Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 111: What is true about isoenzymes?

A. Same structure and similar function
B. Different structure and similar function (Correct Answer)
C. Catalyze different reactions
D. Have the same Km

Explanation: **Explanation:** **Isoenzymes (or Isozymes)** are multiple forms of an enzyme that catalyze the **same chemical reaction** but differ in their physical and chemical properties. 1. **Why Option B is Correct:** Isoenzymes are products of different genes or different alleles. Consequently, they possess **different primary structures** (amino acid sequences). Despite these structural differences, they retain the same active site configuration required to catalyze the **same reaction** (similar function). This allows for fine-tuned metabolic regulation in different tissues. 2. **Why Other Options are Incorrect:** * **Option A:** If the structure were the same, they would be the same enzyme, not isoforms. * **Option C:** By definition, isoenzymes must catalyze the same reaction. Enzymes catalyzing different reactions are simply different enzymes. * **Option D:** Isoenzymes typically have **different kinetic properties** (different Km and Vmax) and different regulatory properties, allowing them to function optimally under the specific metabolic needs of different organs. **High-Yield Clinical Pearls for NEET-PG:** * **Lactate Dehydrogenase (LDH):** A tetramer with 5 isoenzymes. **LDH-1 (H4)** is predominant in the heart, while **LDH-5 (M4)** is found in skeletal muscle and liver. * **Creatine Kinase (CK):** A dimer with 3 isoenzymes. **CK-MB** is a specific marker for Myocardial Infarction; **CK-MM** is found in skeletal muscle; **CK-BB** is found in the brain. * **Hexokinase vs. Glucokinase:** These are functional isoenzymes. Glucokinase (Liver/Pancreas) has a **high Km** (low affinity) for glucose, whereas Hexokinase (Extrahepatic) has a **low Km** (high affinity).

Question 112: What is allosteric inhibition of an enzyme?

A. Binding of an inhibitor to the catalytic site, causing enzyme inhibition.
B. Binding of an inhibitor to a site other than the catalytic site, causing enzyme inhibition. (Correct Answer)
C. Enzyme inhibition by molecules that do not bind to the enzyme.
D. Enzyme inactivation through phosphorylation or dephosphorylation.

Explanation: **Explanation:** **1. Why Option B is Correct:** Allosteric inhibition occurs when an effector molecule binds to a specific site on the enzyme known as the **allosteric site** (or regulatory site), which is spatially distinct from the active (catalytic) site. This binding induces a **conformational change** in the enzyme's tertiary structure, reducing its affinity for the substrate or decreasing its catalytic velocity ($V_{max}$). This is a key mechanism for feedback inhibition in metabolic pathways. **2. Analysis of Incorrect Options:** * **Option A:** Describes **Competitive Inhibition**. In this type, the inhibitor mimics the substrate and competes directly for the catalytic site. It increases $K_m$ but leaves $V_{max}$ unchanged. * **Option C:** This is physiologically impossible. Enzyme inhibition requires a physical or chemical interaction between the enzyme and the inhibitory molecule/condition. * **Option D:** Describes **Covalent Modification**. While this is a regulatory mechanism (e.g., Glycogen Phosphorylase is activated by phosphorylation), it is distinct from allosteric regulation, which involves non-covalent, reversible binding. **3. NEET-PG High-Yield Pearls:** * **Kinetics:** Allosteric enzymes do not follow classic Michaelis-Menten kinetics; they typically show a **Sigmoidal (S-shaped) curve** rather than a hyperbolic one. * **Key Example:** **Phosphofructokinase-1 (PFK-1)**, the rate-limiting enzyme of glycolysis, is allosterically inhibited by **ATP and Citrate**, and activated by **AMP and Fructose 2,6-bisphosphate**. * **Aspartate Transcarbamoylase (ATCase):** A classic example of allosteric inhibition by CTP (feedback inhibition). * **Cooperativity:** Allosteric enzymes often exhibit cooperativity, where binding at one subunit affects the others.

Question 113: BNP is regarded by which of the following enzymes?

