Enzymes Practice Questions

Q: Coenzymes are which type of organic compounds?

Non-protein. **Explanation:** Enzymes are often composed of two parts: a protein portion called the **apoenzyme** and a non-protein component called a **cofactor**. When these two combine, they form the catalytically active **holoenzyme** (Holoenzyme = Apoenzyme + Cofactor). **Coenzymes** are a specific type of cofactor. They are **non-protein, low-molecular-weight organic compounds** that are heat-stable and loosely associated with the apoenzyme. They function by carrying chemical groups, electrons, or hydrogen atoms between reactions (e.g., NAD+, FAD, TPP). **Analysis of Options:** * **Option C (Correct):** Coenzymes are strictly non-protein organic molecules, often derived from vitamins (e.g., NAD+ from Niacin). * **Option B (Incorrect):** The protein part of an enzyme is the **apoenzyme**. If the entire enzyme is just a protein without needing a cofactor, it is a simple enzyme (e.g., Pepsin). * **Option A (Incorrect):** Lipoproteins are conjugated proteins involved in lipid transport (like LDL or HDL) and do not function as coenzymes. **High-Yield NEET-PG Pearls:** 1. **Prosthetic Groups:** If the non-protein organic cofactor is **tightly or covalently bound** to the apoenzyme, it is called a prosthetic group (e.g., Heme in Cytochrome c). 2. **Metal Ions:** Inorganic cofactors (like $Mg^{2+}$, $Zn^{2+}$) are called **metal ion activators**. Carbonic anhydrase requires $Zn^{2+}$. 3. **Vitamin Precursors:** Most coenzymes are derivatives of B-complex vitamins. * **TPP:** Vitamin B1 (Thiamine) * **FAD/FMN:** Vitamin B2 (Riboflavin) * **NAD/NADP:** Vitamin B3 (Niacin) * **Pyridoxal Phosphate (PLP):** Vitamin B6 (Pyridoxine) — essential for transamination.

Q: Selenocysteine is a component of which enzyme?

Thioredoxin reductase. ### Explanation **Correct Answer: C. Thioredoxin reductase** **Understanding the Concept:** Selenocysteine is often referred to as the **21st amino acid**. Unlike other non-standard amino acids, it is incorporated into proteins during translation via a unique mechanism involving the UGA stop codon and a specific tRNA. It contains **selenium** in place of the sulfur found in cysteine. This substitution lowers the pKa of the side chain, making it a highly efficient catalyst for redox reactions. **Thioredoxin reductase** is a key antioxidant enzyme that reduces thioredoxin using NADPH. It contains a selenocysteine residue at its active site, which is essential for its catalytic activity in maintaining the redox state of the cell and DNA synthesis. **Analysis of Incorrect Options:** * **A & B. NADP reductase / NADPH dehydrogenase:** These enzymes typically utilize FAD, FMN, or iron-sulfur clusters as cofactors for electron transfer, but they do not contain selenocysteine. * **D. Pyruvate dehydrogenase:** This is a multi-enzyme complex that requires five specific cofactors: Thiamine pyrophosphate (B1), Lipoic acid, CoA (B5), FAD (B2), and NAD+ (B3). It does not utilize selenium or selenocysteine. **High-Yield Clinical Pearls for NEET-PG:** * **Other Selenoenzymes:** Apart from Thioredoxin reductase, other critical enzymes containing selenocysteine include **Glutathione peroxidase** (scavenges $H_2O_2$), **Iodothyronine deiodinase** (converts T4 to T3), and **Selenoprotein P**. * **Genetic Coding:** Selenocysteine is encoded by the **UGA codon** (normally a stop codon). The "recoding" requires a **SECIS element** (Selenocysteine Insertion Sequence) in the 3' untranslated region of the mRNA. * **Deficiency:** Selenium deficiency can lead to **Keshan disease** (cardiomyopathy) or **Kashin-Beck disease** (osteoarthropathy).

Q: Enzymatic activity is measured in which unit?

