Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 91: Coenzymes are which type of organic compounds?

A. Lipoprotein
B. Proteinaceous
C. Non-protein (Correct Answer)
D. Any of the above

Explanation: **Explanation:** Enzymes are often composed of two parts: a protein portion called the **apoenzyme** and a non-protein component called a **cofactor**. When these two combine, they form the catalytically active **holoenzyme** (Holoenzyme = Apoenzyme + Cofactor). **Coenzymes** are a specific type of cofactor. They are **non-protein, low-molecular-weight organic compounds** that are heat-stable and loosely associated with the apoenzyme. They function by carrying chemical groups, electrons, or hydrogen atoms between reactions (e.g., NAD+, FAD, TPP). **Analysis of Options:** * **Option C (Correct):** Coenzymes are strictly non-protein organic molecules, often derived from vitamins (e.g., NAD+ from Niacin). * **Option B (Incorrect):** The protein part of an enzyme is the **apoenzyme**. If the entire enzyme is just a protein without needing a cofactor, it is a simple enzyme (e.g., Pepsin). * **Option A (Incorrect):** Lipoproteins are conjugated proteins involved in lipid transport (like LDL or HDL) and do not function as coenzymes. **High-Yield NEET-PG Pearls:** 1. **Prosthetic Groups:** If the non-protein organic cofactor is **tightly or covalently bound** to the apoenzyme, it is called a prosthetic group (e.g., Heme in Cytochrome c). 2. **Metal Ions:** Inorganic cofactors (like $Mg^{2+}$, $Zn^{2+}$) are called **metal ion activators**. Carbonic anhydrase requires $Zn^{2+}$. 3. **Vitamin Precursors:** Most coenzymes are derivatives of B-complex vitamins. * **TPP:** Vitamin B1 (Thiamine) * **FAD/FMN:** Vitamin B2 (Riboflavin) * **NAD/NADP:** Vitamin B3 (Niacin) * **Pyridoxal Phosphate (PLP):** Vitamin B6 (Pyridoxine) — essential for transamination.

Question 92: Selenocysteine is a component of which enzyme?

A. NADP reductase
B. NADPH dehydrogenase
C. Thioredoxin reductase (Correct Answer)
D. Pyruvate dehydrogenase

Explanation: ### Explanation **Correct Answer: C. Thioredoxin reductase** **Understanding the Concept:** Selenocysteine is often referred to as the **21st amino acid**. Unlike other non-standard amino acids, it is incorporated into proteins during translation via a unique mechanism involving the UGA stop codon and a specific tRNA. It contains **selenium** in place of the sulfur found in cysteine. This substitution lowers the pKa of the side chain, making it a highly efficient catalyst for redox reactions. **Thioredoxin reductase** is a key antioxidant enzyme that reduces thioredoxin using NADPH. It contains a selenocysteine residue at its active site, which is essential for its catalytic activity in maintaining the redox state of the cell and DNA synthesis. **Analysis of Incorrect Options:** * **A & B. NADP reductase / NADPH dehydrogenase:** These enzymes typically utilize FAD, FMN, or iron-sulfur clusters as cofactors for electron transfer, but they do not contain selenocysteine. * **D. Pyruvate dehydrogenase:** This is a multi-enzyme complex that requires five specific cofactors: Thiamine pyrophosphate (B1), Lipoic acid, CoA (B5), FAD (B2), and NAD+ (B3). It does not utilize selenium or selenocysteine. **High-Yield Clinical Pearls for NEET-PG:** * **Other Selenoenzymes:** Apart from Thioredoxin reductase, other critical enzymes containing selenocysteine include **Glutathione peroxidase** (scavenges $H_2O_2$), **Iodothyronine deiodinase** (converts T4 to T3), and **Selenoprotein P**. * **Genetic Coding:** Selenocysteine is encoded by the **UGA codon** (normally a stop codon). The "recoding" requires a **SECIS element** (Selenocysteine Insertion Sequence) in the 3' untranslated region of the mRNA. * **Deficiency:** Selenium deficiency can lead to **Keshan disease** (cardiomyopathy) or **Kashin-Beck disease** (osteoarthropathy).

