Enzymes Practice Questions

Q: Chymotrypsin cleaves the peptide bond at the carboxyl-terminal side of which amino acid residues?

All of these. **Explanation:** Chymotrypsin is a serine protease secreted by the pancreas that plays a vital role in protein digestion. Its specificity is determined by its **hydrophobic pocket** (S1 pocket), which is large enough to accommodate bulky, non-polar side chains. **1. Why "All of these" is correct:** Chymotrypsin preferentially cleaves peptide bonds on the **carboxyl-terminal side** of large hydrophobic amino acids, specifically the **aromatic amino acids**: * **Phenylalanine (Option A)** * **Tryptophan (Option B)** * **Tyrosine (Option C)** Because all three amino acids possess the bulky aromatic rings required to fit into the enzyme's active site pocket, chymotrypsin acts on all of them, making Option D the correct choice. It can also occasionally cleave at Leucine and Methionine, though with much lower affinity. **2. Comparison with other Proteases (Distinguishing incorrect options):** In the NEET-PG context, it is crucial to differentiate Chymotrypsin from other digestive enzymes: * **Trypsin:** Cleaves at the carboxyl side of basic amino acids (**Lysine and Arginine**). * **Elastase:** Cleaves at the carboxyl side of small neutral amino acids (**Alanine, Glycine, and Serine**). * **Pepsin:** An acidic protease that also favors aromatic residues but acts primarily in the stomach. **3. Clinical Pearls & High-Yield Facts:** * **Zymogen Activation:** Chymotrypsin is secreted as the inactive proenzyme **Chymotrypsinogen**, which is activated by **Trypsin** in the duodenum. * **Catalytic Triad:** The mechanism of action involves a specific "catalytic triad" of three amino acids: **Aspartate, Histidine, and Serine**. * **Diagnostic Use:** Fecal chymotrypsin levels can be measured to assess pancreatic exocrine function (e.g., in Chronic Pancreatitis or Cystic Fibrosis).

Q: Regan enzyme is an isoenzyme of which of the following?

Alkaline phosphatase. **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).

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

Doubles. ### 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.

Q: What is the Michaelis constant (Km)?

4. ***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.

Q: Which of the following methods most accurately estimates blood creatinine level?

Enzyme assay. **Explanation:** The **Enzymatic Assay** (using Creatininase and Creatinase) is considered the most accurate method for estimating blood creatinine because of its high **specificity**. 1. **Why Enzyme Assay is Correct:** Unlike chemical methods, enzymes are highly specific to their substrate. The enzymatic method (often involving a peroxidase-coupled reaction) eliminates interference from "non-creatinine chromogens." It is currently the method of choice in clinical laboratories seeking to minimize errors in Calculated Glomerular Filtration Rate (eGFR). 2. **Analysis of Incorrect Options:** * **Jaffe Method (Option A):** This is the traditional method based on the reaction of creatinine with alkaline picrate to form a red-orange complex. It is notorious for **positive interference** from substances like glucose, ketones, protein, and cephalosporins, leading to overestimation. * **Kinetic Jaffe Method (Option B):** An improvement over the manual Jaffe method, it measures the rate of color formation to reduce interference from slow-reacting chromogens (like glucose). While faster and more common, it still lacks the absolute specificity of enzymatic assays. * **Technicon Method (Option C):** This refers to older automated analyzers (like the AutoAnalyzer) which typically utilized a modified Jaffe reaction. It is not a distinct biochemical principle for accuracy. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** The analytical "Gold Standard" for creatinine measurement is **IDMS** (Isotope Dilution Mass Spectrometry), but among routine laboratory methods, **Enzymatic Assay** is the most accurate. * **Interference:** Bilirubin causes **negative interference** in the Jaffe reaction (falsely low results), while ketones and cephalosporins cause **positive interference**. * **Creatinine Source:** It is an anhydride of creatine, produced at a constant rate proportional to **muscle mass**.

Q: Which of the following is NOT a serine protease?

