Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 561: What is the primary enzyme responsible for the conversion of carbon dioxide to bicarbonate in erythrocytes?

A. The high solubility of CO2 in water
B. The role of hemoglobin in CO2 transport
C. The conversion of carbon dioxide to carbonic acid
D. The action of carbonic anhydrase in erythrocytes (Correct Answer)

Explanation: ***The action of carbonic anhydrase in erythrocytes*** - **Carbonic anhydrase** is an enzyme found in high concentrations within **red blood cells (erythrocytes)**, catalyzing the rapid interconversion of carbon dioxide and water to **carbonic acid**. - This enzyme is crucial for the efficient transport of carbon dioxide from the tissues to the lungs, as carbonic acid quickly dissociates into **bicarbonate ions**, which are easily transported in the plasma. *The high solubility of CO2 in water* - While **CO2** does have some solubility in water, this process is too slow on its own to account for the rapid and efficient transport of the large amounts of metabolic CO2 produced by the body. - The direct dissolution of CO2 in plasma accounts for only a small fraction of its total transport. *The role of hemoglobin in CO2 transport* - **Hemoglobin** does play a role in CO2 transport by forming **carbaminohemoglobin**, binding to the amino groups on the globin chains. - However, this mechanism represents only about 20-30% of CO2 transport and does not involve the conversion to **bicarbonate**. *The conversion of carbon dioxide to carbonic acid* - The conversion of CO2 to **carbonic acid (H2CO3)** is indeed an intermediate step in bicarbonate formation. - However, this reaction is very slow in the absence of an enzyme and does not address the primary catalyst responsible for this rapid conversion.

Question 562: Which of the following statements about the enzyme aldolase B is false?

A. The enzyme cleaves fructose-1-phosphate into dihydroxyacetone phosphate and glyceraldehyde
B. The enzyme involved catalyzes the conversion of fructose to fructose-1 phosphate (Correct Answer)
C. The enzyme is involved in fructose metabolism
D. The enzyme is found primarily in liver, kidney, and intestine

Explanation: ***The enzyme involved catalyzes the conversion of fructose to fructose-1 phosphate*** - This statement is **false** because the enzyme that catalyzes the conversion of **fructose to fructose-1-phosphate** is **fructokinase**, not aldolase B. - Aldolase B acts further down the pathway, cleaving fructose-1-phosphate. *The enzyme is involved in fructose metabolism* - Aldolase B is indeed a crucial enzyme in **fructose metabolism**, specifically in the breakdown of **fructose-1-phosphate**. - Its role is to convert fructose-1-phosphate into usable glycolytic intermediates. *The enzyme cleaves fructose-1-phosphate into dihydroxyacetone phosphate and glyceraldehyde* - This statement accurately describes the primary catalytic action of **aldolase B**, which is the cleavage of **fructose-1-phosphate**. - This cleavage yields **dihydroxyacetone phosphate (DHAP)** and **glyceraldehyde**, which can then enter glycolysis. *The enzyme is found primarily in liver, kidney, and intestine* - Aldolase B is predominantly expressed in the **liver**, **kidney**, and **small intestine**, organs that are key sites for processing dietary fructose. - This distribution reflects its critical role in fructose metabolism in these tissues.

Question 563: Which isoenzyme of LDH is seen in the heart?

A. LDH 1 (Correct Answer)
B. LDH 2
C. LDH 3
D. LDH 4

Explanation: ***LDH 1*** - **LDH 1** is the predominant isoenzyme found in the **heart muscle**, particularly in the ventricles. Its elevation is a key indicator of cardiac tissue damage. - It is also found in **red blood cells** and the **kidneys**. *LDH 2* - **LDH 2** is also present in the **heart**, but it is typically the second most abundant isoenzyme after LDH 1 in cardiac tissue. - It is predominantly found in the **reticuloendothelial system**, including macrophages and lymphoid tissues. *LDH 3* - **LDH 3** is primarily associated with **pulmonary tissue**, the **spleen**, and **lymph nodes**. - Its elevation often suggests conditions affecting these organs, such as pulmonary embolism or pneumonia. *LDH 4* - **LDH 4** is found in the **kidney**, **placenta**, and **pancreas**, with some presence in skeletal muscle. - **LDH 5** (not LDH 4) is the primary isoenzyme in the **liver** and **skeletal muscle**.