Question 93: In which of the following enzymatic reactions is magnesium required?

A. ATPase
B. Dismutase
C. Phosphatase (Correct Answer)
D. Aldolase

Explanation: **Explanation:** **1. Why Phosphatase is the Correct Answer:** Magnesium ($Mg^{2+}$) is the most common intracellular divalent cation and acts as a vital cofactor for enzymes that involve **phosphate group transfer** or metabolism. Phosphatases (and Kinases) require $Mg^{2+}$ because the actual substrate for these enzymes is not just the phosphate molecule, but a **Mg-ATP complex** or a metal-coordinated phosphate group. The $Mg^{2+}$ ion neutralizes the negative charges on the phosphate groups, facilitating a nucleophilic attack and stabilizing the transition state during the hydrolysis of phosphate esters. **2. Analysis of Incorrect Options:** * **ATPase:** While many ATPases do require magnesium, in the context of standard Biochemistry examinations (like NEET-PG), **Phosphatases** and **Kinases** are the classic, high-yield examples of $Mg^{2+}$-dependent enzymes. (Note: If this were a "multiple correct" type, ATPase would be a strong secondary choice, but Phosphatase is the primary textbook association). * **Dismutase:** Superoxide Dismutase (SOD) typically requires **Copper (Cu), Zinc (Zn)**, or **Manganese (Mn)** as cofactors, depending on its cellular location (cytosolic vs. mitochondrial). * **Aldolase:** This is a lyase involved in glycolysis (cleaving Fructose 1,6-bisphosphate). Most mammalian Aldolases (Class I) do **not** require a metal cofactor; they use a Schiff base mechanism. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Phosphate Rule":** Almost all enzymes acting on phosphorylated substrates (Kinases, Phosphatases, Enolase, Phosphofructokinase) require $Mg^{2+}$. * **Hypomagnesemia:** Low magnesium levels can lead to **hypocalcemia** because $Mg^{2+}$ is required for the secretion and action of Parathyroid Hormone (PTH). * **Other Metal Cofactors:** * **Zinc:** Carbonic Anhydrase, Alcohol Dehydrogenase, DNA Polymerase. * **Molybdenum:** Xanthine Oxidase. * **Selenium:** Glutathione Peroxidase. * **Copper:** Cytochrome c Oxidase, Tyrosinase.

Question 94: Enzymatic activity is measured in which unit?

A. mg/dl
B. microgram/litre
C. mg/litre
D. mol/second (Correct Answer)

Explanation: **Explanation:** Enzymatic activity refers to the amount of substrate converted into product per unit of time under specified conditions. The standard SI unit for enzymatic activity is the **Katal (kat)**, defined as the amount of enzyme that catalyzes the conversion of **1 mole of substrate per second (mol/s)**. **1. Why "mol/second" is correct:** In biochemistry, the rate of a reaction is a measure of velocity. Since enzymes are biological catalysts, their activity must be expressed as a rate—specifically, the quantity of substance (moles) transformed over a specific time interval (seconds). While the "International Unit" (IU), defined as 1 μmol/min, is frequently used in clinical labs, the SI unit is mol/s. **2. Why other options are incorrect:** * **Options A, B, and C (mg/dl, μg/L, mg/L):** These are units of **concentration** (mass per volume). They measure the static amount of a substance (like glucose or albumin) in a fluid but do not account for the dynamic rate of a chemical reaction. An enzyme's "activity" is distinct from its "mass concentration." **3. High-Yield Clinical Pearls for NEET-PG:** * **Katal (kat):** 1 kat = 1 mol/s. This is a very large unit; hence, clinical values are often in microkatals (µkat). * **International Unit (IU):** 1 IU = 1 μmol/min. (Conversion: 1 IU ≈ 16.67 nkat). * **Specific Activity:** This is the enzyme activity per milligram of total protein (**μmol/min/mg**). It is the gold standard for measuring **enzyme purity** during protein purification. * **Turnover Number ($k_{cat}$):** The number of substrate molecules converted into product per enzyme active site per second.

Question 95: Which enzyme converts androgen to estrogen?

