Enzymes Practice Questions

Q: An enzyme-substrate interaction has an initial Vmax of 10 and a Km of 5. After adding Drug B, the Km becomes 10. Which of the following statements is false?

Drug B binds to an allosteric site of the enzyme.. ### Explanation The question describes a scenario where the **$V_{max}$ remains constant (10)** while the **$K_m$ increases (from 5 to 10)**. This is the classic kinetic hallmark of **Competitive Inhibition**. **1. Why Option D is the Correct (False) Statement:** In competitive inhibition, the inhibitor competes with the substrate for the **active site** of the enzyme. Binding to an **allosteric site** (a site other than the active site) is characteristic of non-competitive or uncompetitive inhibition. Since the $V_{max}$ did not change, Drug B must be binding to the active site, making statement D false. **2. Analysis of Incorrect Options (True Statements):** * **Option A:** True. An increase in $K_m$ with an unchanged $V_{max}$ defines a competitive inhibitor. * **Option B:** True. Because the inhibitor and substrate compete for the same site, increasing the substrate concentration ($[S]$) "outcompetes" the drug, eventually allowing the reaction to reach its original $V_{max}$. * **Option C:** True. Competitive inhibitors are typically **structural analogs** of the substrate, allowing them to fit into the same active site (e.g., Malonate is a structural analog of Succinate). --- ### High-Yield Clinical Pearls for NEET-PG * **Competitive Inhibition:** $V_{max}$ = Unchanged; $K_m$ = Increased. (Mnemonic: **C**ompetitive = **C**onstant $V_{max}$). * **Non-Competitive Inhibition:** $V_{max}$ = Decreased; $K_m$ = Unchanged. (Inhibitor binds to allosteric site). * **Uncompetitive Inhibition:** Both $V_{max}$ and $K_m$ decrease. (Inhibitor binds only to the Enzyme-Substrate complex). * **Classic Clinical Example:** **Statins** (e.g., Atorvastatin) are competitive inhibitors of HMG-CoA reductase. They are structural analogs of HMG-CoA. * **Methanol Poisoning:** Ethanol acts as a competitive inhibitor of Alcohol Dehydrogenase, preventing the formation of toxic formaldehyde.

Q: Which enzyme acts the fastest?

Catalase. **Explanation:** The efficiency of an enzyme is measured by its **Turnover Number ($K_{cat}$)**, which is defined as the number of substrate molecules converted into product by a single enzyme molecule per unit of time (usually per second) when the enzyme is fully saturated. **1. Why Catalase is the Correct Answer:** Catalase is one of the most efficient enzymes known in biology. It has an exceptionally high turnover number of approximately **$4 \times 10^7$ per second**. This means a single molecule of catalase can decompose 40 million molecules of hydrogen peroxide ($H_2O_2$) into water and oxygen every second. This rapid action is vital to protect cells from oxidative damage caused by reactive oxygen species (ROS). Its catalytic efficiency is so high that it is limited only by the rate at which the substrate can diffuse to the enzyme's active site (diffusion-limited). **2. Why Other Options are Incorrect:** * **Lactate Dehydrogenase (LDH):** While essential for anaerobic glycolysis, its turnover number is significantly lower (approx. $10^3$ per second) compared to catalase. * **Trypsin:** As a digestive protease, its reaction rate is relatively slow (approx. $10^2$ per second) because peptide bond hydrolysis is a more complex chemical process than the decomposition of $H_2O_2$. **3. NEET-PG High-Yield Pearls:** * **Catalytic Efficiency:** Represented by the ratio **$K_{cat}/K_m$**. An enzyme is considered "catalytically perfect" if this ratio is $10^8$ to $10^9$ $M^{-1}s^{-1}$. * **Clinical Correlation:** Catalase is found in **peroxisomes**. Deficiency of catalase (Acatalasemia) can lead to oral ulcerations and gangrene. * **Other Fast Enzymes:** Carbonic Anhydrase and Acetylcholinesterase are also among the fastest enzymes, though Catalase typically tops the list in turnover speed.

