Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 21: Which of the following is a competitive inhibitor of the succinate dehydrogenase enzyme?

A. Succinic acid
B. Fumaric acid
C. Malonic acid (Correct Answer)
D. Oxalic acid

Explanation: **Explanation:** **1. Why Malonic Acid is Correct:** Competitive inhibition occurs when a molecule structurally resembles the substrate and competes for the active site of an enzyme. In the Citric Acid Cycle (TCA cycle), **Succinate Dehydrogenase (SDH)** converts Succinate to Fumarate. **Malonic acid (Malonate)** is a classic example of a competitive inhibitor because its chemical structure is very similar to Succinate (both are dicarboxylic acids). Malonate binds to the active site of SDH, preventing Succinate from binding, thereby inhibiting the reaction. This inhibition can be overcome by increasing the concentration of the substrate (Succinate). **2. Analysis of Incorrect Options:** * **A. Succinic acid:** This is the **substrate** for the enzyme, not an inhibitor. * **B. Fumaric acid:** This is the **product** of the reaction catalyzed by succinate dehydrogenase. While high concentrations of products can sometimes cause feedback inhibition, it is not the classic competitive inhibitor used to describe this mechanism. * **C. Oxalic acid:** While also a dicarboxylic acid, it is not the specific competitive inhibitor for SDH in the context of the TCA cycle. **3. NEET-PG High-Yield Pearls:** * **Kinetics:** In competitive inhibition, the **$V_{max}$ remains unchanged**, but the **$K_m$ increases** (affinity for the substrate decreases). * **Location:** Succinate Dehydrogenase is unique because it is the only TCA cycle enzyme located in the **inner mitochondrial membrane** (it also functions as Complex II in the Electron Transport Chain). * **Other Examples:** Other high-yield competitive inhibitors include **Statins** (inhibiting HMG-CoA reductase) and **Methanol/Ethylene glycol poisoning** (treated with Ethanol or Fomepizole via competitive inhibition of Alcohol Dehydrogenase).

Question 22: Which of the following statements is FALSE regarding Cytochrome P-450 enzymes?

A. They are involved in the production of steroids.
B. They absorb maximum light at 450nm wavelength.
C. They are present in the endoplasmic reticulum of liver cells.
D. They are non-heme proteins. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is the Correct Answer (The False Statement)** Cytochrome P450 (CYP) enzymes are **heme-containing proteins** (hemeproteins). They belong to a superfamily of enzymes that contain a molecule of iron-protoporphyrin IX at their active site. This heme group is essential for their function as it binds oxygen and facilitates the hydroxylation of various substrates. Therefore, stating they are "non-heme proteins" is biochemically incorrect. **2. Analysis of Incorrect Options (True Statements)** * **Option A:** CYPs are crucial for the **biosynthesis of steroid hormones** (e.g., cortisol, aldosterone, and sex steroids) in the adrenal cortex and gonads, as well as the synthesis of bile acids and Vitamin D. * **Option B:** The name "P450" is derived from the fact that when these enzymes are in a reduced state and bound to carbon monoxide (CO), they exhibit a characteristic absorption peak (Soret peak) at **450 nm**. * **Option C:** These enzymes are primarily located in the **smooth endoplasmic reticulum** (microsomes) of hepatocytes, where they play a central role in the Phase I metabolism (hydroxylation) of drugs and xenobiotics. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Reaction Type:** They are **Monooxygenases** (Mixed-function oxidases). They incorporate one atom of oxygen into the substrate and reduce the other atom to water. * **Requirement:** They require **NADPH** and the enzyme **NADPH-cytochrome P450 reductase**. * **Inducers vs. Inhibitors:** * *Inducers:* Rifampicin, Phenytoin, Carbamazepine, Chronic Alcohol (increase drug metabolism). * *Inhibitors:* Ketoconazole, Erythromycin, Cimetidine, Grapefruit juice (decrease drug metabolism, leading to toxicity). * **Polymorphism:** CYP2D6 shows significant genetic polymorphism, affecting the metabolism of drugs like codeine and beta-blockers.

Question 23: Which of the following is an example of a metalloenzyme?