A. Cholesterol Desmolase
B. Aromatase (Correct Answer)
C. 11 beta-hydroxylase
D. 21 beta-hydroxylase

Explanation: **Explanation:** The conversion of androgens to estrogens is a critical step in steroidogenesis, catalyzed by the enzyme **Aromatase** (also known as CYP19A1). **1. Why Aromatase is Correct:** Aromatase is a cytochrome P450 enzyme located in the endoplasmic reticulum of various tissues (granulosa cells of the ovary, adipose tissue, and placenta). It catalyzes the **aromatization** of the 'A' ring of androgens. Specifically, it converts **Androstenedione to Estrone** and **Testosterone to Estradiol**. This is the rate-limiting step in estrogen synthesis. **2. Why Other Options are Incorrect:** * **Cholesterol Desmolase (CYP11A1):** This is the "rate-limiting enzyme" for the entire steroidogenesis pathway. It converts Cholesterol to Pregnenolone in the mitochondria. * **11 beta-hydroxylase:** This enzyme is involved in the adrenal corticosteroid pathway, converting 11-deoxycortisol to Cortisol and 11-deoxycorticosterone to Corticosterone. * **21 beta-hydroxylase:** This is the most common enzyme deficient in **Congenital Adrenal Hyperplasia (CAH)**. It converts Progesterone to 11-deoxycorticosterone and 17-OH-progesterone to 11-deoxycortisol. **3. Clinical Pearls for NEET-PG:** * **Aromatase Inhibitors (e.g., Letrozole, Anastrozole):** These are first-line treatments for ER-positive postmenopausal breast cancer. * **PCOS Connection:** In Polycystic Ovary Syndrome, there is often an altered LH/FSH ratio leading to relative aromatase deficiency in granulosa cells, causing androgen buildup. * **Site of Action:** In males, peripheral aromatization of testosterone in adipose tissue is the primary source of estrogen; excessive activity can lead to gynecomastia.

Question 96: Which amino acids constitute the 'catalytic triad' in the active center of proteases?

A. Serine, Lysine, Arginine
B. Serine, Histidine, Aspartate (Correct Answer)
C. Histidine, Phenylalanine, Tryptophan
D. None of the above

Explanation: ### Explanation The **catalytic triad** is a classic biochemical motif found in the active sites of many hydrolase and transferase enzymes, most notably **Serine Proteases** (e.g., Chymotrypsin, Trypsin, Elastase, and Thrombin). **1. Why Option B is Correct:** The triad consists of **Serine (Ser), Histidine (His), and Aspartate (Asp)**. These three residues work in a coordinated "charge-relay" system: * **Aspartate** uses its carboxylate group to hydrogen-bond with Histidine. * **Histidine** acts as a general base, accepting a proton from the Serine hydroxyl (-OH) group. * **Serine** becomes a highly reactive **alkoxide nucleophile**, which attacks the carbonyl carbon of the peptide bond in the substrate, leading to proteolysis. **2. Why Other Options are Incorrect:** * **Option A:** Lysine and Arginine are basic amino acids but do not participate in the specific charge-relay mechanism required for serine protease activity. * **Option C:** Phenylalanine and Tryptophan are bulky, hydrophobic aromatic amino acids. While they may be involved in substrate binding pockets (like the S1 pocket of chymotrypsin), they do not perform the catalytic chemical transformation. **3. NEET-PG High-Yield Pearls:** * **Mechanism:** The catalytic triad is an example of **Covalent Catalysis** (via the Serine-substrate intermediate). * **Irreversible Inhibition:** Organophosphates (like Malathion or Nerve Gas) inhibit these enzymes by covalently binding to the **Serine** residue of the triad. * **Evolutionary Note:** The Ser-His-Asp triad is a prime example of **convergent evolution**, appearing in both the Chymotrypsin family and the Subtilisin family despite no structural similarity. * **Zymogens:** These proteases are secreted as inactive precursors (e.g., Trypsinogen) to prevent autodigestion of the pancreas.

