Enzymes Practice Questions

Q: Which of the following methods is used for regulating the quantity of an enzyme?

Induction. **Explanation:** Enzyme regulation occurs through two primary mechanisms: **control of enzyme activity** (fast) and **control of enzyme quantity** (slow). **Why Induction is Correct:** **Induction** refers to an increase in the synthesis of enzyme molecules at the genetic level (transcription/translation). By increasing the rate of gene expression, the cell increases the absolute number (quantity) of enzyme molecules available. This is a relatively slow process, taking hours to days. A classic example is the induction of Cytochrome P450 enzymes by drugs like Phenobarbital. Conversely, **Repression** decreases the quantity of an enzyme. **Why Other Options are Incorrect:** * **A. Phosphorylation:** This is a form of **Covalent Modification**. It regulates the *activity* (turning the enzyme 'on' or 'off') of existing enzyme molecules, not their quantity. For example, Glycogen Phosphorylase is activated by phosphorylation. * **C. Acetylation:** Similar to phosphorylation, this is a post-translational covalent modification that alters the *function or affinity* of a protein (e.g., Histone acetylation) rather than its total concentration. * **D. Glycosylation:** This is a post-translational modification primarily involved in protein folding, stability, and targeting to specific organelles, rather than the quantitative regulation of enzyme levels. **High-Yield NEET-PG Pearls:** * **Short-term regulation:** Allosteric regulation and Covalent modification (seconds to minutes). * **Long-term regulation:** Induction and Repression (hours to days). * **Rate-limiting step:** Regulation usually occurs at the first committed step of a metabolic pathway. * **Key Example:** Insulin induces the synthesis of key glycolytic enzymes (Glucokinase, PFK-1) while repressing gluconeogenic enzymes.

Q: Which of the following is a functional plasma enzyme?

Lipoprotein lipase. ### Explanation Enzymes in the plasma are broadly classified into two categories: **Functional** and **Non-functional** plasma enzymes. **1. Why Lipoprotein Lipase (LPL) is the Correct Answer:** Functional plasma enzymes are those that are actively secreted into the blood by specific organs (usually the liver) and perform their primary physiological function within the circulation. **Lipoprotein lipase** is a classic example; it is synthesized by extrahepatic tissues and attached to the capillary endothelium. It acts on circulating chylomicrons and VLDL to hydrolyze triglycerides into free fatty acids and glycerol. Other examples include pseudocholinesterase and enzymes involved in blood coagulation. **2. Why the Other Options are Incorrect:** * **Lactate Dehydrogenase (LDH) & Creatine Phosphokinase (CPK):** These are **non-functional plasma enzymes**. They perform their metabolic roles exclusively *inside* cells. Their presence in the plasma in high concentrations is abnormal and indicates cell damage or necrosis (e.g., myocardial infarction or hepatitis). * **Lipase:** While secreted, pancreatic lipase is an **exocrine enzyme** intended for the digestive tract. Its presence in the blood is a diagnostic marker for pancreatic injury (e.g., acute pancreatitis) rather than a functional component of plasma. **3. High-Yield Clinical Pearls for NEET-PG:** * **Functional Enzymes:** Substrate is always present in the blood; concentration is higher in plasma than in tissues. * **Non-functional Enzymes:** Substrate is absent in the blood; concentration is much higher in tissues than in plasma. * **Heparin Connection:** Intravenous heparin releases LPL from the endothelial wall into the plasma, a phenomenon known as "post-heparin lipolytic activity." * **Diagnostic Value:** Non-functional enzymes are used as **biomarkers** for organ-specific damage (e.g., ALT for liver, CK-MB for heart).

Q: Which of the following coenzymes is NOT a part of the Pyruvate Dehydrogenase Complex?