Pepsin. **Explanation:** The classification of proteases is based on the specific amino acid residue at their active site that facilitates the catalytic mechanism. **Why Pepsin is the correct answer:** Pepsin is an **Aspartic protease** (Acid protease). It utilizes two highly conserved aspartic acid residues in its active site to activate a water molecule, which then attacks the peptide bond of the substrate. Pepsin functions optimally in the highly acidic environment of the stomach (pH 1.5–2.5). **Why the other options are incorrect:** * **Trypsin, Chymotrypsin, and Elastase** are all classic examples of **Serine proteases**. * They share a common catalytic mechanism involving a **"Catalytic Triad"** consisting of **Serine, Histidine, and Aspartate**. * In these enzymes, the Serine residue acts as a nucleophile to attack the carbonyl carbon of the substrate's peptide bond. These enzymes are primarily secreted by the pancreas as zymogens and function in the neutral-to-alkaline pH of the small intestine. **High-Yield NEET-PG Pearls:** 1. **Catalytic Triad:** Remember the mnemonic **"SHA"** (Serine, Histidine, Aspartate) for serine proteases. 2. **Blood Clotting Factors:** Most coagulation factors (II, VII, IX, X, XI, XII) and Thrombin are also Serine proteases. 3. **Substrate Specificity:** * **Trypsin:** Cleaves at Basic AAs (Arg, Lys). * **Chymotrypsin:** Cleaves at Aromatic AAs (Phe, Tyr, Trp). * **Elastase:** Cleaves at Small Neutral AAs (Gly, Ala, Ser). 4. **Other Protease Classes:** * **Cysteine Proteases:** Caspases (apoptosis), Cathepsins. * **Metalloproteases:** Carboxypeptidases (require Zinc), Matrix Metalloproteinases (MMPs).

Q: Which enzyme is not released as a proenzyme?

Amylase. **Explanation:** The core concept tested here is the distinction between **proteolytic enzymes** and **non-proteolytic enzymes**. **Why Amylase is the Correct Answer:** Amylase is a carbohydrate-digesting enzyme (hydrolase) that breaks down starch into maltose. Unlike proteases, amylase does not pose a threat to the structural integrity of the pancreatic cells or ducts because it does not digest cellular proteins. Therefore, it is synthesized and secreted in its **active form**. It only requires calcium ions ($Ca^{2+}$) and chloride ions ($Cl^-$) for optimal activity, rather than proteolytic cleavage for activation. **Why the Other Options are Incorrect:** * **Trypsin, Chymotrypsin, and Elastase** are all **proteases** (enzymes that digest proteins). * If these were secreted in their active forms, they would cause **autodigestion** of the pancreas, leading to acute pancreatitis. * To prevent this, they are secreted as inactive **proenzymes (zymogens)**: Trypsinogen, Chymotrypsinogen, and Proelastase. * **Trypsinogen** is activated to **Trypsin** by the enzyme **Enteropeptidase** (Enterokinase) in the duodenum. Once formed, Trypsin acts as a common activator for Chymotrypsinogen and Proelastase. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogens:** Inactive precursors of enzymes. Most gastrointestinal proteases (including Pepsinogen in the stomach) are zymogens. * **Pancreatitis:** In acute pancreatitis, premature activation of trypsinogen within the pancreas leads to a cascade of zymogen activation and hemorrhagic necrosis. * **Diagnostic Marker:** Serum amylase and lipase are elevated in acute pancreatitis, but **Lipase** is considered more specific. * **Inhibitor:** Pancreatic Secretory Trypsin Inhibitor (PSTI/SPINK1) is a protective protein that inhibits any small amounts of trypsin prematurely formed within the pancreas.

Question 1

Trypsin cleaves which amino acid residue?

Accepted Answer

Arginine

Answer

Glutamate

Answer

Lysine

Answer

Proline

Question 2

Which of the following is a multienzyme complex in humans?

Accepted Answer

Fatty acid synthetase

Answer

Malonyl CoA Carboxylase

Answer

Carbamoyl phosphate synthetase

Answer

Adenosine phosphoribosyl transferase

Question 3

Chymotrypsin cleaves the peptide bond at the carboxyl-terminal side of which amino acid residues?

Accepted Answer

All of these

Answer

Phenylalanine

Answer

Tryptophan

Answer

Tyrosine

Question 4

Regan enzyme is an isoenzyme of which of the following?

Accepted Answer

Alkaline phosphatase

Answer

Lactate dehydrogenase

Answer

Creatine kinase

Answer

Acid phosphatase

Question 5

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

Accepted Answer

Doubles

Answer

Halves

Answer

Quadruples

Answer

Increases 10-fold

Question 6

What is the Michaelis constant (Km)?

Accepted Answer

4

Answer

3

Answer

5

Answer

6

Question 7

Which of the following methods most accurately estimates blood creatinine level?

Accepted Answer

Enzyme assay

Answer

Jaffe method

Answer

Kinetic Jaffe method

Answer

Technicon method

Question 8

In the test given below to estimate blood glucose, which of the following is true?

Accepted Answer

Glucose oxidase is highly specific for beta anomer of Glucose

Answer

Glucose is converted to glucuronic acid

Answer

The terminal Carbon is oxidized.

Answer

This is an example of a reductometric method

Question 9

Which of the following is NOT a serine protease?

Accepted Answer

Pepsin

Answer

Elastase

Answer

Chymotrypsin

Answer

Trypsin

Question 10

Which enzyme is not released as a proenzyme?

Accepted Answer

Amylase

Answer

Elastase

Answer

Trypsin

Answer

Chymotrypsin

Enzymes — MCQs

Enzymes — MCQs

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