Enzymes Practice Questions

Q: Which of the following best describes the role of NAD+ in the reaction catalyzed by aldehyde dehydrogenase?

Coenzyme. **Explanation:** The correct answer is **Coenzyme**. **1. Why Coenzyme is correct:** Enzymes often require non-protein components called **cofactors** to function. Cofactors are broadly divided into inorganic ions (e.g., $Mg^{2+}$, $Zn^{2+}$) and organic molecules. Organic cofactors are specifically termed **coenzymes**. NAD+ (Nicotinamide Adenine Dinucleotide) is an organic molecule derived from Vitamin B3 (Niacin). In the reaction catalyzed by **Aldehyde Dehydrogenase**, NAD+ acts as an electron acceptor (oxidizing agent), facilitating the conversion of acetaldehyde to acetate. Since it is an organic carrier molecule that is loosely bound to the enzyme, it is classified as a coenzyme. **2. Why other options are incorrect:** * **Cofactor:** While "cofactor" is a broad umbrella term that includes coenzymes, "Coenzyme" is the more specific and accurate description for an organic molecule like NAD+. In medical exams, always choose the most specific term. * **Apoenzyme:** This refers to the **protein portion** of the enzyme that is catalytically inactive without its required cofactor. NAD+ is the non-protein component, not the protein part. **High-Yield Clinical Pearls for NEET-PG:** * **Holoenzyme:** The complete, catalytically active unit (Apoenzyme + Cofactor). * **Prosthetic Group:** A coenzyme that is **tightly or covalently** bound to the enzyme (e.g., FAD, Heme, Biotin). NAD+ is NOT a prosthetic group because it binds loosely and dissociates. * **Clinical Correlation:** Aldehyde Dehydrogenase (ALDH) is inhibited by **Disulfiram**, leading to the accumulation of acetaldehyde, which causes the "Disulfiram-like reaction" (flushing, tachycardia, nausea). * **Vitamin Precursor:** Deficiency of Niacin (the precursor to NAD+) leads to **Pellagra** (3Ds: Dermatitis, Diarrhea, Dementia).

Q: Damage to RBC resulting in their lysis increases which isoenzyme in blood?

LDH-1. **Explanation:** **Lactate Dehydrogenase (LDH)** is a tetrameric enzyme composed of two subunits: H (Heart) and M (Muscle). It exists in five isoenzyme forms (LDH-1 to LDH-5), which are distributed tissue-specifically. **1. Why LDH-1 is the correct answer:** LDH-1 (H4) is predominantly found in **Red Blood Cells (RBCs)** and **Cardiac Muscle**. When RBCs undergo lysis (hemolysis), they release large quantities of LDH-1 into the bloodstream. Therefore, an increase in LDH-1 is a hallmark biochemical marker for hemolytic anemia and myocardial infarction. **2. Analysis of Incorrect Options:** * **LDH-3 (H2M2):** Primarily found in the **lungs** and spleen. It increases in conditions like pulmonary embolism or pneumonia. * **LDH-4 (H1M3):** Found in the **kidneys**, placenta, and pancreas. * **LDH-5 (M4):** Predominantly found in **Skeletal Muscle** and the **Liver**. It is a sensitive marker for hepatitis, liver congestion, or skeletal muscle injury (e.g., rhabdomyolysis). **3. High-Yield Clinical Pearls for NEET-PG:** * **LDH Flip:** In normal serum, LDH-2 is higher than LDH-1 (LDH-2 > LDH-1). In cases of **Myocardial Infarction** or **Hemolysis**, LDH-1 levels rise significantly, surpassing LDH-2. This is known as the "Flipped LDH Pattern." * **Total LDH:** While non-specific, total LDH is used as a marker of high cell turnover (e.g., malignancies like Lymphoma or Seminoma). * **Megaloblastic Anemia:** This condition often shows the highest elevations of LDH-1 due to ineffective erythropoiesis (intramedullary hemolysis).

Q: In the study of enzymes, a sigmoidal plot of substrate concentration versus reaction velocity may indicate which of the following?