Question 97: Alkaline phosphatase is raised in all of the following conditions except:

A. Obstructive jaundice
B. Placental tumor
C. Prostatic tumor (Correct Answer)
D. Skeletal tumor

Explanation: **Explanation:** The correct answer is **Prostatic tumor** because it is classically associated with an elevation in **Acid Phosphatase (ACP)** and **Prostate-Specific Antigen (PSA)**, rather than Alkaline Phosphatase (ALP). **1. Why Prostatic Tumor is the exception:** Prostatic carcinoma cells secrete Acid Phosphatase. While advanced prostate cancer with bone metastasis (osteoblastic lesions) can eventually cause a secondary rise in ALP, the primary biochemical marker for the tumor itself is ACP. In contrast, ALP is a marker of high bone turnover or biliary obstruction. **2. Analysis of Incorrect Options:** * **Obstructive Jaundice:** ALP is a marker of cholestasis. It is synthesized by the biliary canalicular membranes; when bile flow is obstructed, ALP synthesis increases and leaks into the sinusoidal blood. * **Placental Tumor:** The placenta is one of the four main sources of ALP isoenzymes (Liver, Bone, Placenta, Intestine). Conditions like germ cell tumors (e.g., seminoma) or placental site trophoblastic tumors can secrete the **Regan isoenzyme**, which is a heat-stable placental-like ALP. * **Skeletal Tumor:** ALP is produced by **osteoblasts**. Any condition involving increased bone formation or remodeling, such as osteosarcoma or bone metastases from other cancers, will significantly raise serum ALP levels. **Clinical Pearls for NEET-PG:** * **Isoenzymes of ALP:** Remember the mnemonic **"BLIP"** (Bone, Liver, Intestine, Placenta). * **Heat Stability:** Placental ALP is the most heat-stable, while Bone ALP is the most heat-labile ("Bone burns"). * **Regan Isoenzyme:** A specific biochemical marker for various carcinomas (lung, GI) that mimics placental ALP. * **Acid Phosphatase (ACP):** Specifically inhibited by **tartrate** (Prostatic fraction), whereas the Gaucher’s disease fraction is tartrate-resistant (TRAP).

Question 98: What is a monoclonal antibody that acts as an enzyme called?

A. Granzyme
B. Abzyme (Correct Answer)
C. Lipozyme
D. None of the above

Explanation: **Explanation:** **Correct Answer: B. Abzyme** An **Abzyme** (a portmanteau of **Ab**antibody and en**zyme**) is a monoclonal antibody with catalytic activity. While traditional antibodies bind to stable antigens, abzymes are designed to bind to the **transition state** of a chemical reaction. According to Pauling’s principle, enzymes catalyze reactions by stabilizing the transition state; therefore, an antibody engineered to fit a transition state analog will function like an enzyme, lowering the activation energy and accelerating the reaction. **Analysis of Incorrect Options:** * **A. Granzyme:** These are serine proteases released by cytoplasmic granules within cytotoxic T cells and Natural Killer (NK) cells. Their primary role is to induce apoptosis in virus-infected or malignant cells, not to act as catalytic antibodies. * **C. Lipozyme:** This is a non-standard term often used in biotechnology to refer to immobilized lipases used for industrial catalysis. It is not a monoclonal antibody. **High-Yield Clinical Pearls for NEET-PG:** * **Transition State Analogs:** Abzymes are produced by immunizing animals with "transition state analogs"—stable molecules that mimic the unstable intermediate of a reaction. * **Potential Applications:** Abzymes are being researched for targeted prodrug activation (converting an inactive drug into an active one at the tumor site) and for neutralizing cocaine toxicity by accelerating its hydrolysis in the bloodstream. * **Ribozyme vs. Abzyme:** Do not confuse the two. A **Ribozyme** is an RNA molecule with catalytic activity (e.g., Peptidyl transferase in ribosomes), whereas an **Abzyme** is a protein (antibody).

Question 99: Which of the following is NOT a feature of a coenzyme?

A. It has high molecular weight. (Correct Answer)
B. It combines loosely with enzyme molecules.
C. It mostly belongs to the vitamin B complex group.
D. It is heat stable.