Thiamine pyrophosphate. The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme system that converts Pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. This complex requires **five specific cofactors** to function. ### **Explanation of the Question** The question as presented contains a technical error in its marking. **Thiamine Pyrophosphate (TPP) IS a part of the PDH complex.** In fact, all four options listed (Lipoic Acid, FAD, TPP, and Vitamin B3/NAD+) are essential components of the complex. If this were a standard NEET-PG question, the correct answer would typically be a vitamin like **B6 (Pyridoxine)** or **B12 (Cobalamin)**, which are NOT involved in this complex. ### **The Five Essential Cofactors (Mnemonic: "Tender Loving Care For Nancy")** 1. **T - Thiamine Pyrophosphate (TPP):** Derived from Vitamin B1; acts as a prosthetic group for E1 (Pyruvate decarboxylase). 2. **L - Lipoic Acid:** Acts as a prosthetic group for E2 (Dihydrolipoyl transacetylase). 3. **C - Coenzyme A (CoA):** Derived from Vitamin B5 (Pantothenic acid); serves as the substrate for E2. 4. **F - FAD:** Derived from Vitamin B2 (Riboflavin); acts as a prosthetic group for E3 (Dihydrolipoyl dehydrogenase). 5. **N - NAD+:** Derived from Vitamin B3 (Niacin); serves as the electron acceptor for E3. ### **Clinical Pearls for NEET-PG** * **Arsenic Poisoning:** Arsenite inhibits the PDH complex by binding to the -SH groups of **Lipoic Acid**, leading to lactic acidosis and neurological symptoms. * **Wernicke-Korsakoff Syndrome:** Caused by B1 deficiency; PDH activity decreases, impairing glucose utilization in the brain. * **Enzyme Components:** E1 = Pyruvate dehydrogenase; E2 = Dihydrolipoyl transacetylase; E3 = Dihydrolipoyl dehydrogenase.

Q: Selenocysteine residues are present in which of the following enzymes?

Deiodinase. **Explanation:** The correct answer is **Deiodinase**. Selenocysteine is often referred to as the "21st amino acid" and is unique because it contains selenium instead of the sulfur found in cysteine. **1. Why Deiodinase is Correct:** The conversion of the prohormone Thyroxine (T4) to the active Triiodothyronine (T3) is catalyzed by **Iodothyronine Deiodinases** (Types 1, 2, and 3). These enzymes are **selenoproteins**, meaning they require selenocysteine at their active site for catalytic activity. Selenium deficiency can lead to impaired thyroid hormone metabolism, mimicking symptoms of hypothyroidism despite normal T4 levels. Other notable selenoproteins include **Glutathione peroxidase** (an antioxidant) and **Thioredoxin reductase**. **2. Why the Other Options are Incorrect:** * **Pyruvate carboxylase:** This enzyme, involved in gluconeogenesis, requires **Biotin (Vitamin B7)** as a cofactor and Manganese/Magnesium for activation. * **Xanthine oxidase:** This enzyme, crucial for purine catabolism (converting hypoxanthine to uric acid), requires **Molybdenum**, Iron, and FAD. * **Lysyl oxidase:** Essential for cross-linking collagen and elastin fibers, this enzyme is a **Copper-dependent** metalloenzyme. Deficiency leads to Menkes disease or Ehlers-Danlos syndrome. **Clinical Pearls for NEET-PG:** * **Codon:** Selenocysteine is encoded by the **UGA** codon, which usually acts as a "stop" codon. A specific mRNA sequence (SECIS element) directs the ribosome to insert selenocysteine instead. * **Antioxidant link:** Glutathione peroxidase (containing selenium) protects cells from oxidative damage by reducing hydrogen peroxide to water. * **Deficiency:** Selenium deficiency is linked to **Keshan disease** (cardiomyopathy) and **Kashin-Beck disease** (osteoarthropathy).

Q: Which enzyme is a NAD+ linked dehydrogenase?

