Enzymes Practice Questions

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

Lipoprotein lipase. ### 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).

Q: Which of the following enzymes is not a protein?

Ribozyme. ### 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.

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

Follow Michaelis-Menten kinetics. ### 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.

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

Muscular dystrophy. **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.

Q: Which enzyme serves as a marker for the Golgi apparatus?

Galactosyl transferase. ### Explanation **Correct Option: C. Galactosyl transferase** The Golgi apparatus is the primary site for the post-translational modification of proteins and lipids, specifically **glycosylation**. **Galactosyl transferase** is an enzyme localized within the Golgi cisternae that catalyzes the addition of galactose to proteins. Because it is highly concentrated and specific to this organelle, it serves as the definitive biochemical marker for identifying the Golgi apparatus in cellular studies. **Analysis of Incorrect Options:** * **A. Peroxidase:** This enzyme is primarily found in **peroxisomes** (and certain specialized granules like those in neutrophils). It is involved in oxidation reactions. * **B. Galactosidase:** This is a hydrolytic enzyme found in **lysosomes**. While the Golgi *adds* sugars, lysosomes contain enzymes to *break them down*. * **C. Catalase:** This is the classic marker for **peroxisomes**. It is responsible for the decomposition of hydrogen peroxide ($H_2O_2$) into water and oxygen, protecting the cell from oxidative damage. **High-Yield Clinical Pearls for NEET-PG:** * **Golgi Functions:** "Packaging and Forwarding center" of the cell; involved in forming lysosomes and acrosomes of sperm. * **I-Cell Disease:** A clinical correlation where a deficiency in a Golgi enzyme (phosphotransferase) leads to the failure of tagging proteins with Mannose-6-Phosphate, causing them to be secreted extracellularly rather than sent to lysosomes. * **Other Organelle Markers:** * **Mitochondria:** ATP synthase / Succinate dehydrogenase. * **Lysosomes:** Acid phosphatase. * **Smooth ER:** Glucose-6-phosphatase. * **Nucleus:** DNA Polymerase.

Q: Which ion is most important in glycolysis?

Mg. **Explanation:** **Why Magnesium (Mg²⁺) is the Correct Answer:** Magnesium is the essential cofactor for almost all enzymes that utilize or synthesize ATP in the glycolytic pathway. The underlying biochemical principle is that Mg²⁺ binds to the anionic phosphate groups of ATP, forming a **Mg-ATP complex**. This complex shields the negative charges of the phosphate groups, allowing the enzyme’s active site to perform a nucleophilic attack more effectively. Key Mg²⁺-dependent enzymes in glycolysis include: * **Hexokinase & Glucokinase** (First step) * **Phosphofructokinase-1 (PFK-1)** (Rate-limiting step) * **Phosphoglycerate Kinase & Pyruvate Kinase** (ATP-generating steps) * **Enolase** (Requires Mg²⁺ for stabilization; inhibited by Fluoride) **Analysis of Incorrect Options:** * **A. Zinc (Zn²⁺):** While Zn is a vital cofactor for enzymes like Carbonic Anhydrase, Alcohol Dehydrogenase, and Carboxypeptidase, it does not play a primary role in the core reactions of glycolysis. * **C. Copper (Cu²⁺):** Copper is primarily involved in redox reactions, such as in Cytochrome c Oxidase (Complex IV) of the Electron Transport Chain and Superoxide Dismutase. * **D. Calcium (Ca²⁺):** Calcium acts as a secondary messenger and is crucial for muscle contraction and blood coagulation, but it is not a cofactor for glycolytic enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **Fluoride Inhibition:** In clinical practice, fluoride (grey-top tubes) is used for blood glucose estimation because it inhibits the enzyme **Enolase** by displacing Mg²⁺, thereby halting glycolysis and preserving glucose levels. * **Kinase Rule:** As a general rule for biochemistry questions, whenever a **Kinase** enzyme is involved (transfer of phosphate), **Mg²⁺** is almost always the required cofactor.

Q: Which of the following is a mitochondrial marker enzyme?

Succinic dehydrogenase. **Explanation:** The correct answer is **Succinic dehydrogenase (SDH)**. Marker enzymes are specific enzymes used to identify and assess the integrity of particular cellular organelles. **Why Succinic Dehydrogenase is the correct answer:** Succinic dehydrogenase is a key enzyme of the **TCA cycle** and is uniquely located in the **inner mitochondrial membrane** (where it also functions as Complex II of the Electron Transport Chain). Because it is structurally bound to the mitochondrial membrane, it serves as a reliable marker for mitochondria in subcellular fractionation studies. **Analysis of Incorrect Options:** * **Aldolase:** This is a glycolytic enzyme located in the **cytosol**. It is used as a clinical marker for muscle damage (e.g., dermatomyositis). * **Amylase:** This is a digestive enzyme secreted by the **pancreas and salivary glands** into the extracellular space. It is a marker for acute pancreatitis, not a specific intracellular organelle. * **Pyruvate dehydrogenase (PDH):** While PDH is located within the mitochondrial matrix, it is a multi-enzyme complex rather than a standard membrane marker. SDH is the more classically cited "marker enzyme" in biochemistry textbooks for mitochondrial identification. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Markers:** Inner Membrane (Succinic dehydrogenase), Matrix (Glutamate dehydrogenase), Outer Membrane (Monoamine oxidase). * **Lysosome Marker:** Acid phosphatase. * **Golgi Apparatus Marker:** Galactosyltransferase. * **Cytosol Marker:** Lactate dehydrogenase (LDH). * **Peroxisome Marker:** Catalase. * **Endoplasmic Reticulum Marker:** Glucose-6-phosphatase.

