Enzymes Practice Questions

Q: Which of the following is NOT a rate-limiting enzyme?

Glyceraldehyde-3-phosphate dehydrogenase. **Explanation:** In metabolic pathways, **rate-limiting enzymes** catalyze the slowest, usually irreversible step that determines the overall flux of the pathway. These enzymes are typically regulated by hormones and allosteric effectors. **Why Glyceraldehyde-3-phosphate dehydrogenase (G3PDH) is the correct answer:** G3PDH is an enzyme involved in **Glycolysis**. It catalyzes the reversible conversion of Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. Because this reaction is **reversible** and exists in near-equilibrium, it does not serve as a regulatory or rate-limiting checkpoint. The rate-limiting step of glycolysis is actually catalyzed by **Phosphofructokinase-1 (PFK-1)**. **Analysis of Incorrect Options:** * **HMG-CoA Reductase:** This is the well-known rate-limiting enzyme for **Cholesterol synthesis**. It converts HMG-CoA to Mevalonate and is the primary target for Statin drugs. * **HMG-CoA Synthase:** This is the rate-limiting enzyme for **Ketogenesis** (specifically the mitochondrial isoform). It condenses Acetoacetyl-CoA with Acetyl-CoA to form HMG-CoA. * **Acetyl-CoA Carboxylase (ACC):** This is the rate-limiting enzyme for **De novo Fatty Acid synthesis**. It converts Acetyl-CoA to Malonyl-CoA and requires Biotin as a cofactor. **High-Yield Clinical Pearls for NEET-PG:** * **Statins:** Competitive inhibitors of HMG-CoA Reductase; they are the first-line treatment for hypercholesterolemia. * **Biotin (B7) Dependency:** Remember the mnemonic **"ABC"** for carboxylases (Acetyl-CoA, Propionyl-CoA, Pyruvate carboxylase)—they all require **A**TP, **B**iotin, and **C**O₂. * **G3PDH Inhibition:** Iodoacetate inhibits G3PDH, while Arsenate can uncouple the reaction, leading to the bypass of ATP synthesis at this step.

Q: Which enzyme in leucocytes is required for the production of hypochlorite?

Myeloperoxidase. ### Explanation The production of hypochlorite is a critical step in the **Respiratory Burst** (oxidative burst), a process used by phagocytes (neutrophils and monocytes) to kill ingested pathogens. **1. Why Myeloperoxidase (MPO) is correct:** Myeloperoxidase is a heme-containing enzyme found in the **azurophilic granules** of neutrophils. During the respiratory burst, it catalyzes the reaction between **hydrogen peroxide (H₂O₂)** and **chloride ions (Cl⁻)** to produce **hypochlorous acid (HOCl)**, commonly known as bleach. HOCl is the most potent bactericidal agent produced by neutrophils. **2. Analysis of Incorrect Options:** * **A. NADPH Oxidase:** This is the *initiating* enzyme of the respiratory burst. It converts molecular oxygen into **superoxide radicals ($O_2^{\cdot-}$)**. A deficiency in this enzyme leads to **Chronic Granulomatous Disease (CGD)**. * **C. Catalase:** This enzyme breaks down hydrogen peroxide into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). It actually acts as a protective mechanism for cells against oxidative damage and is used by "catalase-positive" bacteria to neutralize the host's H₂O₂. * **D. Superoxide Dismutase (SOD):** This enzyme converts the superoxide radical into hydrogen peroxide ($2O_2^{\cdot-} + 2H^+ \rightarrow H_2O_2 + O_2$). It precedes the action of MPO. ### High-Yield Clinical Pearls for NEET-PG: * **MPO Deficiency:** Usually asymptomatic because neutrophils can still kill bacteria using superoxide and H₂O₂, though the process is slower. However, patients may have increased susceptibility to *Candida* infections. * **Green Sputum:** The green color of pus and phlegm is attributed to the heme pigment in Myeloperoxidase. * **CGD Diagnosis:** Diagnosed via the **Nitroblue Tetrazolium (NBT) test** (negative/colorless in CGD) or the more modern **Dihydrorhodamine (DHR) flow cytometry** test.