Pyruvate dehydrogenase (PDH). **Explanation:** **1. Why Pyruvate Dehydrogenase (PDH) is Correct:** The Pyruvate Dehydrogenase Complex (PDH) is a multi-enzyme system that catalyzes the oxidative decarboxylation of pyruvate to Acetyl-CoA. This reaction is a critical link between glycolysis and the TCA cycle. The PDH complex requires five cofactors: **Thiamine pyrophosphate (TPP), Lipoic acid, CoA, FAD, and NAD+**. Specifically, the E3 component (Dihydrolipoyl dehydrogenase) utilizes NAD+ as the final electron acceptor, reducing it to **NADH + H+**. Thus, it is a classic NAD+-linked dehydrogenase. **2. Analysis of Incorrect Options:** * **Glucose-6-phosphate dehydrogenase (G6PD):** This is the rate-limiting enzyme of the Hexose Monophosphate (HMP) Shunt. It is **NADP+-linked**, not NAD+-linked. It reduces NADP+ to **NADPH**, which is essential for reductive biosynthesis and maintaining reduced glutathione. * **Flavin adenine dinucleotide (FAD) & Flavin mononucleotide (FMN):** These are not enzymes; they are **coenzymes** (prosthetic groups) derived from Vitamin B2 (Riboflavin). While they are involved in redox reactions, they are the "tools" used by enzymes, not the dehydrogenases themselves. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for PDH Cofactors:** "**T**ender **L**oving **C**are **F**or **N**ancy" (TPP, Lipoic acid, CoA, FAD, NAD). * **Clinical Correlation:** PDH deficiency is the most common cause of congenital lactic acidosis. * **Inhibitors:** PDH is inhibited by **Arsenite**, which binds to the -SH groups of Lipoic acid. * **Key Distinction:** Generally, NAD+ is used in **catabolic** pathways (energy generation), while NADP+ is used in **anabolic** pathways (synthesis).

Q: Malonate is a competitive inhibitor of succinate dehydrogenase, a key enzyme in the Krebs tricarboxylic acid cycle. How does the presence of malonate affect the kinetic parameters of succinate dehydrogenase?

Increases the apparent Km but does not affect Vmax.. ### Explanation **1. Why Option A is Correct (The Mechanism)** Malonate acts as a **competitive inhibitor** because its chemical structure closely resembles succinate (the natural substrate). In competitive inhibition, the inhibitor and substrate compete for the same **active site** on the enzyme. * **Effect on $K_m$:** Because malonate occupies the active site, a higher concentration of succinate is required to achieve half-maximal velocity ($1/2 V_{max}$). This results in an **increase in the apparent $K_m$** (decreased affinity). * **Effect on $V_{max}$:** Competitive inhibition can be overcome by increasing the substrate concentration. At infinitely high succinate levels, the inhibitor is displaced, allowing the enzyme to reach its original maximum velocity. Thus, **$V_{max}$ remains unchanged**. **2. Why Other Options are Wrong** * **Option B:** Decreased $K_m$ implies increased affinity, which never occurs with inhibitors. * **Option C:** This describes **Non-competitive inhibition**. In this case, the inhibitor binds to an allosteric site, reducing the total catalytic activity ($V_{max}$ decreases) regardless of substrate concentration, while $K_m$ remains constant. * **Option D:** No known inhibitor increases $V_{max}$; this would describe an enzyme activator. **3. Clinical Pearls & High-Yield Facts for NEET-PG** * **Lineweaver-Burk Plot:** In competitive inhibition, the plots intersect at the **Y-axis** ($1/V_{max}$ is constant). * **Classic Example:** The use of **Statins** (HMG-CoA reductase inhibitors) to treat hypercholesterolemia is a clinically vital example of competitive inhibition. * **Ethylene Glycol Poisoning:** Ethanol acts as a competitive inhibitor of Alcohol Dehydrogenase, preventing the formation of toxic metabolites. * **Methanol Poisoning:** Fomepizole is the competitive inhibitor used as an antidote.

Q: Which enzymes are responsible for the synthesis of extracellular glucans and fructans?