Cooperative bindings. **Explanation:** **1. Why "Cooperative Bindings" is correct:** In biochemistry, a **sigmoidal (S-shaped) curve** is the hallmark of **allosteric enzymes**. These enzymes do not follow simple Michaelis-Menten kinetics. Instead, they exhibit **cooperative binding**, where the binding of a substrate molecule to one active site increases the affinity of other active sites for the substrate (positive cooperativity). This results in a slow initial velocity that increases rapidly once the first few substrate molecules bind, creating the characteristic "S" shape. A classic non-enzymatic example of this is the oxygen-dissociation curve of **Hemoglobin**. **2. Why the other options are incorrect:** * **A. Michaelis-Menten kinetics:** These enzymes produce a **hyperbolic curve**, not a sigmoidal one. They typically consist of a single polypeptide chain with one active site (e.g., Myoglobin). * **B & C. Competitive and Noncompetitive inhibition:** Both of these follow Michaelis-Menten kinetics. While they alter the $V_{max}$ or $K_m$, the fundamental shape of the velocity-substrate plot remains **hyperbolic**. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzymes:** Most rate-limiting steps in metabolic pathways (e.g., **PFK-1** in glycolysis) are allosteric and show sigmoidal kinetics. * **Hill Coefficient ($n$):** This measures the degree of cooperativity. If $n > 1$, there is positive cooperativity (sigmoidal curve); if $n = 1$, it is non-cooperative (hyperbolic curve). * **T and R states:** Allosteric enzymes transition from a **T (Tense/Low affinity)** state to an **R (Relaxed/High affinity)** state upon substrate binding. * **Allosteric Effectors:** These shift the sigmoidal curve: **Activators** shift it to the **left** (decreasing $K_{0.5}$), while **inhibitors** shift it to the **right** (increasing $K_{0.5}$).

Q: Which of the following statements is true about glutamate dehydrogenase?

It is used as a marker to assess drug safety.. **Explanation:** **Glutamate Dehydrogenase (GDH)** is a mitochondrial enzyme that catalyzes the reversible oxidative deamination of glutamate to $\alpha$-ketoglutarate and ammonia. **Why Option D is Correct:** GDH is primarily located in the **mitochondrial matrix** of hepatocytes, specifically in the **perivenous (Zone 3)** region of the liver lobule. Because it is sequestered within the mitochondria, its release into the serum indicates severe hepatocellular necrosis or mitochondrial damage. In pharmaceutical research and clinical trials, GDH is used as a **highly specific biomarker for Drug-Induced Liver Injury (DILI)**. It is more specific for mitochondrial damage than ALT, making it a critical marker for assessing drug safety. **Analysis of Incorrect Options:** * **Option A:** GDH is located in the **mitochondrial matrix**, not the inner mitochondrial membrane. * **Option B:** In acute viral hepatitis, **ALT and AST** are the primary markers elevated due to cytoplasmic leakage. GDH is a marker of **centrilobular necrosis** (often seen in toxic injury or ischemia) rather than typical viral hepatitis. * **Option C:** Under physiological conditions, the reaction strongly **favors the oxidative deamination** (formation of $\alpha$-ketoglutarate and ammonia) to facilitate the entry of nitrogen into the urea cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Allosteric Regulation:** GDH is uniquely regulated; it is **inhibited by GTP/ATP** (high energy) and **activated by ADP/GDP** (low energy). * **Coenzyme Versatility:** It is one of the few enzymes that can use either **NAD+** (for catabolism) or **NADP+** (for anabolism). * **Clinical Marker:** A high **GDH/ALT ratio** is often suggestive of alcoholic liver disease or ischemic hepatitis.

Q: Which molecule activates glutamate dehydrogenase in mitochondria?

ADP. **Explanation:** **Glutamate Dehydrogenase (GDH)** is a unique mitochondrial enzyme that catalyzes the reversible oxidative deamination of Glutamate to $\alpha$-ketoglutarate and ammonia. It serves as a critical bridge between amino acid metabolism and the TCA cycle. **Why ADP is the correct answer:** GDH is an allosterically regulated enzyme that responds to the energy status of the cell. When cellular energy levels are low, **ADP** (and GDP) concentrations rise. ADP acts as a potent **allosteric activator** of GDH, promoting the breakdown of glutamate into $\alpha$-ketoglutarate. This $\alpha$-ketoglutarate then enters the TCA cycle to facilitate ATP production. **Why the other options are incorrect:** * **ATP and GTP (Options A & B):** These are indicators of high cellular energy. They act as **allosteric inhibitors** of GDH. When energy is abundant, the cell does not need to divert amino acids into the TCA cycle for fuel. * **NADH (Option C):** As a product of the GDH reaction and a signal of high energy status, NADH also acts as an inhibitor of the enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Dual Coenzyme Specificity:** GDH is one of the few enzymes that can use either **NAD+** (oxidative deamination) or **NADP+** (reductive amination) as a coenzyme. * **Hyperinsulinism-Hyperammonemia Syndrome:** Mutations that lead to the loss of allosteric inhibition of GDH (making it permanently active) result in this syndrome. Overactive GDH increases $\alpha$-ketoglutarate, stimulating the TCA cycle and triggering excessive insulin release (hypoglycemia), while simultaneously increasing ammonia production (hyperammonemia). * **Directionality:** In the liver, the reaction primarily proceeds toward the formation of ammonia for the urea cycle.