A. Lysyl oxidase (Correct Answer)
B. Lysyl hydroxylase
C. Prolyl hydroxylase
D. Glucosyl transferase

Explanation: **Explanation:** The distinction between a **metalloenzyme** and a **metal-activated enzyme** is a high-yield concept in Biochemistry. A metalloenzyme contains a tightly bound metal ion as an integral part of its structure, which is essential for its catalytic activity. **1. Why Lysyl Oxidase is Correct:** Lysyl oxidase is a classic example of a **metalloenzyme** that requires **Copper (Cu²⁺)** as a cofactor. It plays a critical role in the extracellular matrix by catalyzing the oxidative deamination of lysine and hydroxylysine residues in collagen and elastin. This process leads to the formation of allysine, which is necessary for the **cross-linking** of collagen fibers, providing tensile strength to tissues. **2. Analysis of Incorrect Options:** * **Lysyl hydroxylase & Prolyl hydroxylase:** These enzymes are involved in the post-translational modification of collagen *inside* the cell. They are not metalloenzymes in the same category; they require **Ferrous iron (Fe²⁺)** and **Vitamin C (Ascorbic acid)** as cofactors. Deficiency leads to Scurvy. * **Glucosyl transferase:** This enzyme is involved in the glycosylation of hydroxylysine residues during collagen synthesis and does not function as a copper-dependent metalloenzyme. **3. High-Yield Clinical Pearls for NEET-PG:** * **Menkes Disease:** A defect in copper absorption (ATP7A gene) leads to low copper levels, causing decreased **Lysyl oxidase** activity. This results in "kinky hair," connective tissue defects, and arterial tortuosity. * **Wilson Disease:** A defect in copper excretion (ATP7B gene) leading to copper toxicity. * **Other Copper-containing enzymes:** Tyrosinase, Cytochrome c oxidase, Superoxide dismutase (cytosolic), and Ceruloplasmin. * **Zinc-containing enzymes:** Carbonic anhydrase, Alcohol dehydrogenase, and Carboxypeptidase.

Question 24: Glutathione peroxidase is a/an?

A. Catalase
B. Antioxidant (Correct Answer)
C. Microsomal enzyme
D. None of the above

Explanation: **Explanation:** **Glutathione Peroxidase (GPx)** is a critical intracellular enzyme that functions as a major **antioxidant**. Its primary role is to protect cells from oxidative damage by catalyzing the reduction of hydrogen peroxide ($H_2O_2$) and lipid hydroperoxides into water and alcohols, respectively. This reaction uses **Reduced Glutathione (GSH)** as a hydrogen donor, converting it into its oxidized form (GSSG). **Why Option B is Correct:** GPx is the body’s primary defense against oxidative stress in the cytosol and mitochondria. By neutralizing reactive oxygen species (ROS), it prevents lipid peroxidation of cell membranes and DNA damage. **Analysis of Incorrect Options:** * **Option A (Catalase):** While both GPx and Catalase decompose $H_2O_2$, they are distinct enzymes. Catalase is primarily located in **peroxisomes** and handles high concentrations of $H_2O_2$, whereas GPx is found in the cytosol and mitochondria and is effective even at low peroxide concentrations. * **Option C (Microsomal enzyme):** Microsomal enzymes (like the Cytochrome P450 system) are located in the smooth endoplasmic reticulum and are mainly involved in drug metabolism (xenobiotics). GPx is not a microsomal enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Selenium Dependency:** GPx is a **selenoprotein**; it contains the 21st amino acid, **Selenocysteine**, at its active site. Selenium deficiency leads to reduced GPx activity. * **G6PD Link:** The regeneration of GSH (required by GPx) depends on **Glutathione Reductase**, which requires **NADPH**. This NADPH is supplied by the HMP Shunt (G6PD enzyme). This is why G6PD deficiency leads to hemolysis due to oxidative stress. * **Location:** It is the most abundant selenoprotein in mammals, found in almost all tissues.

Question 25: Trypsin cleaves which amino acid residue?

A. Arginine (Correct Answer)
B. Glutamate
C. Lysine
D. Proline

Explanation: **Explanation:** Trypsin is a serine protease found in the digestive system that plays a critical role in protein degradation. It exhibits high specificity for **basic amino acids**. **Why Arginine is Correct:** Trypsin cleaves peptide bonds specifically at the **carboxyl side** of the basic amino acids **Arginine (Arg)** and **Lysine (Lys)**. In the context of this question, Arginine is the primary correct choice. The specificity is due to the enzyme's "specificity pocket" (S1 pocket), which contains a negatively charged Aspartate residue at its bottom, allowing it to attract and bind the positively charged side chains of Arginine and Lysine. **Analysis of Incorrect Options:** * **Glutamate (B):** This is an acidic amino acid. Enzymes like Pepsin or certain bacterial proteases may prefer acidic residues, but Trypsin rejects them due to charge repulsion. * **Lysine (C):** While Trypsin *does* cleave at Lysine, in many competitive exams, if both are listed separately or if a single best answer is required based on specific clinical vignettes, Arginine is often highlighted due to its strongly basic guanidino group. (Note: In many standard texts, both are correct; however, if the question identifies A as the sole key, it follows the convention of prioritizing the most strongly basic residue). * **Proline (D):** Trypsin **cannot** cleave a bond if the succeeding residue is Proline. The rigid ring structure of Proline creates a conformational constraint that prevents the peptide bond from fitting into the enzyme's active site. **Clinical Pearls for NEET-PG:** * **Zymogen Activation:** Trypsin is secreted as inactive **Trypsinogen**. It is activated by **Enteropeptidase (Enterokinase)**, a brush-border enzyme. Once activated, Trypsin acts as the common activator for all other pancreatic zymogens (Chymotrypsinogen, Procarboxypeptidase, etc.). * **Acute Pancreatitis:** Premature activation of Trypsin within the pancreas leads to autodigestion of the organ. * **Diagnostic Marker:** Urinary Trypsinogen Activated Peptide (TAP) is a marker used to assess the severity of acute pancreatitis.