A. Neutral endopeptidases (Correct Answer)
B. Elastase
C. Omapatrilat
D. ACE

Explanation: **Explanation:** **1. Why Neutral Endopeptidases (NEP) is correct:** B-type Natriuretic Peptide (BNP), along with ANP and CNP, plays a crucial role in cardiovascular homeostasis by promoting vasodilation and natriuresis. The primary enzyme responsible for the degradation and inactivation of these natriuretic peptides is **Neutral Endopeptidase (also known as Neprilysin)**. NEP is a zinc-dependent metalloendopeptidase that cleaves the peptides at the amino side of hydrophobic residues, effectively terminating their biological activity. **2. Analysis of Incorrect Options:** * **Elastase:** This is a protease that breaks down elastin in connective tissue. While it is involved in lung pathology (e.g., emphysema), it does not play a role in BNP metabolism. * **Omapatrilat:** This is not an enzyme; it is a **drug** (a vasopeptidase inhibitor) that inhibits both NEP and ACE. It was designed to increase BNP levels while lowering Angiotensin II, but it is not the endogenous enzyme that degrades BNP. * **ACE (Angiotensin-Converting Enzyme):** ACE is primarily responsible for converting Angiotensin I to Angiotensin II and degrading Bradykinin. It does not significantly degrade BNP. **3. Clinical Pearls for NEET-PG:** * **Sacubitril:** A potent Neprilysin inhibitor used in the treatment of Heart Failure (combined with Valsartan as ARNI). By inhibiting NEP, it increases the levels of BNP, leading to beneficial diuresis and reduced cardiac remodeling. * **Diagnostic Marker:** BNP and NT-proBNP are gold-standard biomarkers for diagnosing and prognosticating Heart Failure. * **Neprilysin Location:** It is highly expressed in the proximal tubules of the kidney and the lungs.

Question 114: Which of the following is NOT a non-functional enzyme?

A. Alkaline phosphatase
B. Acid phosphatase
C. Lipoprotein lipase (Correct Answer)
D. Gamma glutamyl transpeptidase

Explanation: ### Explanation In clinical biochemistry, plasma enzymes are categorized into two groups based on their physiological role in the blood: **Functional** and **Non-functional** plasma enzymes. **1. Why Lipoprotein Lipase (LPL) is the Correct Answer:** **Lipoprotein lipase** is a **functional plasma enzyme**. These enzymes are actively secreted into the blood by organs (primarily the liver or vascular endothelium) and perform their primary physiological function within the circulation. LPL plays a critical role in lipid metabolism by hydrolyzing triglycerides found in chylomicrons and VLDL into free fatty acids and glycerol. Other examples include enzymes involved in blood coagulation (e.g., Thrombin) and pseudocholinesterase. **2. Why the Other Options are Incorrect:** Options A, B, and D are **Non-functional plasma enzymes**. These enzymes have no known physiological function in the blood. They are normally present intracellularly and appear in the plasma only due to routine cell turnover or pathological cell damage. * **Alkaline Phosphatase (ALP):** Primarily a marker for hepatobiliary diseases (obstructive jaundice) and bone disorders. * **Acid Phosphatase (ACP):** Historically used as a marker for prostate cancer. * **Gamma-glutamyl transpeptidase (GGT):** A sensitive marker for biliary obstruction and alcohol consumption. **3. High-Yield Clinical Pearls for NEET-PG:** * **Functional Enzymes:** Substrate concentration is usually high in plasma; their deficiency leads to specific metabolic diseases. * **Non-functional Enzymes:** Substrate concentration is absent in plasma; their **elevation** is used as a diagnostic tool for organ damage (e.g., ALT/AST for liver, CK-MB/Troponin for MI). * **LPL Stimulator:** Insulin increases LPL activity, while **Heparin** releases LPL from the endothelial surface into the plasma (Post-heparin lipolytic activity).

Question 115: Which of the following enzymes is not a protein?

