Enzymes Practice Questions

Q: Which one of the following enzymes is not a protein, but an RNA molecule?

Peptidyl transferase. ### Explanation The correct answer is **Peptidyl transferase**. **1. Why Peptidyl Transferase is Correct:** Traditionally, all enzymes were thought to be proteins. However, **ribozymes** are RNA molecules that possess catalytic activity. Peptidyl transferase is a classic example of a ribozyme. It is located in the large ribosomal subunit (28S rRNA in eukaryotes and 23S rRNA in prokaryotes). During translation, it catalyzes the formation of peptide bonds between amino acids. Because the catalytic activity resides in the RNA component rather than a protein, it is classified as a non-protein enzyme. **2. Why the Other Options are Incorrect:** * **RNA Polymerase (B):** This is a complex protein enzyme responsible for synthesizing RNA from a DNA template during transcription. * **Restriction Endonuclease (C):** These are bacterial protein enzymes (often called "molecular scissors") used in recombinant DNA technology to cut DNA at specific palindromic sequences. * **Reverse Transcriptase (D):** This is an RNA-dependent DNA polymerase protein, famously found in retroviruses like HIV, which synthesizes DNA from an RNA template. **3. High-Yield Clinical Pearls for NEET-PG:** * **Other Ribozymes:** Apart from Peptidyl transferase, other notable ribozymes include **RNase P** (involved in tRNA processing) and **SnRNAs** (involved in splicing). * **Antibiotic Link:** Many antibiotics target the 50S subunit (e.g., Chloramphenicol), effectively inhibiting the peptidyl transferase activity of the ribozyme. * **Abzymes:** These are catalytic antibodies (proteins) that mimic enzymes; do not confuse them with ribozymes (RNA).

Q: Plasma cholinesterase is reduced in all of the following conditions except?

Hemolysis. ### Explanation **Plasma Cholinesterase (Pseudocholinesterase)** is a glycoprotein enzyme synthesized by the **liver** and secreted into the blood. It is clinically significant because it hydrolyzes drugs like succinylcholine and mivacurium. **Why Hemolysis is the correct answer:** Hemolysis involves the destruction of Red Blood Cells (RBCs). While RBCs contain **True Cholinesterase** (Acetylcholinesterase), they do not contain Plasma Cholinesterase. Therefore, the breakdown of RBCs does not decrease the levels of plasma cholinesterase; in fact, it has no significant effect on its concentration. **Why the other options are incorrect:** * **Liver Disease:** Since the liver is the primary site of synthesis for plasma cholinesterase, any hepatic dysfunction (cirrhosis, hepatitis) leads to decreased production and lower plasma levels. * **Pregnancy:** Plasma cholinesterase levels naturally decrease by about 20–30% during the first trimester and remain low until delivery, likely due to hemodilution and hormonal changes. * **Malnutrition:** As a protein synthesized by the liver, its levels drop in states of protein-energy malnutrition (like Kwashiorkor) due to a lack of substrate for protein synthesis. **Clinical Pearls for NEET-PG:** 1. **Succinylcholine Apnea:** Patients with inherited or acquired deficiency of plasma cholinesterase cannot metabolize succinylcholine, leading to prolonged neuromuscular blockade and respiratory paralysis. 2. **Organophosphate Poisoning:** Plasma cholinesterase is a sensitive marker for organophosphate poisoning (it decreases earlier than RBC cholinesterase), though RBC cholinesterase is more specific for monitoring chronic exposure. 3. **True vs. Pseudo:** Remember, **True** cholinesterase is found in the **N**erve endings, **G**ray matter, and **R**BCs (Mnemonic: **NGR**), while **Pseudo**cholinesterase is found in the **P**lasma, **L**iver, and **W**hite matter (Mnemonic: **PLW**).

Q: NAD+ acts as a coenzyme for which of the following enzymes?

