Enzymes Practice Questions

Q: Which of the following is NOT a mixed-function oxidase?

Homogentisate oxidase. ### Explanation The key to answering this question lies in distinguishing between **Dioxygenases** and **Monooxygenases (Mixed-Function Oxidases)**. **1. Why Homogentisate Oxidase is the Correct Answer:** Homogentisate oxidase is a **dioxygenase**. In biochemical reactions, dioxygenases incorporate **both atoms** of molecular oxygen ($O_2$) directly into the substrate. In the phenylalanine/tyrosine catabolic pathway, homogentisate oxidase catalyzes the conversion of homogentisate to maleylacetoacetate by incorporating two oxygen atoms. Because it does not require a co-substrate to "accept" the second oxygen atom, it is not a mixed-function oxidase. **2. Why the Other Options are Incorrect:** Options B, C, and D are all **Mixed-Function Oxidases (Monooxygenases)**. These enzymes incorporate only **one atom** of $O_2$ into the substrate (as a hydroxyl group), while the other oxygen atom is reduced to **water ($H_2O$)**. This process requires a second substrate (electron donor) like NADPH or Tetrahydrobiopterin ($BH_4$). * **Cytochrome P-450:** The classic example of a monooxygenase used in drug metabolism and steroid synthesis. * **Phenylalanine Hydroxylase:** Converts Phenylalanine to Tyrosine using $BH_4$ as a co-reductant. * **Tryptophan Hydroxylase:** Converts Tryptophan to 5-Hydroxytryptophan (serotonin precursor) using $BH_4$. **Clinical Pearls for NEET-PG:** * **Alkaptonuria:** Deficiency of **Homogentisate oxidase** leads to the triad of dark urine (on standing), ochronosis (pigmentation of connective tissue), and arthritis. * **Phenylketonuria (PKU):** Deficiency of **Phenylalanine hydroxylase** is the most common cause. * **Cofactor Alert:** Most hydroxylases (Mixed-function oxidases) in amino acid metabolism require **Tetrahydrobiopterin ($BH_4$)** as a cofactor.

Q: What is the number of isoenzymes of Lactate Dehydrogenase (LDH)?

5. **Explanation:** Lactate Dehydrogenase (LDH) is a tetrameric enzyme (composed of four subunits) that catalyzes the reversible conversion of lactate to pyruvate. The correct answer is **5** because LDH is formed by the combination of two distinct polypeptide chains: **H (Heart)** and **M (Muscle)**. Since the enzyme is a tetramer, these two subunits can combine in five different permutations, resulting in five distinct isoenzymes: * **LDH-1 (H4):** Predominantly in the heart and RBCs. * **LDH-2 (H3M1):** Predominantly in the Reticuloendothelial system. * **LDH-3 (H2M2):** Predominantly in the lungs. * **LDH-4 (H1M3):** Predominantly in the kidneys and pancreas. * **LDH-5 (M4):** Predominantly in the liver and skeletal muscle. **Analysis of Incorrect Options:** * **Option A (1):** Incorrect. Enzymes with multiple subunits and tissue-specific expressions usually exist in multiple isoforms. * **Option B (3):** Incorrect. This might be confused with Creatine Kinase (CK), which has 3 isoenzymes (CK-MM, CK-MB, CK-BB). * **Option C (4):** Incorrect. While LDH is a tetramer (4 subunits), the mathematical combinations of two different subunits (H and M) result in five possible arrangements. **High-Yield Clinical Pearls for NEET-PG:** * **Flipped Pattern:** Normally, LDH-2 > LDH-1. In **Myocardial Infarction (MI)** or hemolytic anemia, LDH-1 levels exceed LDH-2 (LDH-1/LDH-2 ratio >1), known as the "flipped pattern." * **LDH-5:** Is the most heat-labile isoenzyme and serves as a marker for liver cell damage or skeletal muscle injury. * **Total LDH:** Is a non-specific marker of cellular turnover; significantly elevated levels are seen in megaloblastic anemia and certain malignancies (e.g., Seminoma, Lymphoma).