Q: What is true about reversible non-competitive inhibitors?

Lowers Vmax. In enzyme kinetics, **Non-competitive inhibition** occurs when an inhibitor binds to an allosteric site (a site other than the active site) on either the free enzyme or the enzyme-substrate (ES) complex. ### Why Option A is Correct Because the inhibitor binds to a site different from the active site, it does not prevent the substrate from binding. However, it renders the enzyme catalytically inactive. Since the inhibitor effectively "takes enzymes out of commission" regardless of how much substrate is added, the **Vmax (Maximum Velocity) decreases**. No amount of substrate can displace a non-competitive inhibitor. ### Why Other Options are Incorrect * **Option B & C:** In pure non-competitive inhibition, the inhibitor has the same affinity for the free enzyme and the ES complex. Therefore, the binding of the inhibitor does not interfere with the binding of the substrate to the active site. Consequently, the **Km (Michaelis constant) remains unchanged**. (Note: While Option C is technically a true statement, in NEET-PG, if both "Vmax decreases" and "Km is unchanged" are present, the primary defining characteristic is the effect on Vmax). * **Option D:** This is incorrect because Vmax must decrease; if Vmax were unaffected while Km increased, it would describe Competitive Inhibition. ### High-Yield Clinical Pearls for NEET-PG * **Competitive Inhibition:** Vmax stays the same, Km increases (e.g., Statins, Methotrexate). * **Non-competitive Inhibition:** Vmax decreases, Km stays the same (e.g., Cyanide poisoning of Cytochrome oxidase, Fluoride inhibition of Enolase). * **Uncompetitive Inhibition:** Both Vmax and Km decrease (e.g., Lithium, Hydrazine). * **Lineweaver-Burk Plot:** In non-competitive inhibition, the plots intersect on the **negative X-axis** (same -1/Km).

Q: Allopurinol specifically inhibits which enzyme?

Xanthine oxidase. **Explanation:** **1. Why Xanthine Oxidase is Correct:** Allopurinol is a structural analog of hypoxanthine. It acts as a **suicide inhibitor** (mechanism-based irreversible inhibition) of **Xanthine Oxidase (XO)**. In the body, XO converts allopurinol into its active metabolite, **oxypurinol** (alloxanthine). Oxypurinol binds tightly to the molybdenum-sulfide complex at the enzyme's active site, effectively "locking" it. This prevents the conversion of hypoxanthine to xanthine and xanthine to **uric acid**, thereby lowering serum urate levels. **2. Why the Other Options are Incorrect:** * **Arginase:** This is a key enzyme in the **Urea Cycle** that converts Arginine into Ornithine and Urea. It is not targeted by allopurinol. * **Carbamoyl Transferase (OTC):** Also part of the Urea Cycle, it catalyzes the reaction between carbamoyl phosphate and ornithine. Deficiency leads to hyperammonemia and orotic aciduria. * **Urease:** This enzyme is produced by certain bacteria (like *H. pylori* and *Proteus*) to convert urea into ammonia and CO₂. It is not a human metabolic enzyme and is not inhibited by allopurinol. **3. Clinical Pearls for NEET-PG:** * **Drug of Choice:** Allopurinol is the first-line agent for **chronic gout** and prophylaxis against **Tumor Lysis Syndrome**. * **Drug Interaction:** Since **6-Mercaptopurine (6-MP)** and **Azathioprine** are metabolized by Xanthine Oxidase, co-administration with Allopurinol leads to toxic levels of these drugs. Reduce their dose by 75%. * **Alternative:** **Febuxostat** is a non-purine selective inhibitor of XO used if patients are intolerant to allopurinol. * **Side Effect:** Watch for **Stevens-Johnson Syndrome (SJS)**, especially in patients with the HLA-B*5801 allele.

Question 1

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

Accepted Answer

Lipoprotein lipase

Answer

Alkaline phosphatase

Answer

Acid phosphatase

Answer

Gamma glutamyl transpeptidase

Question 2

Which of the following enzymes is not a protein?

Accepted Answer

Ribozyme

Answer

DNAse

Answer

Abzyme

Answer

Eco RI

Question 3

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

Accepted Answer

Units of activity per milligram of protein

Answer

Millimoles per liter

Answer

Micromoles per minute

Answer

Units of activity per minute

Question 4

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

Accepted Answer

Follow Michaelis-Menten kinetics

Answer

They are multi enzyme complex

Answer

Presence of modulator site

Answer

Give sigmoid shaped curve

Question 5

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

Accepted Answer

Muscular dystrophy

Answer

Myocardial infarction

Answer

Alcoholic cirrhosis

Answer

Brain tumours

Question 6

Which enzyme serves as a marker for the Golgi apparatus?

Accepted Answer

Galactosyl transferase

Answer

Peroxidase

Answer

Galactosidase

Answer

Catalase

Question 7

Which ion is most important in glycolysis?

Accepted Answer

Mg

Answer

Zn

Answer

Cu

Answer

Ca

Question 8

Which of the following is a mitochondrial marker enzyme?

Accepted Answer

Succinic dehydrogenase

Answer

Aldolase

Answer

Amylase

Answer

Pyruvate dehydrogenase

Question 9

What is true about reversible non-competitive inhibitors?

Accepted Answer

Lowers Vmax

Answer

Lowers Km

Answer

Km is not affected

Answer

Vmax is not affected

Question 10

Allopurinol specifically inhibits which enzyme?

Accepted Answer

Xanthine oxidase

Answer

Arginase

Answer

Carbamoyl transferase

Answer

Urease

Enzymes — MCQs

Enzymes — MCQs

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