Q: Which one of the following statements regarding enzyme kinetics is true?

A low Km indicates high enzyme-substrate affinity.. **Explanation:** **1. Understanding Km and Affinity (The Correct Answer)** The Michaelis constant (**Km**) is defined as the substrate concentration at which the reaction velocity is half of the maximum velocity ($V_{max}$). It is an inverse measure of the enzyme's affinity for its substrate. * **Low Km:** The enzyme reaches half-saturation at a low substrate concentration, indicating **high affinity**. * **High Km:** The enzyme requires a high substrate concentration to reach half-saturation, indicating **low affinity**. **2. Analysis of Incorrect Options** * **Option A:** Incorrect. As explained above, a high Km signifies that the enzyme binds weakly to the substrate (low affinity). * **Options C & D:** In **Non-competitive inhibition**, the inhibitor binds to a site other than the active site (allosteric site). This reduces the overall concentration of functional enzymes, thereby **decreasing $V_{max}$**. However, because the inhibitor does not compete for the active site, the binding of the substrate to the enzyme is unaffected; therefore, **Km remains unchanged**. **3. High-Yield Clinical Pearls for NEET-PG** * **Competitive Inhibition:** The inhibitor resembles the substrate. **Km increases** (affinity decreases), but **$V_{max}$ remains unchanged** (can be overcome by adding more substrate). *Example: Statins inhibiting HMG-CoA reductase.* * **Non-competitive Inhibition:** **$V_{max}$ decreases**, but **Km remains unchanged**. *Example: Cyanide poisoning of Cytochrome oxidase.* * **Lineweaver-Burk Plot:** * X-intercept = $-1/Km$ * Y-intercept = $1/V_{max}$ * **Hexokinase vs. Glucokinase:** Hexokinase has a **low Km** (high affinity) for glucose, allowing it to function even at low blood sugar levels, whereas Glucokinase has a **high Km** (low affinity), functioning primarily after meals (high blood sugar).

Q: Copper is involved in collagen synthesis by which enzyme?

Lysyl oxidase. **Explanation:** **1. Why Lysyl Oxidase is correct:** Collagen synthesis involves several post-translational modifications. **Lysyl oxidase** is a copper-dependent extracellular enzyme responsible for the final step: the formation of **cross-links** between collagen fibrils. It oxidatively deaminates specific lysine and hydroxylysine residues into reactive aldehydes (allysine). These aldehydes then undergo spontaneous condensation to form covalent cross-links, which provide collagen with its structural integrity and high tensile strength. **2. Why other options are incorrect:** * **Lysyl hydroxylase:** This enzyme is responsible for the hydroxylation of lysine residues *inside* the cell. Crucially, it requires **Vitamin C (Ascorbic acid)** and Iron (Fe²⁺) as cofactors, not copper. Deficiency leads to Scurvy. * **Cytochrome oxidase:** While this is a copper-containing enzyme (Complex IV of the electron transport chain), it is involved in cellular respiration and ATP production, not collagen synthesis. * **Tyrosinase:** This is also a copper-containing enzyme, but its role is in the rate-limiting step of **melanin synthesis**. Deficiency results in Albinism. **3. Clinical Pearls & High-Yield Facts:** * **Menkes Disease:** A defect in copper absorption (ATP7A gene) leads to copper deficiency. This results in reduced lysyl oxidase activity, causing "kinky hair," connective tissue defects, and aortic aneurysms. * **Lathyrism:** Consumption of sweet peas (*Lathyrus odoratus*) containing β-aminopropionitrile inhibits lysyl oxidase, leading to skeletal and vascular deformities. * **Cofactor Summary:** Remember **"C for Cross-linking and Copper"** (Lysyl oxidase) vs. **"H for Hydroxylation and Hydroxylase"** (Vitamin C).