Q: All of the following are serine proteases, except?

Carboxypeptidase. **Explanation:** The classification of proteases is based on the functional group present at their active site that initiates the peptide bond cleavage. **1. Why Carboxypeptidase is the correct answer:** Carboxypeptidase is a **Zinc-containing Metalloenzyme** (specifically a metalloprotease). It requires a metal ion (Zn²⁺) to activate a water molecule, which then attacks the peptide bond. Unlike serine proteases, it does not utilize a serine residue for its catalytic mechanism. It is an exopeptidase that cleaves amino acids from the C-terminal end of proteins. **2. Why the other options are incorrect:** * **Chymotrypsin & Trypsin:** These are classic examples of pancreatic serine proteases. They utilize a "Catalytic Triad" consisting of **Serine, Histidine, and Aspartate**. The serine residue acts as a nucleophile to attack the carbonyl carbon of the substrate. * **Thrombin:** This is a critical serine protease in the coagulation cascade. It converts fibrinogen to fibrin. Like trypsin, it cleaves peptide bonds following specific basic amino acid residues. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Catalytic Triad:** Always remember the three amino acids involved in serine proteases: **Ser-His-Asp**. * **Serine Protease Inhibitors (SERPINs):** Deficiencies in these lead to clinical conditions (e.g., **α1-Antitrypsin deficiency** leading to emphysema and liver cirrhosis). * **Other Protease Classes:** * **Cysteine Proteases:** Caspases (apoptosis), Cathepsins. * **Aspartyl Proteases:** Pepsin, Renin, HIV Protease. * **Metalloproteases:** Carboxypeptidase, Matrix Metalloproteinases (MMPs), ACE. * **Zymogens:** Most serine proteases are secreted as inactive precursors (e.g., Trypsinogen) to prevent autodigestion of the pancreas.

Q: Activation energy is the free energy difference between which of the following?

Substrate and transition state. **Explanation:** **1. Why Option B is Correct:** In any biochemical reaction, substrates do not convert into products instantaneously. They must first reach a high-energy, unstable intermediate state known as the **Transition State (T‡)**. The **Activation Energy ($E_a$)** is defined as the threshold energy required to lift the substrate from its ground state to this transition state. From a medical biochemistry perspective, **enzymes function by lowering this activation energy**. By stabilizing the transition state, enzymes allow more substrate molecules to cross the energy barrier at body temperature, thereby increasing the reaction rate without altering the overall equilibrium. **2. Why Other Options are Incorrect:** * **Option A (Substrate and Product):** The free energy difference between the substrate and the product is known as **Gibbs Free Energy ($\Delta G$)**. This determines the spontaneity and direction of the reaction, not its speed. * **Option B (Transition state and Product):** This represents the energy released as the unstable intermediate collapses into the final product. It does not define the barrier to starting the reaction. * **Option D:** Activation energy is a specific measurement between two distinct points on an energy profile; it is not a cumulative sum of all energy changes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Kinetics:** Enzymes **decrease** activation energy but **do not change** the $\Delta G$ or the equilibrium constant ($K_{eq}$). * **Transition State Analogs:** Drugs that mimic the transition state of a substrate (e.g., **Statins** mimicking the HMG-CoA intermediate) act as potent competitive inhibitors because they bind to the enzyme's active site with higher affinity than the substrate itself. * **Arrhenius Equation:** A higher activation energy results in a slower reaction rate.

Q: Which amino acid is responsible for the activation of Thioredoxin reductase?