Q: Enzyme activity is expressed as:

Micromoles/min. ### Explanation **1. Why Option D is Correct:** Enzyme activity refers to the **rate of a reaction** catalyzed by an enzyme. By definition, the standard unit of enzyme activity is the **International Unit (IU)**. One IU is defined as the amount of enzyme that catalyzes the conversion of **1 micromole ($\mu$mol) of substrate per minute** under specified conditions (optimum pH, temperature, and substrate concentration). Since activity measures "work done over time," the unit must include a time component (min) and a quantity of substrate processed ($\mu$mol). **2. Why Other Options are Incorrect:** * **Option A (Millimoles/lit):** This is a unit of **molar concentration**, commonly used to measure electrolytes or metabolites in the blood, not the rate of a reaction. * **Option B (Mg/lit) & Option C (Mg/dl):** These are units of **mass concentration**. While they can be used to measure the total protein mass of an enzyme (e.g., Troponin mass assays), they do not reflect the functional "activity" or catalytic power of the enzyme. **3. High-Yield Clinical Pearls for NEET-PG:** * **Katal (kat):** The SI unit of enzyme activity. 1 Katal = 1 mole of substrate converted per second. (Note: 1 IU = 16.67 nkat). * **Specific Activity:** Defined as the number of enzyme units per milligram of total protein (Units/mg). It is the best indicator of **enzyme purity** during purification processes. * **Turnover Number ($K_{cat}$):** The number of substrate molecules converted into product per enzyme molecule per second when the enzyme is fully saturated. * **Clinical Correlation:** In diagnostic labs, we measure enzyme *activity* (e.g., ALT, AST, ALP) rather than *mass* because activity is a more sensitive indicator of cellular damage and is easier to assay.

Q: Which of the following enzymes is NOT regulated by phosphorylation?

Glucokinase. **Explanation:** The correct answer is **Glucokinase (Option A)**. **Why Glucokinase is the correct answer:** Glucokinase (Hexokinase IV) is primarily regulated by **compartmentalization** and **allosteric inhibition**, not by covalent modification like phosphorylation. In the liver, it is regulated by the **Glucokinase Regulatory Protein (GKRP)**, which sequesters it in the nucleus in its inactive form when glucose levels are low. When glucose levels rise, glucokinase is released into the cytoplasm to initiate glycolysis. Unlike many other metabolic enzymes, its activity is not toggled by the addition or removal of a phosphate group. **Analysis of incorrect options:** * **Glycogen synthase (Option B):** This enzyme is inactivated by phosphorylation (via Protein Kinase A or Glycogen Synthase Kinase-3) and activated by dephosphorylation (via Protein Phosphatase-1). * **Glycogen phosphorylase (Option C):** This is the classic example of covalent regulation. It is activated by phosphorylation (via Phosphorylase Kinase) and inactivated by dephosphorylation. * **Citrate lyase (Option D):** ATP-Citrate Lyase, a key enzyme in fatty acid synthesis, is regulated by phosphorylation. It is activated by insulin-mediated signaling (via Akt/Protein Kinase B). **High-Yield Clinical Pearls for NEET-PG:** * **Glucokinase vs. Hexokinase:** Glucokinase has a **high Km** (low affinity) and **high Vmax**, allowing it to function as a "glucose sensor" in the liver and pancreatic beta cells. * **MODY Type 2:** Mutations in the glucokinase gene lead to Maturity-Onset Diabetes of the Young (MODY) type 2. * **General Rule:** Most rate-limiting enzymes in carbohydrate and lipid metabolism (e.g., Pyruvate Dehydrogenase, HMG-CoA Reductase, Acetyl-CoA Carboxylase) are regulated by phosphorylation/dephosphorylation. Glucokinase is a notable exception.

Q: Fluoride causes inhibition of which of the following enzymes?

