Enzymes Practice Questions

Q: Which of the following is a copper-containing enzyme?

Cytochrome oxidase. **Explanation:** **Cytochrome oxidase** (also known as Complex IV of the Electron Transport Chain) is the correct answer because it contains **two copper centers ($Cu_A$ and $Cu_B$)** in addition to two heme groups ($a$ and $a_3$). These copper ions are essential for the final step of cellular respiration, where electrons are transferred to oxygen to form water. **Analysis of Options:** * **Catalase:** This is a **heme-containing (iron)** enzyme found in peroxisomes. It protects cells from oxidative damage by catalyzing the decomposition of hydrogen peroxide into water and oxygen. * **Lactate dehydrogenase (LDH):** This is a glycolytic enzyme that converts pyruvate to lactate. It does not require a metal cofactor like copper; instead, it utilizes **NAD+/NADH** as a coenzyme. * **None of the above:** Incorrect, as Cytochrome oxidase is a well-known cuproenzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Other Copper-containing enzymes:** Remember the mnemonic **"C-C-S-S-T-L"**: **C**ytochrome oxidase, **C**eruloplasmin (ferroxidase), **S**uperoxide dismutase (cytosolic Zn-Cu SOD), **S**pice (Tyrosinase for melanin), **T**ryptophan pyrrolase, and **L**ysyl oxidase (essential for collagen cross-linking). * **Menkes Disease:** A defect in copper absorption (ATP7A) leading to "kinky hair" and connective tissue defects due to low **Lysyl oxidase** activity. * **Wilson Disease:** A defect in copper excretion (ATP7B) leading to copper toxicity and low levels of **Ceruloplasmin**. * **Cyanide/CO Poisoning:** Both inhibit **Cytochrome oxidase** by binding to the iron/copper centers, halting the ETC and causing cellular asphyxia.

Q: Which enzyme protects the brain from free radical injury?

Superoxide dismutase. **Explanation:** **Correct Answer: B. Superoxide dismutase (SOD)** **Concept:** The brain is highly susceptible to oxidative stress due to its high oxygen consumption and rich lipid content. Free radicals, specifically the **superoxide anion ($O_2^-$)**, are toxic byproducts of aerobic metabolism. **Superoxide dismutase (SOD)** is the primary antioxidant enzyme that neutralizes these radicals by catalyzing the dismutation of superoxide into oxygen and hydrogen peroxide ($H_2O_2$). This prevents the formation of more reactive species like hydroxyl radicals, thereby protecting neuronal membranes from lipid peroxidation. **Analysis of Incorrect Options:** * **A. Myeloperoxidase:** Found primarily in neutrophils, this enzyme produces hypochlorous acid (HOCl) to kill bacteria. In the brain, its overactivity is actually associated with *promoting* oxidative damage and neuroinflammation rather than preventing it. * **C. Monoamine oxidase (MAO):** This enzyme breaks down neurotransmitters (catecholamines). A byproduct of this reaction is hydrogen peroxide, which can actually *increase* oxidative stress. * **D. Hydroxylase:** These enzymes (e.g., Phenylalanine hydroxylase) are involved in the synthesis of neurotransmitters and amino acid metabolism; they do not function as antioxidant scavengers. **High-Yield Clinical Pearls for NEET-PG:** * **Amyotrophic Lateral Sclerosis (ALS):** Mutations in the **SOD1 gene** (encoding Cu/Zn superoxide dismutase) are a classic cause of familial ALS, highlighting the enzyme's critical role in motor neuron survival. * **Antioxidant Trio:** The three main enzymes protecting the body from reactive oxygen species (ROS) are **SOD, Catalase, and Glutathione Peroxidase.** * **Vulnerability:** The brain has relatively lower levels of catalase compared to other organs, making SOD and Glutathione systems even more vital for neuroprotection.

Q: Trypsinogen is converted to trypsin by which of the following?