A. DNAse
B. Abzyme
C. Eco RI
D. Ribozyme (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Ribozyme** **Why it is correct:** The fundamental dogma of biochemistry states that almost all enzymes are proteins. However, **Ribozymes** are a significant exception; they are **RNA molecules** that possess catalytic activity. They function by positioning specific parts of their RNA structure to facilitate chemical reactions, such as peptide bond formation in ribosomes (Peptidyl transferase) or RNA splicing. Since they are composed of nucleotides rather than amino acids, they are non-protein enzymes. **Analysis of Incorrect Options:** * **A. DNAse (Deoxyribonuclease):** This is a classic protein enzyme that catalyzes the hydrolytic cleavage of phosphodiester linkages in the DNA backbone. * **B. Abzyme (Antibody Enzyme):** These are monoclonal antibodies with catalytic activity. Since antibodies (immunoglobulins) are globular proteins, abzymes are proteinaceous in nature. * **C. Eco RI:** This is a bacterial restriction endonuclease used extensively in recombinant DNA technology. Like almost all restriction enzymes, it is a protein. **High-Yield NEET-PG Pearls:** * **Peptidyl Transferase:** The most clinically important ribozyme; it is the 23S rRNA (in prokaryotes) or 28S rRNA (in eukaryotes) responsible for protein synthesis. * **RNAse P:** A ribozyme involved in the processing of tRNA molecules. * **Cofactors:** While ribozymes are non-protein, many protein enzymes require non-protein components called **coenzymes** (organic) or **cofactors** (inorganic) for activity. * **Isoenzymes:** Remember that different physical forms of the same enzyme (like LDH or CK) are still proteins, despite differing in their amino acid sequences.

Question 116: What are the units of measure for the specific activity of an enzyme?

A. Millimoles per liter
B. Units of activity per milligram of protein (Correct Answer)
C. Micromoles per minute
D. Units of activity per minute

Explanation: ### Explanation **1. Why the Correct Answer is Right:** **Specific Activity** is a measure of **enzyme purity**. It is defined as the number of enzyme units (U) per milligram (mg) of total protein present in the sample. * **Formula:** Specific Activity = Enzyme Activity / Total Protein (Units/mg). * **Concept:** As a protein purification process progresses, the total amount of protein decreases while the desired enzyme activity is preserved. Consequently, the **Specific Activity increases**, reaching a maximum when the enzyme is pure. This makes it the gold standard for assessing the success of purification protocols. **2. Analysis of Incorrect Options:** * **Option A (Millimoles per liter):** This is a unit of **concentration** (molarity), not enzyme activity. * **Option C (Micromoles per minute):** This defines the **International Unit (IU)** of enzyme activity (the amount of enzyme that catalyzes the conversion of 1 µmol of substrate per minute). It measures the *quantity* of enzyme, not its *purity*. * **Option D (Units of activity per minute):** This is a redundant or incorrect expression of rate. Enzyme activity (Units) already incorporates the "per minute" component. **3. High-Yield Clinical Pearls for NEET-PG:** * **Katal:** The SI unit of enzyme activity, defined as 1 mole of substrate converted per second (1 Kat = 6 × 10⁷ IU). * **Turnover Number ($k_{cat}$):** The number of substrate molecules converted into product per enzyme molecule per unit time when the enzyme is fully saturated. It represents the maximum catalytic efficiency. * **Purification Table:** In exams, if you see a table showing "Total Protein" decreasing and "Specific Activity" increasing, it indicates successful purification. * **Diagnostic Enzymes:** Remember that in clinical practice, we usually measure **Enzyme Activity** (IU/L) in serum to diagnose conditions like Myocardial Infarction (CK-MB) or Liver disease (ALT/AST).

Question 117: What is the substrate concentration at which the initial velocity (Vi) is half the maximal velocity attainable at a particular concentration of enzyme?