Enolase. **Explanation:** **Correct Option: B. Enolase** Fluoride is a potent inhibitor of **Enolase**, the ninth enzyme in the glycolytic pathway. Enolase catalyzes the dehydration of 2-phosphoglycerate to phosphoenolpyruvate (PEP). The inhibition occurs because fluoride ions combine with magnesium (Mg²⁺) and phosphate to form a **magnesium-fluorophosphate complex**. This complex displaces the essential Mg²⁺ ions from the enzyme's active site, effectively halting glycolysis. **Analysis of Incorrect Options:** * **A. Pyruvate dehydrogenase (PDH):** This multi-enzyme complex converts pyruvate to Acetyl-CoA. It is primarily inhibited by high ratios of ATP/ADP and NADH/NAD⁺, or by Arsenite (which binds to lipoic acid), but not by fluoride. * **C. Glucose-6-phosphate dehydrogenase (G6PD):** This is the rate-limiting enzyme of the Pentose Phosphate Pathway (HMP Shunt). It is regulated by the availability of NADP⁺ and inhibited by high levels of NADPH. * **D. Pyruvate kinase:** This enzyme catalyzes the final step of glycolysis. It is inhibited by ATP, Alanine, and Glucagon (via phosphorylation), but is not the target of fluoride. **Clinical Pearls for NEET-PG:** * **Blood Glucose Estimation:** In clinical practice, blood samples for glucose estimation are collected in **Grey-topped vials** containing **Sodium Fluoride (NaF)**. This prevents "in vitro" glycolysis by RBCs, ensuring the measured glucose level reflects the patient's actual blood sugar at the time of draw. * **Anticoagulant Pairing:** NaF is usually paired with **Potassium Oxalate** (an anticoagulant) because fluoride alone is a poor anticoagulant. * **Water Fluoridation:** At low concentrations, fluoride prevents dental caries by inhibiting bacterial enolase in oral plaque.

Q: 5'-Nucleotidase activity is increased in which of the following conditions?

Cholestatic disorders. **Explanation:** **5'-Nucleotidase (5'-NT)** is a glycoprotein enzyme found predominantly in the plasma membranes of hepatocytes, specifically localized to the canalicular and sinusoidal membranes. **Why Cholestatic Disorders is correct:** In **cholestatic disorders** (obstructive jaundice or intrahepatic cholestasis), bile salts exert a detergent effect on the hepatocyte membranes, causing the release and solubilization of 5'-NT into the circulation. Its clinical significance lies in its high specificity for **hepatobiliary disease**. While Alkaline Phosphatase (ALP) is also elevated in cholestasis, it is also found in bone. Therefore, 5'-NT is used to differentiate whether an elevated ALP is of hepatic or bony origin. **Why other options are incorrect:** * **A. Bone diseases:** Unlike ALP, 5'-NT levels remain **normal** in bone diseases (e.g., Rickets, Paget’s disease, or bone metastases). This makes it a superior marker for confirming liver pathology. * **B. Prostate cancer:** The specific marker for prostate cancer is **Prostate-Specific Antigen (PSA)** and historically, Acid Phosphatase (ACP). 5'-NT has no diagnostic value here. * **C. Chronic renal failure:** While many enzymes can be deranged in uremia, 5'-NT is not a marker for renal function or damage. **High-Yield Clinical Pearls for NEET-PG:** * **ALP vs. 5'-NT:** If both are elevated → Hepatobiliary origin. If ALP is elevated but 5'-NT is normal → Bone origin. * **Gamma-Glutamyl Transferase (GGT):** Similar to 5'-NT, GGT is also elevated in cholestasis and normal in bone disease. However, GGT is also induced by **alcohol** and drugs (enzyme induction), whereas 5'-NT is not. * **Pregnancy:** 5'-NT levels remain normal during pregnancy, whereas ALP levels increase due to the placental isoenzyme.

Q: Serum acid phosphatase is increased in which of the following conditions?

Osteopetrosis. **Explanation:** **Serum Acid Phosphatase (ACP)** is an enzyme primarily found in the lysosomes of various tissues, including the prostate, liver, spleen, and bone. In the context of bone metabolism, the **Tartrate-Resistant Acid Phosphatase (TRAP)** isoenzyme is a specific marker of **osteoclast activity**. 1. **Why Osteopetrosis is correct:** Osteopetrosis (Marble Bone Disease) is characterized by defective osteoclasts that fail to resorb bone. Despite their functional failure, there is often a compensatory **increase in the number and metabolic activity of osteoclasts**. This leads to an increased leakage of the TRAP isoenzyme into the circulation, resulting in elevated serum ACP levels. 2. **Why the other options are incorrect:** * **Paget’s Disease:** This is characterized by high bone turnover with a predominant increase in **osteoblastic** activity. Therefore, the hallmark biochemical marker is a significantly elevated **Serum Alkaline Phosphatase (ALP)**, not ACP. * **Osteogenesis Imperfecta:** This is a genetic disorder of Type I collagen synthesis. While bone fragility is present, it does not typically present with a diagnostic elevation of serum ACP. **High-Yield Clinical Pearls for NEET-PG:** * **Prostatic ACP:** Historically used for diagnosing and monitoring **Prostate Cancer** (metastatic), though largely replaced by PSA. * **TRAP Marker:** Apart from Osteopetrosis, TRAP is a highly specific marker for **Hairy Cell Leukemia**. * **Gaucher’s Disease:** Serum ACP is also significantly elevated due to the release of lysosomal enzymes from "Gaucher cells" (lipid-laden macrophages). * **Differentiation:** Remember: **ALP** = Osteoblast activity (Bone formation); **ACP/TRAP** = Osteoclast activity (Bone resorption).