Malate dehydrogenase. ### Explanation **1. Why Malate Dehydrogenase is Correct:** Malate dehydrogenase is a key enzyme in the **TCA cycle** and the **Malate-Aspartate shuttle**. It catalyzes the reversible oxidation of L-malate to oxaloacetate. This reaction involves the transfer of two electrons and a proton to **NAD+**, reducing it to NADH + H⁺. In biochemistry, most dehydrogenases involved in the oxidation of hydroxyl groups (like malate or lactate) to carbonyl groups utilize NAD+ as the electron acceptor. **2. Analysis of Incorrect Options:** * **Xanthine oxidase (Option A):** This enzyme, involved in purine catabolism, is a metalloflavoprotein. It utilizes **FAD** and Molybdenum as cofactors, not NAD+. * **L-amino acid oxidase (Option B):** This enzyme catalyzes the oxidative deamination of amino acids. It is a flavoprotein that uses **FMN** (Flavin Mononucleotide) as its coenzyme. * **Succinate dehydrogenase (Option C):** A unique enzyme that is part of both the TCA cycle and Complex II of the Electron Transport Chain. It utilizes **FAD** (covalently bound) because the free energy change of succinate oxidation is insufficient to reduce NAD+. **3. Clinical Pearls & High-Yield Facts:** * **Niacin (Vitamin B3):** NAD+ and NADP+ are derived from Niacin. Deficiency leads to **Pellagra** (3Ds: Dermatitis, Diarrhea, Dementia). * **NAD+ vs. NADPH:** Generally, **NAD+** is used in **catabolic** pathways (oxidative), while **NADPH** is used in **anabolic** pathways (reductive biosynthesis like fatty acid synthesis) and to maintain reduced glutathione. * **Mnemonic:** Most "Dehydrogenases" use NAD+, EXCEPT for **Succinate Dehydrogenase**, **Acyl-CoA Dehydrogenase**, and **Glycerol-3-Phosphate Dehydrogenase (mitochondrial)**, which use FAD.

Q: After adding Drug A to an enzyme-substrate reaction, the Km remains the same and Vmax decreases. Drug A is a:

Non-competitive inhibitor. ### Explanation The correct answer is **Non-competitive inhibitor**. **1. Why Non-competitive inhibition is correct:** In non-competitive inhibition, the inhibitor binds to an **allosteric site** (a site other than the active site) on both the free enzyme and the enzyme-substrate (ES) complex. * **Effect on Vmax:** Because the inhibitor effectively "takes the enzyme out of commission" regardless of how much substrate is present, the maximum velocity (**Vmax**) of the reaction **decreases**. * **Effect on Km:** Since the inhibitor does not compete for the active site, the affinity of the remaining functional enzymes for the substrate remains unchanged. Therefore, the **Km remains the same**. **2. Why other options are incorrect:** * **Competitive inhibitor:** These drugs compete with the substrate for the **active site**. Increasing substrate concentration can overcome this inhibition. Thus, **Vmax remains constant**, but **Km increases** (lower affinity). * **Agonist:** This is a pharmacological term for a drug that binds to a receptor and activates it to produce a biological response. It is not a type of enzyme inhibition. * **Uncompetitive inhibitor (High-yield contrast):** Here, the inhibitor binds only to the ES complex. This results in a **decrease in both Vmax and Km**. **3. NEET-PG High-Yield Pearls:** * **Lineweaver-Burk Plot:** In non-competitive inhibition, the lines intersect on the **negative X-axis** ($-1/Km$ is constant). * **Classic Examples:** * **Non-competitive:** Cyanide (inhibits Cytochrome Oxidase), Heavy metals (Lead, Mercury). * **Competitive:** Statins (HMG-CoA Reductase), Methotrexate (Dihydrofolate Reductase). * **Memory Aid:** **C**ompetitive = **C**hanges Km; **N**on-competitive = **N**o change in Km.

Q: Which of the following enzymes exhibits absolute specificity for its substrate?

Urease. ### Explanation **Concept: Enzyme Specificity** Enzyme specificity refers to the ability of an enzyme to choose a specific substrate from a group of similar chemical compounds. **Absolute specificity** is the highest level of selectivity, where an enzyme acts on **only one specific substrate** to catalyze a single reaction. **Why Urease is Correct:** **Urease** is the classic example of absolute specificity. It acts exclusively on **urea** to produce ammonia and carbon dioxide. It will not catalyze the hydrolysis of any other substituted ureas or related compounds, regardless of structural similarity. **Analysis of Incorrect Options:** * **Lactate Dehydrogenase (LDH):** Exhibits **optical/stereo-specificity**. It acts specifically on the L-isomer of lactate but not the D-isomer. However, it is not "absolute" in the strictest sense as it can interact with other α-keto acids. * **L-amino acid oxidase:** Exhibits **group specificity**. It acts on a group of structurally related compounds—specifically, various L-amino acids—rather than a single molecule. * **Hexokinase:** Exhibits **group specificity**. It catalyzes the phosphorylation of several six-carbon sugars (hexoses), including glucose, fructose, and mannose. (Note: *Glucokinase* is more specific to glucose but still lacks the absolute exclusivity of Urease). **NEET-PG High-Yield Pearls:** 1. **Bond Specificity:** Enzymes like **pepsin** or **trypsin** act on specific types of chemical bonds (e.g., peptide bonds) regardless of the surrounding structure. 2. **Stereospecificity:** Most enzymes in the human body are specific to **L-amino acids** and **D-sugars**. 3. **Clinical Correlation:** Urease is clinically significant in the **Urea Breath Test** used to diagnose *H. pylori* infections, as the bacteria produce urease to neutralize gastric acid.