mol/second. **Explanation:** Enzymatic activity refers to the amount of substrate converted into product per unit of time under specified conditions. The standard SI unit for enzymatic activity is the **Katal (kat)**, defined as the amount of enzyme that catalyzes the conversion of **1 mole of substrate per second (mol/s)**. **1. Why "mol/second" is correct:** In biochemistry, the rate of a reaction is a measure of velocity. Since enzymes are biological catalysts, their activity must be expressed as a rate—specifically, the quantity of substance (moles) transformed over a specific time interval (seconds). While the "International Unit" (IU), defined as 1 μmol/min, is frequently used in clinical labs, the SI unit is mol/s. **2. Why other options are incorrect:** * **Options A, B, and C (mg/dl, μg/L, mg/L):** These are units of **concentration** (mass per volume). They measure the static amount of a substance (like glucose or albumin) in a fluid but do not account for the dynamic rate of a chemical reaction. An enzyme's "activity" is distinct from its "mass concentration." **3. High-Yield Clinical Pearls for NEET-PG:** * **Katal (kat):** 1 kat = 1 mol/s. This is a very large unit; hence, clinical values are often in microkatals (µkat). * **International Unit (IU):** 1 IU = 1 μmol/min. (Conversion: 1 IU ≈ 16.67 nkat). * **Specific Activity:** This is the enzyme activity per milligram of total protein (**μmol/min/mg**). It is the gold standard for measuring **enzyme purity** during protein purification. * **Turnover Number ($k_{cat}$):** The number of substrate molecules converted into product per enzyme active site per second.

Q: Which enzyme converts androgen to estrogen?

Aromatase. **Explanation:** The conversion of androgens to estrogens is a critical step in steroidogenesis, catalyzed by the enzyme **Aromatase** (also known as CYP19A1). **1. Why Aromatase is Correct:** Aromatase is a cytochrome P450 enzyme located in the endoplasmic reticulum of various tissues (granulosa cells of the ovary, adipose tissue, and placenta). It catalyzes the **aromatization** of the 'A' ring of androgens. Specifically, it converts **Androstenedione to Estrone** and **Testosterone to Estradiol**. This is the rate-limiting step in estrogen synthesis. **2. Why Other Options are Incorrect:** * **Cholesterol Desmolase (CYP11A1):** This is the "rate-limiting enzyme" for the entire steroidogenesis pathway. It converts Cholesterol to Pregnenolone in the mitochondria. * **11 beta-hydroxylase:** This enzyme is involved in the adrenal corticosteroid pathway, converting 11-deoxycortisol to Cortisol and 11-deoxycorticosterone to Corticosterone. * **21 beta-hydroxylase:** This is the most common enzyme deficient in **Congenital Adrenal Hyperplasia (CAH)**. It converts Progesterone to 11-deoxycorticosterone and 17-OH-progesterone to 11-deoxycortisol. **3. Clinical Pearls for NEET-PG:** * **Aromatase Inhibitors (e.g., Letrozole, Anastrozole):** These are first-line treatments for ER-positive postmenopausal breast cancer. * **PCOS Connection:** In Polycystic Ovary Syndrome, there is often an altered LH/FSH ratio leading to relative aromatase deficiency in granulosa cells, causing androgen buildup. * **Site of Action:** In males, peripheral aromatization of testosterone in adipose tissue is the primary source of estrogen; excessive activity can lead to gynecomastia.

Q: Which amino acids constitute the 'catalytic triad' in the active center of proteases?

Serine, Histidine, Aspartate. ### Explanation The **catalytic triad** is a classic biochemical motif found in the active sites of many hydrolase and transferase enzymes, most notably **Serine Proteases** (e.g., Chymotrypsin, Trypsin, Elastase, and Thrombin). **1. Why Option B is Correct:** The triad consists of **Serine (Ser), Histidine (His), and Aspartate (Asp)**. These three residues work in a coordinated "charge-relay" system: * **Aspartate** uses its carboxylate group to hydrogen-bond with Histidine. * **Histidine** acts as a general base, accepting a proton from the Serine hydroxyl (-OH) group. * **Serine** becomes a highly reactive **alkoxide nucleophile**, which attacks the carbonyl carbon of the peptide bond in the substrate, leading to proteolysis. **2. Why Other Options are Incorrect:** * **Option A:** Lysine and Arginine are basic amino acids but do not participate in the specific charge-relay mechanism required for serine protease activity. * **Option C:** Phenylalanine and Tryptophan are bulky, hydrophobic aromatic amino acids. While they may be involved in substrate binding pockets (like the S1 pocket of chymotrypsin), they do not perform the catalytic chemical transformation. **3. NEET-PG High-Yield Pearls:** * **Mechanism:** The catalytic triad is an example of **Covalent Catalysis** (via the Serine-substrate intermediate). * **Irreversible Inhibition:** Organophosphates (like Malathion or Nerve Gas) inhibit these enzymes by covalently binding to the **Serine** residue of the triad. * **Evolutionary Note:** The Ser-His-Asp triad is a prime example of **convergent evolution**, appearing in both the Chymotrypsin family and the Subtilisin family despite no structural similarity. * **Zymogens:** These proteases are secreted as inactive precursors (e.g., Trypsinogen) to prevent autodigestion of the pancreas.