Question 564: If a chymotrypsin molecule undergoes a ser-195-ala mutation, then

A. Chymotrypsin will not bind the substrate
B. Chymotrypsin will bind the substrate as well as cause cleavage
C. Chymotrypsin will bind the substrate but will not cause cleavage (Correct Answer)
D. No effect will be observed

Explanation: ***Chymotrypsin will bind the substrate but will not cause cleavage*** - The **Ser195** residue is critical for the **catalytic mechanism** of chymotrypsin, acting as the **nucleophile** that attacks the substrate's carbonyl group. - A mutation to **alanine** at this position removes the **hydroxyl group** necessary for this nucleophilic attack, thereby **abolishing catalysis** while leaving the substrate binding site intact. *Chymotrypsin will not bind the substrate* - The **Ser195** residue is part of the **catalytic triad** and is not directly involved in forming the primary binding pocket for the substrate. - The enzyme's ability to **recognize and bind** appropriate substrates would remain largely unaffected by this specific mutation. *Chymotrypsin will bind the substrate as well as cause cleavage* - This statement is incorrect because the **Ser195 residue** is absolutely essential for the **cleavage mechanism**. - Its mutation to **alanine** eliminates the **nucleophilic attack** required for peptide bond hydrolysis, making cleavage impossible. *No effect will be observed* - The **Ser195 residue** is a key component of the **catalytic triad** (**His57, Asp102, Ser195**) of chymotrypsin. - A mutation in such a crucial residue will have a **significant impact** on the enzyme's function, specifically its catalytic activity.

Question 565: The rate-limiting enzyme in the synthesis of dopamine is which of the following?

A. Tyrosine hydroxylase (Correct Answer)
B. Dopa decarboxylase
C. Dopamine beta-hydroxylase
D. Monoamine oxidase

Explanation: ***Tyrosine hydroxylase*** - **Tyrosine hydroxylase (TH)** catalyzes the conversion of **L-tyrosine to L-DOPA**, the first and **rate-limiting step** in the synthesis of catecholamines, including dopamine [1] - This enzyme's activity is crucial for regulating the overall **production rate** of dopamine and other catecholamines [1] - The rate-limiting nature means this is the slowest step that controls the overall pathway flux *Dopa decarboxylase* - **Dopa decarboxylase** converts **L-DOPA to dopamine**, which is a subsequent step in the synthesis pathway [1] - While essential for dopamine production, it is **not the rate-limiting enzyme** as its activity is generally higher than that of tyrosine hydroxylase [1] *Monoamine oxidase* - **Monoamine oxidase (MAO)** is an enzyme involved in the **breakdown and inactivation** of monoamine neurotransmitters, including dopamine, rather than its synthesis - It plays a role in regulating the **levels of dopamine** in the synapse but does not contribute to its production *Dopamine beta-hydroxylase* - **Dopamine beta-hydroxylase (DBH)** converts dopamine to **norepinephrine** (noradrenaline) - This enzyme is important for the synthesis of norepinephrine from dopamine but is not involved in the actual synthesis of dopamine itself

Question 566: Carbonic anhydrase requires which of the following?

A. Copper
B. Zinc (Correct Answer)
C. Iron
D. No cofactor required

Explanation: ***Zinc*** - **Zinc** is an essential **cofactor** for the enzyme carbonic anhydrase, playing a crucial role in its catalytic activity. - It directly participates in the enzyme's mechanism by coordinating a water molecule, facilitating the rapid interconversion of **carbon dioxide** and **bicarbonate**. *Copper* - **Copper** is a cofactor for several enzymes, such as **cytochrome c oxidase** and **superoxide dismutase**, but not for carbonic anhydrase. - Its presence is not required for the catalytic function of carbonic anhydrase. *Iron* - **Iron** is a vital component of many proteins, including hemoglobin and cytochromes, involved in **oxygen transport** and **electron transfer**. - However, **iron** does not serve as a cofactor for carbonic anhydrase. *No cofactor required* - This statement is incorrect because carbonic anhydrase is a **metalloenzyme** that absolutely requires a **metal ion cofactor** for its function. - Without a **cofactor**, specifically **zinc**, the enzyme would be catalytically inactive.

Question 567: Which of the following enzymes is NAD+ dependent?

A. HMG CoA reductase
B. Glycerol-3-phosphate dehydrogenase (Correct Answer)
C. Acyl CoA dehydrogenase
D. Succinate dehydrogenase

Explanation: ***Glycerol-3-phosphate dehydrogenase*** - This enzyme catalyzes the interconversion of **dihydroxyacetone phosphate (DHAP)** and **glycerol-3-phosphate**, using **NAD+** as a cofactor to oxidize glycerol-3-phosphate. - In the **glycerol-phosphate shuttle**, it enables the transfer of reducing equivalents from cytosolic NADH to the mitochondrial electron transport chain. *HMG CoA reductase* - This enzyme is a key regulatory step in **cholesterol biosynthesis** and utilizes **NADPH** as a reducing agent, not NAD+. - It catalyzes the reduction of HMG-CoA to **mevalonate**. *Acyl CoA dehydrogenase* - This enzyme is involved in the first step of **beta-oxidation of fatty acids** and uses **FAD** as a prosthetic group, not NAD+, which is reduced to FADH2. - It catalyzes the formation of a double bond between the alpha and beta carbons of the acyl-CoA. *Succinate dehydrogenase* - This enzyme is part of the **TCA cycle** (Complex II of the electron transport chain) and uses **FAD** as its electron acceptor, converting succinate to fumarate. - It is unique among TCA cycle enzymes as it is membrane-bound and directly links the cycle to the **electron transport chain**.

Question 568: Which of the following statements about cofactors and metalloenzymes is true?