Q: Glutathione synthetase requires which cofactor?

Magnesium. **Explanation:** The correct answer is **Magnesium (Mg²⁺)**. **1. Why Magnesium is correct:** Glutathione synthetase is the second enzyme in the synthesis of Glutathione (GSH), catalyzing the condensation of γ-glutamylcysteine and glycine. This reaction is **ATP-dependent**. In biochemistry, almost all enzymes that utilize ATP (kinases and synthetases) require **Magnesium** as a mandatory cofactor. Magnesium ions complex with ATP (Mg-ATP) to shield the negative charges of the phosphate groups, facilitating the nucleophilic attack required for the reaction to proceed. **2. Why other options are incorrect:** * **Selenium (B):** This is a common distractor. Selenium is the essential cofactor for **Glutathione Peroxidase**, the enzyme responsible for neutralizing hydrogen peroxide using reduced glutathione. It is not required for the synthesis of glutathione itself. * **Copper (A) & Iron (D):** These are involved in redox reactions (e.g., Cytochrome c oxidase, Catalase, Superoxide Dismutase) but do not play a role in the ligase activity of glutathione synthetase. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Glutathione Components:** It is a tripeptide made of **Glutamate, Cysteine, and Glycine**. Cysteine is the rate-limiting amino acid. * **Two-Step Synthesis:** 1. γ-glutamylcysteine synthetase (requires Mg²⁺/ATP). 2. Glutathione synthetase (requires Mg²⁺/ATP). * **Glutathione Peroxidase vs. Reductase:** Peroxidase requires **Selenium**; Reductase requires **NADPH** (derived from the HMP Shunt) and **FAD**. * **Clinical Correlation:** Deficiency of glutathione synthetase leads to **5-oxoprolinuria** (pyroglutamic aciduria), characterized by metabolic acidosis, hemolytic anemia, and neurological symptoms.

Q: Alkaline phosphatase levels are elevated in all of the following conditions except?

Brain tumors. **Explanation:** Alkaline Phosphatase (ALP) is a group of isoenzymes that hydrolyze organic phosphates at an alkaline pH. It is primarily found in tissues with high metabolic activity or transport functions, specifically the **liver (bile canalicular membrane), bone (osteoblasts), placenta, and intestinal epithelium.** **Why "Brain Tumors" is the correct answer:** Brain tissue contains negligible amounts of ALP. Therefore, primary or secondary brain tumors do not typically cause an elevation in serum ALP levels. In neurological cases, enzymes like LDH or specific markers might rise, but ALP is not a diagnostic marker for CNS pathologies. **Analysis of Incorrect Options:** * **Obstructive Jaundice:** This is the most common cause of significant ALP elevation. Obstruction of the bile duct triggers increased synthesis of ALP by the canalicular cells and its subsequent leakage into the bloodstream. * **Bone Tumors:** ALP is a marker of **osteoblastic activity**. Conditions like osteosarcoma, bone metastases (prostate or breast cancer), and Paget’s disease show markedly elevated ALP due to increased bone remodeling. * **Placental Tumors:** The placenta produces a specific heat-stable isoenzyme of ALP (Regan isoenzyme). Levels rise physiologically during the third trimester of pregnancy and pathologically in germ cell tumors (e.g., dysgerminoma) or gestational trophoblastic diseases. **High-Yield Clinical Pearls for NEET-PG:** 1. **Isoenzymes:** Remember the mnemonic **"BLIP"** for sources: **B**one, **L**iver, **I**ntestine, **P**lacenta. 2. **Heat Stability:** Placental ALP is the most heat-stable, while Bone ALP is the most heat-labile (**"Bone Burns"**). 3. **Regan Isoenzyme:** A placental-like ALP found in various carcinomas (especially lung and GGTs). 4. **GGT Correlation:** To differentiate liver vs. bone origin of elevated ALP, check **Gamma-glutamyl transferase (GGT)**. GGT is elevated in liver disease but normal in bone disease.