Q: Alkaline phosphatase is raised in all of the following conditions except:

Prostatic tumor. **Explanation:** The correct answer is **Prostatic tumor** because it is classically associated with an elevation in **Acid Phosphatase (ACP)** and **Prostate-Specific Antigen (PSA)**, rather than Alkaline Phosphatase (ALP). **1. Why Prostatic Tumor is the exception:** Prostatic carcinoma cells secrete Acid Phosphatase. While advanced prostate cancer with bone metastasis (osteoblastic lesions) can eventually cause a secondary rise in ALP, the primary biochemical marker for the tumor itself is ACP. In contrast, ALP is a marker of high bone turnover or biliary obstruction. **2. Analysis of Incorrect Options:** * **Obstructive Jaundice:** ALP is a marker of cholestasis. It is synthesized by the biliary canalicular membranes; when bile flow is obstructed, ALP synthesis increases and leaks into the sinusoidal blood. * **Placental Tumor:** The placenta is one of the four main sources of ALP isoenzymes (Liver, Bone, Placenta, Intestine). Conditions like germ cell tumors (e.g., seminoma) or placental site trophoblastic tumors can secrete the **Regan isoenzyme**, which is a heat-stable placental-like ALP. * **Skeletal Tumor:** ALP is produced by **osteoblasts**. Any condition involving increased bone formation or remodeling, such as osteosarcoma or bone metastases from other cancers, will significantly raise serum ALP levels. **Clinical Pearls for NEET-PG:** * **Isoenzymes of ALP:** Remember the mnemonic **"BLIP"** (Bone, Liver, Intestine, Placenta). * **Heat Stability:** Placental ALP is the most heat-stable, while Bone ALP is the most heat-labile ("Bone burns"). * **Regan Isoenzyme:** A specific biochemical marker for various carcinomas (lung, GI) that mimics placental ALP. * **Acid Phosphatase (ACP):** Specifically inhibited by **tartrate** (Prostatic fraction), whereas the Gaucher’s disease fraction is tartrate-resistant (TRAP).

Q: What is a monoclonal antibody that acts as an enzyme called?

Abzyme. **Explanation:** **Correct Answer: B. Abzyme** An **Abzyme** (a portmanteau of **Ab**antibody and en**zyme**) is a monoclonal antibody with catalytic activity. While traditional antibodies bind to stable antigens, abzymes are designed to bind to the **transition state** of a chemical reaction. According to Pauling’s principle, enzymes catalyze reactions by stabilizing the transition state; therefore, an antibody engineered to fit a transition state analog will function like an enzyme, lowering the activation energy and accelerating the reaction. **Analysis of Incorrect Options:** * **A. Granzyme:** These are serine proteases released by cytoplasmic granules within cytotoxic T cells and Natural Killer (NK) cells. Their primary role is to induce apoptosis in virus-infected or malignant cells, not to act as catalytic antibodies. * **C. Lipozyme:** This is a non-standard term often used in biotechnology to refer to immobilized lipases used for industrial catalysis. It is not a monoclonal antibody. **High-Yield Clinical Pearls for NEET-PG:** * **Transition State Analogs:** Abzymes are produced by immunizing animals with "transition state analogs"—stable molecules that mimic the unstable intermediate of a reaction. * **Potential Applications:** Abzymes are being researched for targeted prodrug activation (converting an inactive drug into an active one at the tumor site) and for neutralizing cocaine toxicity by accelerating its hydrolysis in the bloodstream. * **Ribozyme vs. Abzyme:** Do not confuse the two. A **Ribozyme** is an RNA molecule with catalytic activity (e.g., Peptidyl transferase in ribosomes), whereas an **Abzyme** is a protein (antibody).

Q: Which of the following is NOT a feature of a coenzyme?

It has high molecular weight.. ### Explanation Enzymes often require non-protein components called **cofactors** for catalytic activity. When the cofactor is an organic molecule, it is termed a **coenzyme**. **Why Option A is the Correct Answer:** Coenzymes are **low molecular weight**, non-protein organic molecules. They are dialyzable and much smaller than the apoenzyme (the protein part). Therefore, the statement that they have a "high molecular weight" is incorrect. **Analysis of Other Options:** * **Option B (Combines loosely):** Coenzymes typically associate with the apoenzyme via non-covalent bonds (hydrogen bonds or Van der Waals forces) and can be easily separated by dialysis. If a cofactor is covalently and tightly bound, it is specifically called a **prosthetic group** (e.g., Heme in Cytochrome). * **Option C (Vitamin B complex):** Most coenzymes are derivatives of water-soluble B-complex vitamins. For example, NAD+ is derived from Niacin (B3), FAD from Riboflavin (B2), and TPP from Thiamine (B1). * **Option D (Heat stable):** Unlike the protein part of the enzyme (apoenzyme), which denatures at high temperatures, coenzymes are generally **heat-stable**. **High-Yield NEET-PG Pearls:** 1. **Holoenzyme:** The complete, catalytically active unit consisting of the Apoenzyme (protein) + Cofactor (non-protein). 2. **Metal-activated enzymes:** Require metal ions (like $K^+$, $Mg^{2+}$) that are loosely bound. 3. **Metalloenzymes:** Contain tightly bound metal ions (e.g., Zinc in Carbonic Anhydrase and Alcohol Dehydrogenase; Iron in Cytochromes). 4. **Key Coenzyme Pairs:** * **NAD/NADP:** Niacin (B3) * **FAD/FMN:** Riboflavin (B2) * **Coenzyme A:** Pantothenic acid (B5) * **PLP:** Pyridoxine (B6)