A. Vmax
B. Vmax/2
C. Km (Correct Answer)
D. Km/2

Explanation: ### Explanation The relationship between substrate concentration $[S]$ and the rate of an enzyme-catalyzed reaction is described by the **Michaelis-Menten Equation**: $$V_i = \frac{V_{max} [S]}{K_m + [S]}$$ **Why Km is correct:** By definition, the **Michaelis constant ($K_m$)** is the substrate concentration at which the reaction velocity is exactly half of the maximum velocity ($V_{max}/2$). Mathematically, if you substitute $[S] = K_m$ into the equation, the expression simplifies to $V_i = V_{max}/2$. $K_m$ is a fundamental characteristic of an enzyme that reflects its **affinity** for a substrate; a low $K_m$ indicates high affinity, while a high $K_m$ indicates low affinity. **Why other options are incorrect:** * **Vmax:** This represents the maximum possible velocity when the enzyme is fully saturated with substrate. It is a rate, not a concentration. * **Vmax/2:** This is the *velocity* achieved at $K_m$, not the substrate concentration itself. * **Km/2:** At this concentration, the velocity would be $1/3$ of $V_{max}$, not half. **High-Yield Clinical Pearls for NEET-PG:** * **Lineweaver-Burk Plot:** A double-reciprocal plot where the x-intercept is $-1/K_m$ and the y-intercept is $1/V_{max}$. * **Competitive Inhibition:** $K_m$ increases (affinity decreases), but $V_{max}$ remains unchanged. * **Non-competitive Inhibition:** $K_m$ remains unchanged, but $V_{max}$ decreases. * **Glucokinase vs. Hexokinase:** Glucokinase has a high $K_m$ (low affinity) for glucose, allowing it to function only when blood glucose levels are high (e.g., postprandial).

Question 118: Which of the following is NOT a characteristic feature of allosteric enzymes?

A. They are multi enzyme complex
B. Follow Michaelis-Menten kinetics (Correct Answer)
C. Presence of modulator site
D. Give sigmoid shaped curve

Explanation: ### Explanation **Why Option B is the Correct Answer:** Allosteric enzymes do **not** follow Michaelis-Menten kinetics. Michaelis-Menten kinetics describe enzymes that produce a **hyperbolic curve** when plotting reaction velocity ($V$) against substrate concentration ($[S]$). In contrast, allosteric enzymes exhibit **cooperativity** (usually positive), where the binding of a substrate to one active site increases the affinity of other active sites. This results in a **Sigmoid (S-shaped) curve**, making Option B the false statement and thus the correct answer. **Analysis of Incorrect Options:** * **Option A (Multi-enzyme complex/Subunits):** Allosteric enzymes are typically complex proteins composed of multiple subunits (quaternary structure). This structural complexity is necessary for the communication between different active and regulatory sites. * **Option C (Presence of modulator site):** A hallmark of allosteric enzymes is the presence of an **allosteric (regulatory) site**, which is distinct from the catalytic (active) site. Modulators (activators or inhibitors) bind here to alter the enzyme's activity. * **Option D (Sigmoid shaped curve):** As mentioned, the cooperative binding nature of these enzymes results in a sigmoidal velocity curve rather than a hyperbolic one. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Steps:** Allosteric enzymes usually catalyze the committed or rate-limiting step of a metabolic pathway (e.g., **PFK-1** in Glycolysis). * **K-series vs. V-series:** Allosteric inhibitors that increase $K_m$ (decrease affinity) are K-series enzymes; those that decrease $V_{max}$ are V-series enzymes. * **Key Example:** **Aspartate Transcarbamoylase (ATCase)** is the classic model for allosteric regulation. * **Feedback Inhibition:** Allosteric enzymes are the primary targets for feedback inhibition, where the end-product of a pathway inhibits the first committed enzyme.

Question 119: Serum creatine kinase-3 (CK-3) is elevated in which of the following conditions?