Enteropeptidase. **Explanation:** **1. Why Enteropeptidase is Correct:** Trypsinogen is an inactive zymogen secreted by the exocrine pancreas. To prevent autodigestion of the pancreas, it must only be activated once it reaches the duodenum. **Enteropeptidase** (also known as enterokinase), an enzyme secreted by the duodenal mucosal cells (Brunner’s glands), acts as the specific "molecular switch." It cleaves a hexapeptide from the N-terminal end of trypsinogen, converting it into its active form, **trypsin**. Once a small amount of trypsin is formed, it acts autocatalytically to activate more trypsinogen and other pancreatic zymogens (chymotrypsinogen, procarboxypeptidase, and proelastase). **2. Why Other Options are Incorrect:** * **B. Acidic pH:** Acidic pH (gastric acid) is responsible for activating **pepsinogen to pepsin** in the stomach. In contrast, pancreatic enzymes like trypsin require an alkaline pH (provided by bicarbonate) to function optimally. * **C. Elastase:** Elastase is a protease itself, but it is secreted as the zymogen **proelastase**, which requires active trypsin for its activation. It does not activate trypsinogen. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "Master Switch":** Trypsin is considered the master activator of all pancreatic proteases. * **Deficiency:** Congenital enteropeptidase deficiency leads to severe protein malabsorption and failure to thrive because no pancreatic proteases can be activated. * **Protection Mechanism:** The pancreas also secretes **PSTI (Pancreatic Secretory Trypsin Inhibitor)** to neutralize any trypsin prematurely activated within the pancreatic ducts, preventing acute pancreatitis. * **Localization:** Enteropeptidase is located on the brush border of the duodenal enterocytes.

Q: Which of the following is a non-competitive inhibitor of intestinal alkaline phosphatase?

L-Phenylalanine. **Explanation:** Alkaline Phosphatase (ALP) exists in several tissue-specific isoenzymes (liver, bone, kidney, placenta, and intestine). A unique biochemical characteristic of these isoenzymes is their susceptibility to specific amino acid inhibitors, which is often used in laboratory medicine to differentiate the source of elevated ALP levels. **1. Why L-Phenylalanine is correct:** L-Phenylalanine is a classic **uncompetitive/non-competitive inhibitor** specifically for the **intestinal** and **placental** isoenzymes of ALP. It binds to the enzyme-substrate complex, preventing the reaction from proceeding. This inhibition is highly specific; it does not affect the liver, bone, or kidney (tissue non-specific) isoenzymes. In clinical biochemistry, adding L-phenylalanine to a serum sample helps quantify the proportion of intestinal ALP present. **2. Why other options are incorrect:** * **L-Alanine:** This amino acid is a specific inhibitor of the **liver and kidney** isoenzymes of ALP, rather than the intestinal form. * **L-Tyrosine & L-Tryptophan:** While these are aromatic amino acids like phenylalanine, they do not exhibit the same potent or specific inhibitory effect on intestinal ALP isoenzymes and are not used as diagnostic markers in this context. **High-Yield Clinical Pearls for NEET-PG:** * **Heat Stability Test:** This is another method to differentiate ALP. **Placental ALP** is the most heat-stable ("Regan Isoenzyme"), while **Bone ALP** is the most heat-labile ("Bone burns"). * **Levamisole:** Inhibits all ALP isoenzymes *except* intestinal and placental forms. * **Clinical Significance:** Intestinal ALP is often elevated in individuals with blood groups B or O after a fatty meal and in patients with cirrhosis.

Q: Which of the following enzymes is typically elevated in prostate cancer?

Acid phosphatase. ***Acid phosphatase***- While largely replaced by **Prostate-Specific Antigen (PSA)** for screening, **Prostatic Acid Phosphatase (PAP)** remains a strong enzymatic marker for prostate cancer, particularly useful in monitoring metastatic disease.- Significant elevations are often indicative of **extra-capsular extension** or **bone metastases** in advanced disease.*Alkaline phosphatase*- While generalized **Alkaline phosphatase (ALP)** can be elevated in prostate cancer, this is primarily due to **osteoblastic bone metastases**, not the tumor cells directly.- ALP is a non-specific marker primarily associated with **liver** or **generalized bone disease**.*Lactate dehydrogenase*- **Lactate dehydrogenase (LDH)** is a non-specific indicator of generalized **tissue injury** or high **tumor burden** across various malignancies (e.g., lymphomas, melanoma, germ cell tumors).- Its elevation reflects high cellular turnover/necrosis but is not a primary diagnostic marker specific to prostate cancer.*Alpha-fetoprotein*- **Alpha-fetoprotein (AFP)** is a crucial tumor marker for **hepatocellular carcinoma (HCC)** and **nonseminomatous testicular germ cell tumors** (like yolk sac tumors).- It is not typically elevated in prostate cancer and should not be used in its diagnosis or monitoring.