Q: Which statement is false about covalent modification?

It uses the same enzyme for activation and inactivation. ### Explanation **1. Why Option C is the Correct Answer (The False Statement):** Covalent modification involves the addition or removal of a chemical group (most commonly a phosphate) to alter an enzyme's activity. Crucially, this process is **reciprocal but mediated by two different enzymes**. For example, in phosphorylation, a **Kinase** adds a phosphate group, while a **Phosphatase** removes it. Using the same enzyme for both directions would create a futile cycle and prevent effective metabolic switching. **2. Analysis of Incorrect Options (True Statements):** * **Option A (Reversible):** Unlike proteolytic cleavage (e.g., pepsinogen to pepsin), covalent modifications like phosphorylation, methylation, or adenylation are reversible, allowing the cell to toggle enzymes "on" and "off" as needed. * **Option B (Slower than Allosteric):** Allosteric regulation occurs almost instantaneously (milliseconds) via conformational changes. Covalent modification is slightly slower (seconds to minutes) as it requires an enzymatic reaction to occur. * **Option D (Phosphorylation):** This is the most common and high-yield example of covalent modification in human metabolism (e.g., Glycogen Phosphorylase). **3. High-Yield NEET-PG Clinical Pearls:** * **The "Rule of Phosphorylation":** In the **fasting state** (Glucagon/Epinephrine), key regulatory enzymes are usually **phosphorylated**. In the **fed state** (Insulin), they are **dephosphorylated**. * **Exception:** Most enzymes are *inactivated* by phosphorylation, but **Glycogen Phosphorylase** and **Hormone Sensitive Lipase** are *activated* by it. * **Amino Acids involved:** Phosphorylation typically occurs on the hydroxyl (-OH) groups of **Serine, Threonine, or Tyrosine** residues.

Q: Which of the following enzymes contains Zinc?

Carboxypeptidase. **Explanation:** **1. Why Carboxypeptidase is correct:** Carboxypeptidase (specifically Carboxypeptidase A and B) is a classic example of a **metalloenzyme** that requires **Zinc ($Zn^{2+}$)** for its catalytic activity. Zinc acts as a Lewis acid, coordinating with the carbonyl oxygen of the peptide bond to facilitate its cleavage. Zinc is a crucial cofactor for several other enzymes, including Carbonic anhydrase, Alcohol dehydrogenase, and Alkaline phosphatase. **2. Analysis of Incorrect Options:** * **Cytochrome oxidase (Option A):** This is a complex enzyme (Complex IV of the ETC) that contains **Copper ($Cu$)** and **Iron ($Fe$ in heme)**. It is responsible for the final reduction of oxygen to water. * **Glutathione peroxidase (Option B):** This is a high-yield enzyme because it contains the rare amino acid **Selenocysteine**, making **Selenium ($Se$)** its essential cofactor. It plays a vital role in protecting cells from oxidative damage. * **Catalase (Option C):** This enzyme, found in peroxisomes, contains **Iron ($Fe$)** in the form of a heme group. It decomposes hydrogen peroxide into water and oxygen. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Zinc Deficiency:** Presents as **Acrodermatitis enteropathica**, characterized by alopecia, dermatitis (periorificial and acral), and diarrhea. It also causes poor wound healing and hypogeusia (decreased taste). * **Zinc Finger Motifs:** Zinc is essential for the structure of "Zinc fingers," which are common DNA-binding domains in transcription factors (e.g., Steroid hormone receptors). * **Mnemonic for Zinc Enzymes:** "**A**lcoholic **C**ats **C**an **C**limb **R**eally **H**igh" — **A**lcohol dehydrogenase, **C**arbonic anhydrase, **C**arboxypeptidase, **C**u-Zn SOD, **R**NA polymerase, **H**istone deacetylase.

Q: Alcohol dehydrogenase belongs to which class of enzymes?