Selenocysteine. **Explanation:** **Thioredoxin reductase** is a critical flavoprotein enzyme that reduces thioredoxin, a protein essential for DNA synthesis (via ribonucleotide reductase) and the defense against oxidative stress. **Why Selenocysteine is correct:** The catalytic activity of Thioredoxin reductase is dependent on **Selenocysteine (Sec)**, often referred to as the **21st amino acid**. Selenocysteine contains a selenium atom in place of the sulfur found in cysteine. Because selenium has a lower pKa and higher nucleophilicity than sulfur, it allows the enzyme to carry out redox reactions more efficiently at physiological pH. The Selenocysteine residue is located at the C-terminal active site of the enzyme; without it, the enzyme loses its catalytic power. **Why incorrect options are wrong:** * **Cysteine:** While cysteine is structurally similar and involved in disulfide bond formation within the enzyme, it cannot replace the specific redox efficiency provided by the selenium atom in the active site. * **Serine:** Serine contains a hydroxyl group (-OH) which is not redox-active and cannot facilitate the electron transfer required by this enzyme. * **Alanine:** Alanine is a non-polar amino acid with a simple methyl side chain; it lacks the functional groups necessary for catalysis. **Clinical Pearls for NEET-PG:** * **Genetic Coding:** Selenocysteine is encoded by the **UGA stop codon**, requiring a specific mRNA structure called the **SECIS element** (Selenocysteine Insertion Sequence). * **Other Selenoenzymes:** Glutathione peroxidase, Deiodinases (converting T4 to T3), and Selenoprotein P. * **Trace Element:** Selenium deficiency can lead to **Keshan disease** (cardiomyopathy) or **Kashin-Beck disease** (osteoarthropathy).

Q: The Rossmann fold-associated NADH domain is a structural motif found in which of the following enzymes?

Isocitrate Dehydrogenase. **Explanation:** The **Rossmann fold** is a classic structural motif found in proteins that bind nucleotides, particularly the cofactor **NAD+ (or NADH)**. It consists of a series of alternating alpha-helices and beta-strands (typically $\beta-\alpha-\beta-\alpha-\beta$ units) that form a stable domain for nucleotide binding. **Why Isocitrate Dehydrogenase (ICD) is Correct:** Isocitrate Dehydrogenase is a key rate-limiting enzyme of the TCA cycle that catalyzes the oxidative decarboxylation of isocitrate to $\alpha$-ketoglutarate. This reaction requires **NAD+** (in the mitochondria) or **NADP+** (in the cytosol) as an electron acceptor. The enzyme utilizes the **Rossmann fold** to specifically bind these nicotinamide adenine dinucleotides. **Analysis of Incorrect Options:** * **Pyruvate Dehydrogenase (PDH):** This is a multi-enzyme complex. While it uses NAD+, its primary structural domains are specialized for its three subunits (E1, E2, E3) and involve lipoamide and TPP binding rather than the classic Rossmann fold for its primary catalytic action. * **Citrate Synthase:** This enzyme does not involve a redox reaction and **does not bind NAD+/NADH**. It catalyzes the condensation of Acetyl-CoA and Oxaloacetate. * **Succinate Dehydrogenase (SDH):** This is unique because it is the only TCA cycle enzyme that uses **FAD** (covalently bound) instead of NAD+. It is also part of Complex II of the Electron Transport Chain. **High-Yield NEET-PG Pearls:** * **Rossmann Fold:** Always associate this with **NAD/NADH binding** domains in dehydrogenases (e.g., Lactate Dehydrogenase, Alcohol Dehydrogenase, Isocitrate Dehydrogenase). * **Rate-Limiting Step:** Isocitrate Dehydrogenase is the primary rate-limiting enzyme of the TCA cycle, inhibited by ATP/NADH and activated by ADP/$\text{Ca}^{2+}$. * **SDH Fact:** Succinate Dehydrogenase is the only TCA enzyme embedded in the inner mitochondrial membrane; all others are in the matrix.

Q: Which molecule inhibits the conversion of citrate to cis-aconitate?