Explanation: ### Explanation Enzymes often require non-protein components called **cofactors** for catalytic activity. When the cofactor is an organic molecule, it is termed a **coenzyme**. **Why Option A is the Correct Answer:** Coenzymes are **low molecular weight**, non-protein organic molecules. They are dialyzable and much smaller than the apoenzyme (the protein part). Therefore, the statement that they have a "high molecular weight" is incorrect. **Analysis of Other Options:** * **Option B (Combines loosely):** Coenzymes typically associate with the apoenzyme via non-covalent bonds (hydrogen bonds or Van der Waals forces) and can be easily separated by dialysis. If a cofactor is covalently and tightly bound, it is specifically called a **prosthetic group** (e.g., Heme in Cytochrome). * **Option C (Vitamin B complex):** Most coenzymes are derivatives of water-soluble B-complex vitamins. For example, NAD+ is derived from Niacin (B3), FAD from Riboflavin (B2), and TPP from Thiamine (B1). * **Option D (Heat stable):** Unlike the protein part of the enzyme (apoenzyme), which denatures at high temperatures, coenzymes are generally **heat-stable**. **High-Yield NEET-PG Pearls:** 1. **Holoenzyme:** The complete, catalytically active unit consisting of the Apoenzyme (protein) + Cofactor (non-protein). 2. **Metal-activated enzymes:** Require metal ions (like $K^+$, $Mg^{2+}$) that are loosely bound. 3. **Metalloenzymes:** Contain tightly bound metal ions (e.g., Zinc in Carbonic Anhydrase and Alcohol Dehydrogenase; Iron in Cytochromes). 4. **Key Coenzyme Pairs:** * **NAD/NADP:** Niacin (B3) * **FAD/FMN:** Riboflavin (B2) * **Coenzyme A:** Pantothenic acid (B5) * **PLP:** Pyridoxine (B6)

Question 100: Which of the following is not a component of coenzyme A?

A. Pantothenic acid
B. Adenylic acid
C. Sulfhydryl group
D. Acetic acid (Correct Answer)

Explanation: **Explanation:** Coenzyme A (CoA-SH) is a vital cofactor derived from Vitamin B5, primarily involved in the metabolism of fatty acids and the citric acid cycle. It functions as a carrier of acyl groups, forming high-energy thioester bonds. **Why Acetic Acid is the correct answer:** Acetic acid is **not** a structural component of Coenzyme A itself. Instead, acetic acid (as an acetyl group) binds to the pre-existing Coenzyme A molecule to form **Acetyl-CoA**. Acetic acid is a substrate that CoA carries, rather than a building block of the cofactor's structure. **Analysis of incorrect options:** * **Pantothenic Acid (Vitamin B5):** This is the core vitamin component of CoA. It is linked to beta-mercaptoethylamine and ADP. * **Adenylic Acid (Adenosine 3', 5'-bisphosphate):** CoA contains an adenine ring, a ribose sugar, and phosphate groups, which together constitute the adenylic acid portion of the molecule. * **Sulfhydryl Group (-SH):** This is the functional part of the molecule provided by **beta-mercaptoethylamine**. It is where acyl groups (like acetate) attach; hence CoA is often abbreviated as CoA-SH. **NEET-PG High-Yield Pearls:** 1. **Precursor:** Pantothenic acid (B5) is the essential vitamin precursor. Deficiency is rare but can cause "Burning Foot Syndrome." 2. **Functional Group:** The reactive part of CoA is the terminal **thiol (-SH) group**. 3. **Synthesis:** The rate-limiting step in CoA synthesis is the phosphorylation of pantothenate by **pantothenate kinase**. 4. **Key Derivatives:** Acetyl-CoA (2 carbons), Succinyl-CoA (4 carbons - used in heme synthesis), and HMG-CoA (cholesterol synthesis).

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

Practice Questions

Enzyme Inhibition: Competitive and Non-competitive

Practice Questions

Allosteric Regulation

Practice Questions

Coenzymes and Cofactors

Practice Questions

Isoenzymes and Clinical Significance

Practice Questions

Enzyme Regulation: Covalent Modification

Practice Questions

Enzyme Regulation: Zymogen Activation

Practice Questions

Enzyme Induction and Repression

Practice Questions

Ribozymes and Catalytic RNA

Practice Questions

Enzyme Diagnostic Applications

Practice Questions

Enzyme Therapy and Inhibitors as Drugs

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?