Oxidoreductase. **Explanation:** The International Union of Biochemistry (IUB) classifies enzymes into six major classes (EC 1 to EC 6). **Alcohol Dehydrogenase (ADH)** belongs to **Class 1: Oxidoreductases**. **1. Why Oxidoreductase is correct:** Oxidoreductases catalyze oxidation-reduction reactions involving the transfer of electrons or hydrogen atoms. Alcohol dehydrogenase facilitates the conversion of primary or secondary alcohols to aldehydes or ketones. In this reaction, the alcohol is oxidized while the coenzyme **NAD+** is reduced to **NADH**. Since it involves a redox reaction, it is fundamentally an oxidoreductase. **2. Analysis of Incorrect Options:** * **B. Dehydrogenase:** While ADH is indeed a dehydrogenase, this is a *sub-class*, not a primary IUB class. In NEET-PG, when asked for the "class," you must select from the six primary categories (Oxidoreductase, Transferase, Hydrolase, Lyase, Isomerase, Ligase). * **C. Hydrolase:** These enzymes (Class 3) catalyze the cleavage of bonds (C-O, C-N, C-C) by the addition of water (e.g., Pepsin, Urease). ADH does not use water to break bonds. * **D. Oxidase:** This is a sub-class of oxidoreductases where oxygen serves as the electron acceptor, often producing water or hydrogen peroxide (e.g., Cytochrome oxidase). ADH uses NAD+, not oxygen, as the primary electron acceptor. **Clinical Pearls for NEET-PG:** * **Metabolism:** ADH is the rate-limiting enzyme in ethanol metabolism, primarily located in the cytosol of hepatocytes. * **Inhibitor:** **Fomepizole** inhibits ADH and is used as an antidote in methanol or ethylene glycol poisoning to prevent the formation of toxic metabolites (formaldehyde/glycolic acid). * **Kinetics:** Alcohol metabolism follows **zero-order kinetics** because ADH becomes saturated at low ethanol concentrations.

Q: All of the following enzymes are active within a cell except:

Trypsin. **Explanation:** The core concept tested here is the distinction between **intracellular enzymes** and **extracellular (secretory) enzymes**. **Why Trypsin is the correct answer:** Trypsin is a digestive protease synthesized in the pancreas as an inactive precursor called **trypsinogen**. It is secreted into the duodenum, where it is activated by enteropeptidase. Because active trypsin is highly proteolytic and would cause **autodigestion** (pancreatitis) if active within the pancreatic acinar cells, it remains inactive (as a zymogen) while inside the cell. Therefore, it is not "active" within the cell. **Why the other options are incorrect:** * **Fumarase:** An essential enzyme of the **TCA cycle** located in the mitochondrial matrix. It must be active within the cell to facilitate cellular respiration. * **Hexokinase:** The first enzyme of **glycolysis**, active in the cytosol of almost all cells. It phosphorylates glucose to glucose-6-phosphate, trapping it inside the cell. * **Alcohol Dehydrogenase:** Primarily located in the cytosol of **hepatocytes**. It is active within the cell to metabolize ethanol into acetaldehyde. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** Enzymes secreted in inactive forms (e.g., pepsinogen, chymotrypsinogen) to protect the site of synthesis. * **Pancreatitis:** Occurs when trypsin is prematurely activated within the pancreatic cells, often due to ductal obstruction or alcohol-induced injury. * **Alpha-1 Antitrypsin:** A critical serum protein that inhibits proteases like trypsin and elastase, preventing tissue damage. * **Marker Enzymes:** Remember specific localizations: **ALT/AST** (Cytosol/Mitochondria), **Acid Phosphatase** (Lysosomes), and **Catalase** (Peroxisomes).

Q: Which enzyme is a marker for mitochondria?