Q: Identify the type of inhibition shown in the graph.

Noncompetitive inhibition. ***Noncompetitive inhibition***- This inhibition type is characterized by a decrease in **$V_{max}$** but no change in the **$K_m$** value, meaning the inhibitor reduces the enzyme's efficiency but not its affinity for the substrate.- The inhibitor typically binds reversibly to an **allosteric site** (not the active site), affecting the enzyme's catalytic functionality whether the substrate is bound or not.*Competitive inhibition*- Competitive inhibition is characterized by an **increased $K_m$** (decreased apparent affinity) while the **$V_{max}$** remains unchanged.- The inhibitor binds directly to the **active site**, competing with the substrate, and the effect can be overcome by increasing substrate concentration.*Allosteric Inhibition*- Allosteric inhibition is a general mechanism where a molecule binds to a site other than the active site (**allosteric site**), changing the enzyme's conformation and activity.- While noncompetitive and uncompetitive inhibitions are types of allosteric regulation, "noncompetitive inhibition" is the specific and most accurate term for the observed kinetic behavior (decreased $V_{max}$, constant $K_m$).*Uncompetitive inhibition*- This type involves the inhibitor binding only to the **enzyme-substrate complex (ES)**, resulting in a proportional decrease in both **$V_{max}$** and **$K_m$**.- On a Lineweaver-Burk plot, this is shown by parallel lines, highly distinguishing it from noncompetitive inhibition where the lines intersect on the X-axis.

Q: Name the enzyme marked as X in the reaction shown below.

HMG-CoA lyase. ***HMG CoA Lyase*** - The reaction shown converts **HMG-CoA** into **acetoacetate** and **acetyl-CoA**. This cleavage reaction is catalyzed by **HMG-CoA lyase**. - This enzyme is crucial in **ketogenesis**, the metabolic pathway that produces ketone bodies. *HMG COA reductase* - **HMG-CoA reductase** is involved in the synthesis of **cholesterol**, not the breakdown of HMG-CoA into acetoacetate. - It catalyzes the reduction of HMG-CoA to **mevalonate**, which is a precursor for cholesterol and isoprenoid synthesis. *HMG COA oxidase* - This enzyme is not a recognized enzyme in the metabolism of HMG-CoA. - The name suggests an oxidation reaction, which is not what occurs when HMG-CoA is converted to acetoacetate. *HMG COA Trans-peptidase* - This enzyme name does not correspond to any known enzyme involved in HMG-CoA metabolism. - Transpeptidases are typically involved in peptide bond formation or rearrangement, which is unrelated to ketone body synthesis.

Q: Name the plot dealing with kinetics of enzyme inhibition?

Dixon plot. ***Dixon plot*** - The **Dixon plot** is specifically used to determine the type of enzyme inhibition and to calculate the **inhibition constant (Ki)**. - It plots the reciprocal reaction velocity (1/V) against the **inhibitor concentration ([I])** at different substrate concentrations. *Donnan-Gibbs equation plot* - The **Donnan-Gibbs equation** describes the distribution of charged particles across a semi-permeable membrane at equilibrium. - It is irrelevant to enzyme kinetics or inhibition. *Lineweaver-Burk plot* - While the Lineweaver-Burk plot is used in enzyme kinetics to determine **Km** and **Vmax** and analyze inhibition, it plots **1/V against 1/[S]**. - The image provided, showing 1/V against [I], is characteristic of a Dixon plot, not a Lineweaver-Burk plot which plots 1/V against 1/[S]. *Hanes-Woolf plot* - The **Hanes-Woolf plot** is another linearization of the Michaelis-Menten equation, plotting **[S]/V against [S]**. - Like the Lineweaver-Burk plot, it is used for analyzing general enzyme kinetics but not specifically for determining Ki from varying inhibitor concentrations in the manner shown.