Glucosyltransferase, fructosyltransferase. ### Explanation **1. Why Option A is Correct:** The synthesis of extracellular polysaccharides, specifically **glucans** (like dextran) and **fructans** (like levan), is catalyzed by the enzymes **glucosyltransferase** and **fructosyltransferase**, respectively. These enzymes are primarily produced by oral bacteria, most notably *Streptococcus mutans*. The underlying mechanism involves the cleavage of **sucrose** (a disaccharide). These enzymes use the energy released from breaking the glycosidic bond of sucrose to transfer a monosaccharide unit to a growing polymer chain: * **Glucosyltransferase:** Transfers glucose from sucrose to form glucans. * **Fructosyltransferase:** Transfers fructose from sucrose to form fructans. **2. Why Other Options are Incorrect:** * **Options B & C:** The terms "Glucosylconvertase" and "Fructosylconvertase" are not standard biochemical nomenclature for these biosynthetic pathways. While "convertase" is a general term for enzymes that convert one substance to another (e.g., Proprotein convertase), it does not describe the specific transferase activity required for polysaccharide synthesis from sucrose. * **Option D:** Since B and C are incorrect, "Both of the above" is invalid. **3. Clinical Pearls for NEET-PG:** * **Dental Caries:** This is the most high-yield clinical correlation. *Streptococcus mutans* uses these extracellular glucans to form a sticky **biofilm (dental plaque)**, allowing bacteria to adhere to tooth enamel. * **Substrate Specificity:** Sucrose is the *only* sugar that can be used as a substrate for these specific extracellular enzymes, which is why high sucrose intake is directly linked to tooth decay. * **Lactic Acid:** Once the biofilm is formed, bacteria ferment other carbohydrates into lactic acid, which demineralizes the enamel.

Q: Which of the following is a multi-enzyme complex?

Fatty acid synthase. ### Explanation **Correct Answer: C. Fatty acid synthase** **Why it is correct:** A **multi-enzyme complex** is a stable assembly of multiple enzymes that catalyze sequential steps in a metabolic pathway. In eukaryotes (including humans), **Fatty Acid Synthase (FAS)** is a classic example. It is a **homodimer**, where each monomer is a single large polypeptide chain containing **seven different catalytic activities** and an **Acyl Carrier Protein (ACP)** domain. This structural arrangement allows for "substrate channeling," where the growing fatty acid chain remains covalently attached to the complex, increasing catalytic efficiency and preventing the loss of intermediates. **Why the other options are incorrect:** * **A. HMG-CoA synthase:** This is a single enzyme involved in ketogenesis (mitochondria) and cholesterol synthesis (cytoplasm). It is not a multi-enzyme complex. * **B. Malic enzyme:** This is a single oxidative decarboxylase that converts malate to pyruvate, generating NADPH. * **C. Fatty acid oxidase:** This refers to the enzymes of **β-oxidation**. Unlike fatty acid synthesis, the enzymes for β-oxidation are **independent, soluble proteins** located in the mitochondrial matrix (except for the trifunctional protein in the inner membrane, but the system as a whole is not classified as a single multi-enzyme complex like FAS). **High-Yield Clinical Pearls for NEET-PG:** * **Other Multi-enzyme Complexes:** Pyruvate Dehydrogenase (PDH), α-Ketoglutarate Dehydrogenase, and Branched-chain α-keto acid Dehydrogenase. * **FAS End-product:** The primary product of the FAS complex is **Palmitate (C16)**. * **Requirement:** FAS requires **NADPH** as a reducing equivalent, primarily sourced from the Pentose Phosphate Pathway (HMP Shunt). * **Prokaryotic Difference:** In *E. coli* (Type II FAS), the enzymes are separate individual proteins, unlike the integrated Type I system in humans.

Q: Zinc is essential for which of the following enzymes?