Question 26: Which of the following amino acids is a component of Thioredoxin reductase?

A. Selenocysteine (Correct Answer)
B. Cysteine
C. Methionine
D. Homocysteine

Explanation: ### Explanation **Correct Answer: A. Selenocysteine** **Why Selenocysteine is correct:** Selenocysteine is known as the **21st amino acid**. It is a unique amino acid where the sulfur atom of cysteine is replaced by **Selenium**. It is incorporated into proteins during translation via a specialized mechanism involving the UGA stop codon and a specific tRNA. **Thioredoxin reductase** is a critical antioxidant enzyme that relies on the selenocysteine residue at its active site to reduce thioredoxin. This process is essential for DNA synthesis (via ribonucleotide reductase) and for protecting cells against oxidative stress. **Why the other options are incorrect:** * **B. Cysteine:** While cysteine is structurally similar and contains sulfur, it lacks the redox potential provided by selenium required for the specific catalytic activity of Thioredoxin reductase. * **C. Methionine:** This is an essential sulfur-containing amino acid primarily involved in initiation of translation and methyl group donation (as S-adenosylmethionine), not the catalytic redox center of this enzyme. * **D. Homocysteine:** This is an intermediary metabolite in the methionine cycle. Elevated levels are a risk factor for cardiovascular disease, but it is not a standard component of functional enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **Other Selenoenzymes:** Glutathione peroxidase (converts $H_2O_2$ to $H_2O$), Deiodinase (converts $T_4$ to $T_3$), and Selenoprotein P. * **Coding:** Selenocysteine is encoded by the **UGA codon** (normally a stop codon) in the presence of a **SECIS** (Selenocysteine Insertion Sequence) element in the mRNA. * **Deficiency:** Selenium deficiency can lead to **Keshan disease** (cardiomyopathy) or **Kashin-Beck disease** (osteoarthropathy).

Question 27: Regan enzyme is an isoenzyme of which of the following?

A. Lactate dehydrogenase
B. Creatine kinase
C. Acid phosphatase
D. Alkaline phosphatase (Correct Answer)

Explanation: **Explanation:** **Regan isoenzyme** is a biochemical marker of significant clinical importance in oncology. It is a heat-stable, placental-like isoenzyme of **Alkaline Phosphatase (ALP)**. ### Why Alkaline Phosphatase is Correct: Alkaline Phosphatase exists in several isoforms (Liver, Bone, Intestine, and Placenta). The **Regan enzyme** is an ectopic form of the placental ALP isoenzyme. It is produced by certain malignant tumors, most notably **carcinoma of the lung (bronchogenic)**, ovary, and pancreas. Its primary characteristic is that it is **heat-stable** (resists denaturation at 65°C), mimicking the properties of normal placental ALP but occurring in non-pregnant individuals as a paraneoplastic marker. ### Why Other Options are Incorrect: * **Lactate Dehydrogenase (LDH):** LDH has five major isoenzymes (LDH1-LDH5) used to identify tissue damage (e.g., LDH1 in MI, LDH5 in liver disease), but none are termed Regan. * **Creatine Kinase (CK):** CK has three main isoenzymes: MM (muscle), MB (heart), and BB (brain). It does not have a placental-like ectopic variant associated with the Regan name. * **Acid Phosphatase (ACP):** ACP is primarily a marker for prostatic carcinoma (specifically the tartrate-inhibitable fraction) and bone resorption, not related to the Regan variant. ### NEET-PG High-Yield Pearls: * **Nagao Isoenzyme:** Another ALP variant, similar to Regan but inhibited by **L-leucine**. * **Heat Stability Rule:** "Bone is Burned, Placenta is Persistent." (Bone ALP is heat-labile; Placental/Regan ALP is heat-stable). * **Clinical Association:** Regan enzyme is a classic example of **ectopic protein production** by cancer cells (paraneoplastic syndrome).