Q: Which of the following is not a component of coenzyme A?

Acetic acid. **Explanation:** Coenzyme A (CoA-SH) is a vital cofactor derived from Vitamin B5, primarily involved in the metabolism of fatty acids and the citric acid cycle. It functions as a carrier of acyl groups, forming high-energy thioester bonds. **Why Acetic Acid is the correct answer:** Acetic acid is **not** a structural component of Coenzyme A itself. Instead, acetic acid (as an acetyl group) binds to the pre-existing Coenzyme A molecule to form **Acetyl-CoA**. Acetic acid is a substrate that CoA carries, rather than a building block of the cofactor's structure. **Analysis of incorrect options:** * **Pantothenic Acid (Vitamin B5):** This is the core vitamin component of CoA. It is linked to beta-mercaptoethylamine and ADP. * **Adenylic Acid (Adenosine 3', 5'-bisphosphate):** CoA contains an adenine ring, a ribose sugar, and phosphate groups, which together constitute the adenylic acid portion of the molecule. * **Sulfhydryl Group (-SH):** This is the functional part of the molecule provided by **beta-mercaptoethylamine**. It is where acyl groups (like acetate) attach; hence CoA is often abbreviated as CoA-SH. **NEET-PG High-Yield Pearls:** 1. **Precursor:** Pantothenic acid (B5) is the essential vitamin precursor. Deficiency is rare but can cause "Burning Foot Syndrome." 2. **Functional Group:** The reactive part of CoA is the terminal **thiol (-SH) group**. 3. **Synthesis:** The rate-limiting step in CoA synthesis is the phosphorylation of pantothenate by **pantothenate kinase**. 4. **Key Derivatives:** Acetyl-CoA (2 carbons), Succinyl-CoA (4 carbons - used in heme synthesis), and HMG-CoA (cholesterol synthesis).

Question 1

Coenzymes are which type of organic compounds?

Accepted Answer

Non-protein

Answer

Lipoprotein

Answer

Proteinaceous

Answer

Any of the above

Question 2

Selenocysteine is a component of which enzyme?

Accepted Answer

Thioredoxin reductase

Answer

NADP reductase

Answer

NADPH dehydrogenase

Answer

Pyruvate dehydrogenase

Question 3

In which of the following enzymatic reactions is magnesium required?

Accepted Answer

Phosphatase

Answer

ATPase

Answer

Dismutase

Answer

Aldolase

Question 4

Enzymatic activity is measured in which unit?

Accepted Answer

mol/second

Answer

mg/dl

Answer

microgram/litre

Answer

mg/litre

Question 5

Which enzyme converts androgen to estrogen?

Accepted Answer

Aromatase

Answer

Cholesterol Desmolase

Answer

11 beta-hydroxylase

Answer

21 beta-hydroxylase

Question 6

Which amino acids constitute the 'catalytic triad' in the active center of proteases?

Accepted Answer

Serine, Histidine, Aspartate

Answer

Serine, Lysine, Arginine

Answer

Histidine, Phenylalanine, Tryptophan

Answer

None of the above

Question 7

Alkaline phosphatase is raised in all of the following conditions except:

Accepted Answer

Prostatic tumor

Answer

Obstructive jaundice

Answer

Placental tumor

Answer

Skeletal tumor

Question 8

What is a monoclonal antibody that acts as an enzyme called?

Accepted Answer

Abzyme

Answer

Granzyme

Answer

Lipozyme

Answer

None of the above

Question 9

Which of the following is NOT a feature of a coenzyme?

Accepted Answer

It has high molecular weight.

Answer

It combines loosely with enzyme molecules.

Answer

It mostly belongs to the vitamin B complex group.

Answer

It is heat stable.

Question 10

Which of the following is not a component of coenzyme A?

Accepted Answer

Acetic acid

Answer

Pantothenic acid

Answer

Adenylic acid

Answer

Sulfhydryl group

Enzymes — MCQs

Enzymes — MCQs

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