Carbonic anhydrase. **Explanation:** **1. Why Carbonic Anhydrase is Correct:** Carbonic anhydrase is a classic example of a **metalloenzyme** where Zinc ($Zn^{2+}$) is an essential structural and functional component. The zinc ion is coordinated to three histidine residues and a water molecule at the enzyme's active site. It facilitates the rapid interconversion of carbon dioxide and water into bicarbonate and protons ($CO_2 + H_2O \rightleftharpoons HCO_3^- + H^+$), a process vital for acid-base balance, respiration, and renal function. **2. Analysis of Incorrect Options:** * **Pyruvate Kinase:** This glycolytic enzyme requires **Potassium ($K^+$)** as a monovalent activator and **Magnesium ($Mg^{2+}$)** or Manganese ($Mn^{2+}$) as divalent activators. * **Cytochrome Oxidase:** This is the terminal enzyme of the Electron Transport Chain (Complex IV). It contains **Copper ($Cu^{2+}$)** and **Iron ($Fe^{2+}/Fe^{3+}$)** in its heme groups. * **Xanthine Oxidase:** This enzyme, involved in purine catabolism (converting hypoxanthine to xanthine and then to uric acid), requires **Molybdenum (Mo)**, Iron, and FAD. **3. High-Yield Clinical Pearls for NEET-PG:** * **Other Zinc-containing enzymes:** Alcohol dehydrogenase, Carboxypeptidase, DNA/RNA Polymerase, Alkaline Phosphatase (ALP), and Superoxide Dismutase (cytosolic form). * **Zinc Finger Motifs:** Zinc is crucial for the structural stability of many transcription factors. * **Clinical Correlation:** Zinc deficiency leads to **Acrodermatitis Enteropathica**, characterized by periorificial dermatitis, alopecia, and diarrhea. It also causes impaired wound healing and hypogeusia (decreased taste). * **Mnemonic for Zinc Enzymes:** "**Z**inc **C**an **A**lways **A**id **P**olymerase" (**Z**inc: **C**arbonic anhydrase, **A**LP/Alcohol dehydrogenase, **A**CE, **P**olymerase).

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.

Question 1

Which of the following methods is used for regulating the quantity of an enzyme?

Accepted Answer

Induction

Answer

Phosphorylation

Answer

Acetylation

Answer

Glycosylation

Question 2

Which of the following is a functional plasma enzyme?

Accepted Answer

Lipoprotein lipase

Answer

Lactate dehydrogenase (LDH)

Answer

Creatine phosphokinase (CPK)

Answer

Lipase

Question 3

Which of the following coenzymes is NOT a part of the Pyruvate Dehydrogenase Complex?

Accepted Answer

Thiamine pyrophosphate

Answer

Lipoic Acid

Answer

FAD

Answer

Vitamin B3

Question 4

Selenocysteine residues are present in which of the following enzymes?

Accepted Answer

Deiodinase

Answer

Pyruvate carboxylase

Answer

Xanthine oxidase

Answer

Lysyl oxidase

Question 5

Which enzyme is a NAD+ linked dehydrogenase?

Accepted Answer

Pyruvate dehydrogenase (PDH)

Answer

Glucose-6-phosphate dehydrogenase (G6PD)

Answer

Flavin adenine dinucleotide (FAD)

Answer

Flavin mononucleotide (FMN)

Question 6

Malonate is a competitive inhibitor of succinate dehydrogenase, a key enzyme in the Krebs tricarboxylic acid cycle. How does the presence of malonate affect the kinetic parameters of succinate dehydrogenase?

Accepted Answer

Increases the apparent Km but does not affect Vmax.

Answer

Decreases the apparent Km but does not affect Vmax.

Answer

Decreases Vmax but does not affect the apparent Km.

Answer

Increases Vmax but does not affect the apparent Km.

Question 7

Which enzymes are responsible for the synthesis of extracellular glucans and fructans?

Accepted Answer

Glucosyltransferase, fructosyltransferase

Answer

Glucosylconvertase, fructosylconvertase

Answer

Convertase

Answer

Both of the above

Question 8

Which of the following is a multi-enzyme complex?

Accepted Answer

Fatty acid synthase

Answer

HMG-CoA synthase

Answer

Malic enzyme

Answer

Fatty acid oxidase

Question 9

Zinc is essential for which of the following enzymes?

Accepted Answer

Carbonic anhydrase

Answer

Pyruvate kinase

Answer

Cytochrome oxidase

Answer

Xanthine oxidase

Question 10

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

Accepted Answer

Homogentisate oxidase

Answer

Cytochrome P-450

Answer

Phenylalanine hydroxylase

Answer

Tryptophan hydroxylase

Enzymes — MCQs

Enzymes — MCQs

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