Q: Alkaline phosphatase is found in all organs, except?

Heart. **Explanation:** Alkaline Phosphatase (ALP) is a group of isoenzymes that hydrolyze phosphate esters at an alkaline pH. It is primarily associated with tissues involved in high metabolic activity or transport across membranes. **Why Heart is the correct answer:** ALP is notably **absent or present in negligible amounts in the Heart**. The primary enzymes found in cardiac tissue are Creatine Kinase (CK-MB), Aspartate Aminotransferase (AST), and Lactate Dehydrogenase (LDH). Therefore, ALP is not used as a biomarker for myocardial injury. **Analysis of Incorrect Options:** * **Bone (Option A):** ALP is highly concentrated in **osteoblasts**. It is a marker of bone formation and is elevated in conditions like Paget’s disease, rickets, and bone metastases. * **Placenta (Option C):** The placenta produces a heat-stable isoenzyme of ALP. It rises during the third trimester of pregnancy. (Note: The **Regan isoenzyme** is a placental-like ALP seen in some cancers). * **Lungs (Option D):** ALP is present in the vascular endothelium and alveolar cells of the lungs. While not a primary diagnostic source, it is physiologically present. **High-Yield Clinical Pearls for NEET-PG:** 1. **Major Sources:** The mnemonic **"L-B-P"** (Liver, Bone, Placenta) helps remember the primary sites. The Liver (biliary canaliculi) and Bone are the most clinically significant sources. 2. **Heat Stability Test:** To differentiate sources, remember: *"Placenta is Stable, Bone is Blown"* (Placental ALP is heat-stable at 65°C; Bone ALP is heat-labile at 56°C). 3. **Intestinal ALP:** Found in individuals with blood groups B or O, especially after a fatty meal. 4. **Clinical Marker:** ALP is the most sensitive marker for **obstructive jaundice** (cholestasis) and space-occupying lesions in the liver.

Question 1

The enzyme trypsin is specific for peptide bonds of which type of amino acids?

Accepted Answer

Basic amino acids

Answer

Acidic amino acids

Answer

Aromatic amino acids

Answer

Amino acids adjacent to proline residues

Question 2

Which of the following best describes the role of NAD+ in the reaction catalyzed by aldehyde dehydrogenase?

Accepted Answer

Coenzyme

Answer

Cofactor

Answer

Apoenzyme

Answer

None of the above

Question 3

Which of the following statements regarding the Michaelis constant (Km) is TRUE?

Accepted Answer

The Km value is inversely proportional to the affinity of the enzyme for its substrate.

Answer

Km is dependent on enzyme concentration.

Answer

The Km value denotes the product concentration at half maximal velocity.

Answer

The Km value can be the same for two different enzymes.

Question 4

Damage to RBC resulting in their lysis increases which isoenzyme in blood?

Accepted Answer

LDH-1

Answer

LDH-3

Answer

LDH-4

Answer

LDH-5

Question 5

In the study of enzymes, a sigmoidal plot of substrate concentration versus reaction velocity may indicate which of the following?

Accepted Answer

Cooperative bindings

Answer

Michaelis-Menten kinetics

Answer

Competitive inhibition

Answer

Noncompetitive inhibition

Question 6

Which of the following statements is true about glutamate dehydrogenase?

Accepted Answer

It is used as a marker to assess drug safety.

Answer

It is present in the inner mitochondrial membrane.

Answer

It is elevated in acute viral hepatitis.

Answer

It favours the formation of glutamate.

Question 7

Which molecule activates glutamate dehydrogenase in mitochondria?

Accepted Answer

ADP

Answer

ATP

Answer

GTP

Answer

NADH

Question 8

Which one of the following statements regarding enzyme kinetics is true?

Accepted Answer

A low Km indicates high enzyme-substrate affinity.

Answer

A high Km indicates high enzyme-substrate affinity.

Answer

Non-competitive inhibition decreases Km.

Answer

Non-competitive inhibition increases Km.

Question 9

Copper is involved in collagen synthesis by which enzyme?

Accepted Answer

Lysyl oxidase

Answer

Lysyl hydroxylase

Answer

Cytochrome oxidase

Answer

Tyrosinase

Question 10

Alkaline phosphatase is found in all organs, except?

Accepted Answer

Heart

Answer

Bone

Answer

Placenta

Answer

Lungs

Enzymes — MCQs

Enzymes — MCQs

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