Q: Various enzymes are used in clinical laboratories for the estimation of various substances. Peroxidase enzyme is used in estimating which of the following?

Glucose. **Explanation:** The estimation of blood glucose is most commonly performed using the **GOD-POD method** (Glucose Oxidase - Peroxidase method). This is a coupled enzymatic reaction: 1. **Glucose Oxidase (GOD)** catalyzes the oxidation of glucose to gluconic acid and **hydrogen peroxide ($H_2O_2$)**. 2. **Peroxidase (POD)** then catalyzes the reaction between $H_2O_2$ and a chromogen (e.g., 4-aminophenazone and phenol) to form a colored complex (Quinoneimine). The intensity of the color formed is directly proportional to the glucose concentration. **Analysis of Incorrect Options:** * **Ammonia:** Usually estimated using the **Glutamate Dehydrogenase (GLDH)** method, where ammonia reacts with $\alpha$-ketoglutarate to form glutamate, with a concomitant decrease in NADH absorbance. * **Creatinine:** Most commonly measured by the **Jaffe’s Reaction** (reaction with alkaline picrate) or enzymatically using **Creatininase/Creatinase**, but not typically peroxidase. * **Haemoglobin:** Estimated via the **Drabkin’s method**, which converts hemoglobin to cyanmethemoglobin using potassium ferricyanide and cyanide. **High-Yield Clinical Pearls for NEET-PG:** * **Hexokinase Method:** This is the **Reference Method** for glucose estimation because it is more specific than the GOD-POD method. * **Fluoride (Grey top tube):** Used for blood collection to inhibit **Enolase**, preventing glycolysis so that glucose levels remain stable for estimation. * **Trinder’s Reaction:** The specific name for the color-producing reaction involving Peroxidase and a chromogen.

Question 1

An enzyme-substrate interaction has an initial Vmax of 10 and a Km of 5. After adding Drug B, the Km becomes 10. Which of the following statements is false?

Accepted Answer

Drug B binds to an allosteric site of the enzyme.

Answer

Drug B is a competitive inhibitor.

Answer

Increasing the substrate concentration can reverse the inhibition by Drug B.

Answer

Drug B is structurally similar to the substrate.

Question 2

Which enzyme acts the fastest?

Accepted Answer

Catalase

Answer

Lactate dehydrogenase (LDH)

Answer

Trypsin

Answer

None of the above

Question 3

Enzyme activity is expressed as:

Accepted Answer

Micromoles/min

Answer

Millimoles/lit

Answer

Mg/lit

Answer

Mg/dl

Question 4

Which of the following enzymes is NOT regulated by phosphorylation?

Accepted Answer

Glucokinase

Answer

Glycogen synthase

Answer

Glycogen phosphorylase

Answer

Citrate lyase

Question 5

Fluoride causes inhibition of which of the following enzymes?

Accepted Answer

Enolase

Answer

Pyruvate dehydrogenase (PDH)

Answer

Glucose-6-phosphate dehydrogenase (G6PD)

Answer

Pyruvate kinase

Question 6

5'-Nucleotidase activity is increased in which of the following conditions?

Accepted Answer

Cholestatic disorders

Answer

Bone diseases

Answer

Prostate cancer

Answer

Chronic renal failure

Question 7

Serum acid phosphatase is increased in which of the following conditions?

Accepted Answer

Osteopetrosis

Answer

Paget's disease

Answer

Osteogenesis imperfecta

Answer

None of the above

Question 8

Glutathione synthetase requires which cofactor?

Accepted Answer

Magnesium

Answer

Copper

Answer

Selenium

Answer

Iron

Question 9

Alkaline phosphatase levels are elevated in all of the following conditions except?

Accepted Answer

Brain tumors

Answer

Obstructive jaundice

Answer

Bone tumors

Answer

Placental tumors

Question 10

Various enzymes are used in clinical laboratories for the estimation of various substances. Peroxidase enzyme is used in estimating which of the following?

Accepted Answer

Glucose

Answer

Haemoglobin

Answer

Ammonia

Answer

Creatinine

Enzymes — MCQs

Enzymes — MCQs

On this page

Practice by Chapter

Want unlimited practice?