Fluoroacetate. **Explanation:** The conversion of **Citrate to Cis-aconitate** (and subsequently to Isocitrate) is catalyzed by the enzyme **Aconitase** in the TCA cycle. **Why Fluoroacetate is correct:** Fluoroacetate itself is not the inhibitor; it undergoes "lethal synthesis." It reacts with Coenzyme A to form Fluoroacetyl-CoA, which then condenses with oxaloacetate to form **Fluorocitrate**. Fluorocitrate is a potent **suicide inhibitor of Aconitase**. By binding irreversibly to the enzyme, it halts the TCA cycle, leading to a buildup of citrate and a failure of cellular respiration. **Why other options are incorrect:** * **Malonate:** A classic example of a **competitive inhibitor** that inhibits **Succinate Dehydrogenase** (converting succinate to fumarate). * **Fluoride:** Inhibits **Enolase** in the Glycolytic pathway by complexing with magnesium and phosphate. It is used in blood collection tubes (grey top) to prevent glucose breakdown. * **Arsenite:** Inhibits enzymes requiring **Lipoic acid** as a cofactor, specifically the **Pyruvate Dehydrogenase (PDH) complex** and **$\alpha$-ketoglutarate dehydrogenase**. **High-Yield Clinical Pearls for NEET-PG:** * **Suicide Inhibition:** Also known as mechanism-based inhibition (e.g., Allopurinol for Xanthine Oxidase, Aspirin for COX). * **Aconitase** contains an **Iron-Sulfur (Fe-S) cluster** which is essential for its catalytic activity. * **Arsenic Poisoning:** Presents with "garlic breath" and rice-water stools; it inhibits PDH, leading to lactic acidosis.

Question 1

Which of the following is NOT a mixed-function oxidase?

Accepted Answer

Homogentisate oxidase

Answer

Cytochrome P-450

Answer

Phenylalanine hydroxylase

Answer

Tryptophan hydroxylase

Question 2

What is the number of isoenzymes of Lactate Dehydrogenase (LDH)?

Accepted Answer

5

Answer

1

Answer

3

Answer

4

Question 3

All of the following are serine proteases, except?

Accepted Answer

Carboxypeptidase

Answer

Chymotrypsin

Answer

Trypsin

Answer

Thrombin

Question 4

Activation energy is the free energy difference between which of the following?

Accepted Answer

Substrate and transition state

Answer

Substrate and product

Answer

Transition state and product

Answer

Sum of all the above

Question 5

Which amino acid is responsible for the activation of Thioredoxin reductase?

Accepted Answer

Selenocysteine

Answer

Serine

Answer

Cysteine

Answer

Alanine

Question 6

A hepatic enzyme undergoes phosphorylation from a dephosphorylated state. Which of the following is true regarding this process?

Accepted Answer

Always activated by cAMP-dependent protein kinase

Answer

Affected by the level of catecholamines

Answer

Occurs in starvation rather than a well-fed state

Answer

Always activates the enzyme

Question 7

B-Carotene, the precursor of vitamin A, is oxidatively cleaved by which enzyme?

Accepted Answer

beta-Carotene dioxygenase

Answer

Oxygenase

Answer

Hydroxylase

Answer

Transferase

Question 8

The Rossmann fold-associated NADH domain is a structural motif found in which of the following enzymes?

Accepted Answer

Isocitrate Dehydrogenase

Answer

Pyruvate Dehydrogenase

Answer

Citrate synthase

Answer

Succinate Dehydrogenase

Question 9

What is the definition of enzyme specificity?

Accepted Answer

The amount of enzyme causing the transformation of 1 micromole of substrate per minute under standard conditions.

Answer

The amount of enzyme required per second for the formation of one mole of product.

Answer

The number of binding sites on an enzyme for a substrate.

Answer

The ability of an enzyme to bind with various substrates.

Question 10

Which molecule inhibits the conversion of citrate to cis-aconitate?

Accepted Answer

Fluoroacetate

Answer

Malonate

Answer

Fluoride

Answer

Arsenite

Enzymes — MCQs

Enzymes — MCQs

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