Q: The curve below characterizes an allosteric enzyme system. Which of the following is correct about the given curve?

Modifier at allosteric site can affect the catalytic site. ***Modifier at allosteric site can affect the catalytic site*** - Allosteric inhibitors bind to a site other than the active site, causing a **conformational change** in the enzyme that **alters the activity of the catalytic site**, thereby reducing its affinity for the substrate or its maximal velocity, as seen by the lower reaction rate with the modifier. - The graph clearly shows that the presence of an **allosteric modifier** (red curve) leads to a lower initial reaction rate compared to when the substrate is alone (green curve), indicating that the modifier has an effect on the enzyme's function. *Substrate to enzyme is concentration independent* - Enzyme kinetics, including **Michaelis-Menten kinetics** and allosteric regulation, are fundamentally dependent on the **concentrations of substrate and enzyme**. - The graph itself shows reaction rate as a function of **substrate concentration**, demonstrating concentration dependency. *Allosteric modifier can be displaced by adding more substrate to the enzyme* - Allosteric modifiers bind to a **site distinct from the active site** and typically are **not competitively displaced** by increased substrate concentration. - While increasing substrate can eventually saturate the enzyme, it does not typically remove or displace the allosteric modifier from its binding site. *Allosteric modifier does not affect the velocity of reaction* - The graph explicitly demonstrates that the **allosteric modifier reduces the initial reaction rate** at all substrate concentrations shown until saturation. - This reduction in velocity is precisely how allosteric inhibition is characterized, contrasting with the "substrate alone" curve which shows a higher reaction rate.

Question 1

Which of the following is a copper-containing enzyme?

Accepted Answer

Cytochrome oxidase

Answer

Catalase

Answer

Lactate dehydrogenase (LDH)

Answer

None of the above

Question 2

Which enzyme protects the brain from free radical injury?

Accepted Answer

Superoxide dismutase

Answer

Myeloperoxidase

Answer

Monoamine oxidase

Answer

Hydroxylase

Question 3

Which force does NOT act in an enzyme-substrate complex?

Accepted Answer

Van der Waals

Answer

Electrostatic

Answer

Covalent

Answer

Hydrogen

Question 4

Trypsinogen is converted to trypsin by which of the following?

Accepted Answer

Enteropeptidase

Answer

Acidic pH

Answer

Elastase

Answer

None of the above

Question 5

Which of the following is a non-competitive inhibitor of intestinal alkaline phosphatase?

Accepted Answer

L-Phenylalanine

Answer

L-Alanine

Answer

L-Tyrosine

Answer

L-Tryptophan

Question 6

Which of the following enzymes is typically elevated in prostate cancer?

Accepted Answer

Acid phosphatase

Answer

Alpha-fetoprotein

Answer

Lactate dehydrogenase

Answer

Alkaline phosphatase

Question 7

Identify the type of inhibition shown in the graph.

Accepted Answer

Noncompetitive inhibition

Answer

Competitive inhibition

Answer

Allosteric Inhibition

Answer

Uncompetitive inhibition

Question 8

Name the enzyme marked as X in the reaction shown below.

Accepted Answer

HMG-CoA lyase

Answer

HMG-CoA reductase

Answer

HMG-CoA oxidase

Answer

HMG-CoA transpeptidase

Question 9

Name the plot dealing with kinetics of enzyme inhibition?

Accepted Answer

Dixon plot

Answer

Donnan-Gibbs equation plot

Answer

Lineweaver-Burk plot

Answer

Hanes-Woolf plot

Question 10

The curve below characterizes an allosteric enzyme system. Which of the following is correct about the given curve?

Accepted Answer

Modifier at allosteric site can affect the catalytic site

Answer

Substrate to enzyme is concentration independent

Answer

Allosteric modifier can be displaced by adding more substrate to the enzyme

Answer

Allosteric modifier does not affect the velocity of reaction

Enzymes — MCQs

Enzymes — MCQs

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