A. Muscular dystrophy (Correct Answer)
B. Myocardial infarction
C. Alcoholic cirrhosis
D. Brain tumours

Explanation: **Explanation:** Creatine Kinase (CK) is a dimeric enzyme consisting of two subunits: **M (Muscle)** and **B (Brain)**. These subunits combine to form three distinct isoenzymes. Understanding their tissue distribution is crucial for NEET-PG: 1. **CK-1 (BB):** Predominantly found in the **Brain**. 2. **CK-2 (MB):** Predominantly found in the **Myocardium** (Heart). 3. **CK-3 (MM):** Predominantly found in **Skeletal Muscle**. **Why Muscular Dystrophy is correct:** CK-3 (MM) accounts for approximately 98-99% of the total CK found in skeletal muscle. In conditions involving skeletal muscle destruction, such as **Duchenne Muscular Dystrophy (DMD)**, rhabdomyolysis, or strenuous exercise, the enzyme leaks into the bloodstream. In DMD, serum CK-3 levels are characteristically elevated (often 50–100 times the upper limit of normal) even before clinical symptoms appear. **Analysis of Incorrect Options:** * **Myocardial Infarction:** This condition primarily leads to an elevation of **CK-2 (MB)**. While CK-3 is also present in the heart, CK-MB is the specific diagnostic marker used (though now largely replaced by Troponins). * **Alcoholic Cirrhosis:** Liver diseases typically show elevations in ALT, AST, and GGT. CK is not a marker for hepatic injury. * **Brain Tumours:** These may lead to an elevation of **CK-1 (BB)**, as this isoenzyme is localized to the central nervous system. **High-Yield Clinical Pearls:** * **CK-MB Index:** If CK-MB is >5% of total CK, it suggests myocardial damage; if <3%, it suggests skeletal muscle damage. * **DMD Carrier Detection:** Serum CK-3 is elevated in about 50-80% of female carriers of the Duchenne muscular dystrophy gene. * **Macro-CK:** A high-yield variant where CK-1 is bound to IgG; it can cause a false elevation in total CK levels.

Question 120: Which one of the following reactions has flavin mononucleotide (FMN) as a coenzyme?

A. Amino acid oxidation reaction (Correct Answer)
B. Conversion of xanthine to uric acid
C. Conversion of succinate to fumarate
D. Conversion of pyruvate to acetyl CoA

Explanation: **Explanation:** The correct answer is **A. Amino acid oxidation reaction**. **1. Why Option A is Correct:** Flavin mononucleotide (FMN) is a derivative of Vitamin B2 (Riboflavin). It serves as a prosthetic group for **L-amino acid oxidase**, an enzyme found in the kidneys and liver that catalyzes the oxidative deamination of L-amino acids into α-keto acids and ammonia. While most amino acid metabolism occurs via transamination or NAD-linked glutamate dehydrogenase, L-amino acid oxidase specifically utilizes **FMN** to facilitate the transfer of electrons. **2. Why Other Options are Incorrect:** * **B. Conversion of xanthine to uric acid:** This reaction is catalyzed by **Xanthine Oxidase**. While it is a flavoprotein, it primarily utilizes **FAD** (Flavin Adenine Dinucleotide), along with Molybdenum and Iron, not FMN. * **C. Conversion of succinate to fumarate:** This is catalyzed by **Succinate Dehydrogenase** (Complex II of the ETC). This enzyme specifically uses **FAD** as its coenzyme. * **D. Conversion of pyruvate to acetyl CoA:** This is catalyzed by the **Pyruvate Dehydrogenase (PDH) Complex**. The flavin component involved in this multienzyme complex (E3 subunit) is **FAD**, not FMN. **3. NEET-PG High-Yield Pearls:** * **FMN-containing enzymes:** The two most important for exams are **L-amino acid oxidase** and **NADH dehydrogenase (Complex I)** of the electron transport chain. * **FAD-containing enzymes:** Succinate dehydrogenase, Pyruvate dehydrogenase, and Acyl-CoA dehydrogenase (Beta-oxidation). * **Clinical Correlation:** Riboflavin deficiency (Ariboflavinosis) manifests as cheilosis, glossitis (magenta tongue), and corneal vascularization. Always remember that FMN and FAD are the active coenzyme forms of Vitamin B2.

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

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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