Q: Which of the following enzyme assays is useful to diagnose alcoholism?

SGOT. In clinical biochemistry, the ratio and specific elevation of aminotransferases are key to diagnosing alcoholic liver disease. **Why SGOT (AST) is the correct answer:** In alcoholic hepatitis, **SGOT (Aspartate Aminotransferase)** is typically elevated more than SGPT (Alanine Aminotransferase). This occurs because alcohol is a direct mitochondrial toxin. Since SGOT has both cytosolic and mitochondrial isoenzymes, mitochondrial damage leads to a greater release of SGOT. Furthermore, alcoholics often have a deficiency of **Pyridoxal-5-phosphate (Vitamin B6)**; while both enzymes require B6 as a cofactor, the synthesis of SGPT is much more sensitive to this deficiency, leading to lower SGPT levels compared to SGOT. A **De Ritis Ratio (AST:ALT) > 2:1** is highly suggestive of alcoholic liver disease. **Analysis of Incorrect Options:** * **SGPT (ALT):** This is a more specific marker for general liver cell injury (as it is primarily found in the liver cytosol), but in alcoholism, it remains lower than SGOT. * **Alkaline Phosphatase (ALP):** This is primarily a marker for cholestasis or bone turnover. While it may rise in alcoholic cirrhosis, it is not a specific diagnostic marker for alcoholism itself. * **Glutamyl aminotransferase:** This is a distractor term. The relevant enzyme is **Gamma-glutamyl transferase (GGT)**, which is the most sensitive (though less specific) screening marker for chronic alcohol consumption. **High-Yield Clinical Pearls for NEET-PG:** * **AST:ALT > 2:1** = Alcoholic Liver Disease. * **AST:ALT < 1** = Non-Alcoholic Fatty Liver Disease (NAFLD) or Viral Hepatitis. * **GGT** is the most sensitive marker for detecting occult alcohol use and monitoring abstinence. * Alcoholism also causes an increase in **MCV (Macrocytosis)** and **Serum Triglycerides**.

Question 1

All of the following are true about allosteric enzymes, EXCEPT:

Accepted Answer

Allosteric enzymes can have quaternary structure.

Answer

Allosteric enzymes have a regulatory site other than the active site.

Answer

Allosteric enzymes do not show Michaelis-Menten kinetics.

Answer

Allosteric modulators bind noncovalently to the allosteric site.

Question 2

Which of the following justifies the statement that not all enzymes are proteins?

Accepted Answer

Ribonucleic acids (RNAs) can function as enzymes (ribozymes).

Answer

Antibodies participate in the catalysis of many reactions.

Answer

Not all enzymes adhere to the Michaelis-Menten kinetics.

Answer

Metals are involved in the binding to enzymes and act as catalysts.

Question 3

Which of the following acts as a coenzyme and not as a co-factor?

Accepted Answer

Ascorbic acid

Answer

Biotin

Answer

Thiamine

Answer

Folic acid

Question 4

Which one of the following enzymes is not a protein, but an RNA molecule?

Accepted Answer

Peptidyl transferase

Answer

RNA polymerase

Answer

Restriction endonuclease

Answer

Reverse transcriptase

Question 5

Plasma cholinesterase is reduced in all of the following conditions except?

Accepted Answer

Hemolysis

Answer

Pregnancy

Answer

Liver disease

Answer

Malnutrition

Question 6

NAD+ acts as a coenzyme for which of the following enzymes?

Accepted Answer

Malate dehydrogenase

Answer

Xanthine oxidase

Answer

L-amino acid oxidase

Answer

Succinate dehydrogenase

Question 7

What is true about allosteric enzymes?

Accepted Answer

Hill's equation is used to study the kinetics of allosteric enzymes.

Answer

They are single-unit enzymes.

Answer

They follow Michaelis-Menten saturation kinetics.

Answer

Allosteric activity is most prominent in the active site of the enzyme.

Question 8

After adding Drug A to an enzyme-substrate reaction, the Km remains the same and Vmax decreases. Drug A is a:

Accepted Answer

Non-competitive inhibitor

Answer

Competitive inhibitor

Answer

Agonist

Answer

None of the above

Question 9

Which of the following enzymes exhibits absolute specificity for its substrate?

Accepted Answer

Urease

Answer

Lactate dehydrogenase

Answer

L-amino acid oxidase

Answer

Hexokinase

Question 10

Which of the following enzyme assays is useful to diagnose alcoholism?

Accepted Answer

SGOT

Answer

SGPT

Answer

Alkaline phosphatase

Answer

Glutamyl aminotransferase

Enzymes — MCQs

Enzymes — MCQs

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