Question 28: What is the action of adenylate cyclase?

A. Conversion of ATP to ADP
B. Conversion of ADP to ATP
C. Conversion of ATP to cAMP (Correct Answer)
D. Conversion of AMP to ADP

Explanation: **Explanation:** **Adenylate cyclase** (also known as adenylyl cyclase) is a membrane-bound enzyme that plays a pivotal role in the G-protein coupled receptor (GPCR) signaling pathway. Its primary function is to catalyze the conversion of **ATP (Adenosine Triphosphate) into cAMP (cyclic Adenosine Monophosphate)** and pyrophosphate. This reaction is triggered when a ligand (like Glucagon or Epinephrine) binds to a Gs-protein-coupled receptor, activating the alpha subunit which then stimulates adenylate cyclase. cAMP acts as a crucial **second messenger**, activating Protein Kinase A (PKA) to mediate various cellular responses. **Analysis of Incorrect Options:** * **Options A & B:** The interconversion of ATP and ADP is primarily managed by **ATPases** (hydrolysis) or **ATP Synthase/Kinases** (phosphorylation). These are involved in energy metabolism rather than second messenger signaling. * **Option D:** The conversion of AMP to ADP is catalyzed by **Adenylate Kinase** (also known as myokinase), which maintains equilibrium among adenine nucleotides. **High-Yield Clinical Pearls for NEET-PG:** * **Termination of Signal:** The action of cAMP is terminated by **Phosphodiesterase (PDE)**, which converts cAMP into 5'-AMP. Drugs like Theophylline and Caffeine inhibit PDE, thereby increasing cAMP levels. * **Bacterial Toxins:** **Cholera toxin** and **E. coli (LT)** cause permanent activation of adenylate cyclase by ADP-ribosylation of the Gs subunit, leading to massive secretion of water and electrolytes (diarrhea). * **Inhibition:** Adenylate cyclase is inhibited by the **Gi (inhibitory)** protein, which is the target of the Pertussis toxin.

Question 29: For every 10-degree Celsius raise in temperature, the rate of most enzymatic reactions approximately:

A. Halves
B. Doubles (Correct Answer)
C. Quadruples
D. Increases 10-fold

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The relationship between temperature and reaction rate is defined by the **Temperature Coefficient ($Q_{10}$)**. For most biological systems and enzymatic reactions, the $Q_{10}$ value is approximately **2**. This means that for every 10°C rise in temperature (within the physiological range), the kinetic energy of the substrate molecules increases, leading to more frequent and effective collisions with the enzyme's active site. This results in a **doubling** of the reaction velocity. **2. Why the Incorrect Options are Wrong:** * **A. Halves:** Increasing temperature generally increases kinetic energy; a decrease in rate only occurs if the temperature exceeds the optimum point, causing protein denaturation. * **C & D. Quadruples / Increases 10-fold:** While the rate increases, it is not exponential to this degree. A 4-fold or 10-fold increase would require a much higher $Q_{10}$ value, which is not characteristic of standard human metabolic enzymes. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Optimum Temperature:** For most human enzymes, the optimum temperature is **37°C**. Beyond 40–50°C, enzymes undergo **denaturation** (loss of tertiary structure), leading to a precipitous drop in reaction rate. * **The Bell-Shaped Curve:** The graph of enzyme velocity vs. temperature is typically bell-shaped. * **Exception:** Certain bacteria (e.g., *Thermus aquaticus*) have heat-stable enzymes like **Taq polymerase**, which can function at near-boiling temperatures—a property exploited in PCR. * **Hypothermia:** Clinically, this principle explains why induced hypothermia (lowering body temp) is used during cardiac surgeries to decrease the metabolic rate and oxygen demand of tissues.

Question 30: What is the Michaelis constant (Km)?

A. 3
B. 4 (Correct Answer)
C. 5
D. 6

Explanation: ***4*** - The **Michaelis constant (Km)** represents the substrate concentration at which the reaction velocity is **half of Vmax**. - From the graph, when Vmax/2 intersects the hyperbolic curve, the corresponding **[S] = 4 mM**, making Km = 4 mM. *3* - At **[S] = 3 mM**, the reaction velocity is **less than half of Vmax** based on the hyperbolic curve. - This value falls **below the Km point** where the enzyme achieves half-maximal velocity. *5* - At **[S] = 5 mM**, the reaction velocity is **greater than half of Vmax** according to the graph. - This substrate concentration **exceeds the Km value** where half-saturation occurs. *6* - At **[S] = 6 mM**, the reaction velocity is **significantly higher than Vmax/2** on the curve. - This concentration is **well above the Km point** and approaches enzyme saturation levels.

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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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