A. The most common cofactors are metal ions
B. Enzymes that require metal ion cofactors are termed as metalloenzymes
C. Coenzymes are loosely bound cofactors that transiently bind to enzymes, while prosthetic groups are tightly bound cofactors (Correct Answer)
D. Cofactors bind in a transient, dissociable manner to enzymes

Explanation: ***Coenzymes are loosely bound cofactors that transiently bind to enzymes, while prosthetic groups are tightly bound cofactors*** - **Cofactors** are non-protein chemical compounds required by enzymes for biological activity - Cofactors can be subdivided into **two main categories based on binding**: - **Coenzymes**: Organic cofactors that bind *loosely and transiently* (e.g., NAD+, FAD, Coenzyme A) - **Prosthetic groups**: Cofactors that bind *tightly or covalently* to enzymes (e.g., heme in hemoglobin, FAD in succinate dehydrogenase) - This distinction is fundamental to understanding enzyme kinetics and cofactor recycling *The most common cofactors are metal ions* - While **metal ions** (e.g., Mg2+, Zn2+, Fe2+) are important cofactors, **organic coenzymes** are equally prevalent and essential - Major organic coenzymes include NAD+, NADP+, FAD, thiamine pyrophosphate, and coenzyme A - The statement incorrectly suggests metal ions predominate *Enzymes that require metal ion cofactors are termed as metalloenzymes* - This is **imprecise terminology** - **Metalloenzymes** specifically contain *tightly bound metal ions* as integral parts of their structure (e.g., catalase with Fe3+, carbonic anhydrase with Zn2+) - **Metal-activated enzymes** require *loosely bound metal ions* that are not permanently associated with the enzyme - The statement fails to make this critical distinction *Cofactors bind in a transient, dissociable manner to enzymes* - This is **only true for some cofactors** (coenzymes), not all - **Prosthetic groups**, which are also cofactors, bind *tightly or covalently* and are not easily dissociable - The blanket statement is too absolute and ignores the diversity of cofactor binding modes

Question 569: Which of the following enzymes is not classified as an oxidoreductase?

A. Alcohol dehydrogenase
B. Catalase
C. Peroxidase
D. Glucokinase (Correct Answer)

Explanation: ***Glucokinase*** - **Glucokinase** is a **transferase** enzyme that catalyzes the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate. - Its function is primarily in **glucose metabolism** and **insulin secretion**, not in oxidation or reduction reactions. *Catalase* - **Catalase** is an **oxidoreductase** that catalyzes the decomposition of **hydrogen peroxide** into water and oxygen. - This reaction involves the **oxidation and reduction** of substrates, fitting the definition of an oxidoreductase. *Alcohol dehydrogenase* - **Alcohol dehydrogenase** is an **oxidoreductase** that catalyzes the interconversion between alcohols and aldehydes or ketones with the concomitant reduction and oxidation of **NAD+** to **NADH**. - This enzyme is crucial in **detoxifying alcohol** by oxidizing it and is a classic example of an oxidoreductase. *Peroxidase* - **Peroxidase** is an **oxidoreductase** that catalyzes the oxidation of a substrate by **hydrogen peroxide**. - Peroxidases work by using hydrogen peroxide to accept electrons from another molecule, thereby **oxidizing** that molecule.

Question 570: Which of the following enzymes is not considered a rate-limiting enzyme in metabolic pathways?

A. HMG CoA reductase
B. Succinate dehydrogenase (Correct Answer)
C. Phosphofructokinase
D. Acetyl CoA carboxylase

Explanation: ***Succinate dehydrogenase*** - **Succinate dehydrogenase** is an enzyme of the **Krebs cycle** and **electron transport chain** but is not considered a primary rate-limiting enzyme. - Its activity is generally high, and it operates close to its maximum velocity under most physiological conditions, thus not typically controlling the overall flux of its respective pathways in a rate-limiting manner. *HMG CoA reductase* - **HMG CoA reductase** is the **rate-limiting enzyme in cholesterol biosynthesis**. - It is a key target for **statins**, drugs that lower cholesterol by inhibiting this enzyme. *Phosphofructokinase* - **Phosphofructokinase-1 (PFK-1)** is the **rate-limiting enzyme in glycolysis**. - Its activity is tightly regulated by **allosteric modulators** like ATP, AMP, and citrate to control the flux of glucose metabolism. *Acetyl CoA carboxylase* - **Acetyl CoA carboxylase (ACC)** is the **rate-limiting enzyme in fatty acid synthesis**. - It catalyzes the irreversible carboxylation of acetyl-CoA to **malonyl-CoA**, a crucial committed step in the pathway.

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Enzyme Classification and Nomenclature

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Enzyme Kinetics and Michaelis-Menten Equation

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Enzyme Inhibition: Competitive and Non-competitive

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Allosteric Regulation

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Coenzymes and Cofactors

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Isoenzymes and Clinical Significance

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Enzyme Regulation: Covalent Modification

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Enzyme Regulation: Zymogen Activation

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Enzyme Induction and Repression

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Ribozymes and Catalytic RNA

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Enzyme Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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