the energy provided by a food, expressed in **kilocalories (kcal)** or **Joules**. - While important for overall energy balance, it does not directly reflect the rate or magnitude of **blood sugar elevation**. - Two foods with identical caloric content can have vastly different glycemic indices. *The nutritional value of the food* - **Nutritional value** encompasses a broad range of components including **vitamins, minerals, fiber, protein, and fats**, in addition to carbohydrates. - The GI focuses solely on the **carbohydrate impact** on blood glucose and does not provide a comprehensive assessment of a food's overall nutritional benefits. *The fiber content of the food* - **Fiber content** is a separate nutritional measurement expressed in grams per serving. - While high fiber content can **lower** the glycemic index of a food by slowing carbohydrate digestion, the GI itself does not directly measure fiber. - Fiber is an important nutritional component but represents a distinct parameter from glycemic response.

Question 830: In a primary health care setting, which anticoagulant combination is recommended for sending blood samples for accurate blood glucose estimation?

A. EDTA (Ethylenediaminetetraacetic acid)
B. Heparin (Unfractionated Heparin)
C. Potassium oxalate (alone)
D. Potassium oxalate + sodium fluoride (Correct Answer)

Explanation: ***Potassium oxalate + sodium fluoride*** - This combination is crucial for **accurate glucose measurement** because **sodium fluoride prevents glycolysis** (glucose breakdown by red blood cells) by inhibiting enolase. - **Potassium oxalate** acts as an **anticoagulant** by precipitating calcium, preventing clotting without interfering with glucose stability. *EDTA (Ethylenediaminetetraacetic acid)* - While EDTA is a common **anticoagulant** that works by chelating calcium, it does not prevent **glycolysis**. - If glucose estimation is delayed, EDTA tubes will show **falsely low glucose levels** due to red blood cell metabolism. *Heparin (Unfractionated Heparin)* - **Heparin** is an anticoagulant that inhibits thrombin, but it also **does not prevent glycolysis**. - Samples collected in heparin tubes will experience **glucose degradation** over time, leading to inaccurate results if not processed immediately. *Potassium oxalate (alone)* - **Potassium oxalate** acts as an **anticoagulant**, but it **does not prevent glycolysis**. - Therefore, without a glycolytic inhibitor like sodium fluoride, glucose levels will **decrease over time** after blood collection.

Practice by Chapter

Carbohydrate Chemistry and Classification

Practice Questions

Glycolysis: Reactions and Regulation

Practice Questions

Gluconeogenesis: Reactions and Regulation

Practice Questions

Glycogen Metabolism: Synthesis and Breakdown

Practice Questions

Glycogen Storage Diseases

Practice Questions

Pentose Phosphate Pathway

Practice Questions

Metabolism of Fructose and Galactose

Practice Questions

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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