Glutamate dehydrogenase. **Explanation:** In cell biology and biochemistry, specific enzymes are localized within particular organelles, serving as "biochemical markers" to identify the purity of subcellular fractions during centrifugation. **1. Why Glutamate Dehydrogenase (GDH) is correct:** Glutamate dehydrogenase is a key enzyme involved in nitrogen metabolism (oxidative deamination). It is located exclusively within the **mitochondrial matrix**. Since it is not found in the cytosol or other organelles, its presence in a cellular fraction confirms the presence of mitochondria. Other common mitochondrial markers include **Succinate Dehydrogenase (SDH)** (inner membrane) and **Cytochrome Oxidase**. **2. Analysis of Incorrect Options:** * **Acid Phosphatase:** This is the classic marker for **Lysosomes**. It is used clinically to detect lysosomal storage diseases and was historically used as a marker for prostatic carcinoma. * **Alkaline Phosphatase:** This is a marker for the **Plasma Membrane** (and also found in the endoplasmic reticulum). Clinically, it is elevated in obstructive jaundice and bone diseases. * **Hexokinase:** This is a key glycolytic enzyme located in the **Cytosol**. Note: While some isoforms can bind to the outer mitochondrial membrane, it is primarily considered a cytosolic marker. **3. High-Yield NEET-PG Clinical Pearls:** * **Marker for Peroxisomes:** Catalase. * **Marker for Golgi Apparatus:** Galactosyl transferase. * **Marker for Nucleus:** DNA Polymerase / RNA Polymerase. * **Marker for Microsomes (ER):** Glucose-6-phosphatase. * **Mitochondrial DNA:** It is circular, double-stranded, and maternally inherited (Mitochondrial Eve concept).

Question 1

Which of the following is NOT a rate-limiting enzyme?

Accepted Answer

Glyceraldehyde-3-phosphate dehydrogenase

Answer

HMG-CoA reductase

Answer

HMG-CoA synthase

Answer

Acetyl-CoA carboxylase

Question 2

Which enzyme in leucocytes is required for the production of hypochlorite?

Accepted Answer

Myeloperoxidase

Answer

NADPH oxidase

Answer

Catalase

Answer

Superoxide dismutase

Question 3

Which statement is false about covalent modification?

Accepted Answer

It uses the same enzyme for activation and inactivation

Answer

It is reversible

Answer

It is slower than allosteric regulation

Answer

Phosphorylation is a common covalent modification

Question 4

Which of the following enzymes contains Zinc?

Accepted Answer

Carboxypeptidase

Answer

Cytochrome oxidase

Answer

Glutathione peroxidase

Answer

Catalase

Question 5

Which of the following is a characteristic of serine proteases?

Accepted Answer

Are characterized by having several active sites per molecule, each containing a serine residue.

Answer

Hydrolyze peptide bonds involving carboxyl groups of serine residues.

Answer

Are inactivated by reacting with one molecule of di-isopropyl-fluorophosphate per molecule of protein.

Answer

Are exopeptidases.

Question 6

What is the temporal profile of detection of serum enzymes in acute myocardial infarction?

Accepted Answer

CPK, SGOT, LDH

Answer

SGOT, CPK, LDH

Answer

CPK, LDH, SGOT

Answer

SGOT, LDH, CPK

Question 7

Which of the following is NOT a mechanism for regulating enzyme activity?

Accepted Answer

Competitive inhibition

Answer

Covalent modification

Answer

Allosteric activation

Answer

Induction of genes for enzyme synthesis

Question 8

Alcohol dehydrogenase belongs to which class of enzymes?

Accepted Answer

Oxidoreductase

Answer

Dehydrogenase

Answer

Hydrolase

Answer

Oxidase

Question 9

All of the following enzymes are active within a cell except:

Accepted Answer

Trypsin

Answer

Fumarase

Answer

Hexokinase

Answer

Alcohol dehydrogenase

Question 10

Which enzyme is a marker for mitochondria?

Accepted Answer

Glutamate dehydrogenase

Answer

Acid phosphatase

Answer

Alkaline phosphatase

Answer

Hexokinase

Enzymes — MCQs

Enzymes — MCQs

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