Enzymes Practice Questions

Q: Alkaline phosphatase (ALP) is not found in which of the following?

Red blood cells. **Explanation:** Alkaline phosphatase (ALP) is a group of isoenzymes that catalyze the hydrolysis of organic phosphate esters at an alkaline pH. It is primarily a membrane-bound enzyme found in tissues associated with high metabolic activity or transport processes. **Why Red Blood Cells (RBCs) is the correct answer:** RBCs do not contain alkaline phosphatase. Instead, the characteristic phosphatase found in RBCs is **Acid Phosphatase (ACP)**. This distinction is clinically significant; while ALP levels are used to monitor bone and liver diseases, they are unaffected by hemolysis, unlike enzymes like LDH or Potassium which are abundant in RBCs. **Analysis of Incorrect Options:** * **Liver:** ALP is located on the canalicular membranes of hepatocytes. It is a sensitive marker for **cholestasis** (obstructive jaundice), where its synthesis increases in response to bile duct obstruction. * **Bone:** ALP is produced by **osteoblasts** and is essential for bone mineralization. Elevated levels are seen in high bone turnover states like Paget’s disease, rickets, and hyperparathyroidism. * **Prostate:** While the prostate is famously associated with Acid Phosphatase (PSA/PAP), it does contain small amounts of ALP. However, compared to RBCs (which have zero ALP), the prostate is not the correct choice for "not found." **High-Yield Clinical Pearls for NEET-PG:** 1. **Sources of ALP (Mnemonic: BLIP):** **B**one (Osteoblasts), **L**iver (Canalicular), **I**ntestine, and **P**lacenta (Regan Isoenzyme). 2. **Heat Stability:** The placental isoenzyme is the most heat-stable ("**P**lacenta is **P**ersistent"), while the bone isoenzyme is the most heat-labile ("**B**one is **B**urnable"). 3. **Zinc Dependency:** ALP is a **zinc-containing metalloenzyme**; deficiency of zinc can lead to low ALP levels. 4. **Clinical Marker:** ALP is the most sensitive enzyme for detecting space-occupying lesions (SOLs) in the liver.

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: 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 acts as a free radical scavenger?

Catalase. **Explanation:** Free radicals and Reactive Oxygen Species (ROS) are highly reactive molecules that can cause cellular damage. To counter this, the body utilizes **antioxidant enzymes** (scavengers) to neutralize these species. **Why Catalase is Correct:** Catalase is a crucial heme-containing antioxidant enzyme primarily located in **peroxisomes**. It specifically targets **hydrogen peroxide ($H_2O_2$)**, a potent ROS, and converts it into water and molecular oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). By rapidly decomposing $H_2O_2$, it prevents the formation of the highly toxic hydroxyl radical ($\cdot OH$) via the Fenton reaction. **Analysis of Incorrect Options:** * **Peroxidase:** While some specific peroxidases (like Glutathione Peroxidase) act as scavengers, "Peroxidase" as a general class is often involved in synthetic pathways or using $H_2O_2$ to oxidize other substrates rather than primarily acting as a universal scavenger in the same capacity as Catalase or SOD. In the context of standard medical exams, Catalase is the definitive "scavenger" enzyme for $H_2O_2$. * **Dehydrogenase:** These enzymes catalyze oxidation-reduction reactions by transferring hydrogen atoms from a substrate to an electron acceptor (like $NAD^+$ or $FAD$). They are central to metabolic pathways (e.g., Glycolysis, TCA cycle) but do not function as free radical scavengers. **High-Yield Clinical Pearls for NEET-PG:** * **Superoxide Dismutase (SOD):** Converts Superoxide ($\cdot O_2^-$) to $H_2O_2$. It is the "first line" of enzymatic defense. * **Glutathione Peroxidase:** Requires **Selenium** as a cofactor; it neutralizes $H_2O_2$ while converting reduced glutathione (GSH) to oxidized glutathione (GSSG). * **Acatalasia:** A rare genetic deficiency of catalase leading to oral ulcers and gangrene. * **Vitamin Scavengers:** Remember **ACE** (Vitamin A, C, and E) as the primary non-enzymatic antioxidants.

Q: All of the following are cofactors for Branched chain ketoacid dehydrogenase, EXCEPT?

NADP. **Explanation:** The **Branched-chain α-keto acid dehydrogenase (BCKDH)** complex is a multi-enzyme mitochondrial complex responsible for the oxidative decarboxylation of branched-chain amino acids (Leucine, Isoleucine, and Valine). BCKDH belongs to a family of "α-ketoacid dehydrogenases" which also includes **Pyruvate Dehydrogenase (PDH)** and **α-Ketoglutarate Dehydrogenase**. All members of this family require the same **five essential cofactors** (mnemonic: **T**ender **L**oving **C**are **F**or **N**ancy): 1. **T**hiamine pyrophosphate (TPP/Vitamin B1) 2. **L**ipoamide (Lipoic acid) 3. **C**oenzyme A (CoA/Vitamin B5) 4. **F**AD (Riboflavin/Vitamin B2) 5. **N**AD+ (Niacin/Vitamin B3) **Why NADP is the correct answer:** The complex utilizes **NAD+** as the final electron acceptor to produce NADH. **NADP** (Nicotinamide adenine dinucleotide phosphate) is primarily used in reductive biosynthesis (like fatty acid synthesis) and the pentose phosphate pathway, but it is **not** a cofactor for the BCKDH complex. **Analysis of Incorrect Options:** * **Thiamin pyrophosphate (TPP):** Acts as a cofactor for the E1 subunit (decarboxylase). * **Lipoamide:** Acts as a cofactor for the E2 subunit (transacylase), swinging the acyl group between active sites. * **FAD:** Acts as a cofactor for the E3 subunit (dihydrolipoyl dehydrogenase) to re-oxidize lipoamide. **High-Yield Clinical Pearls for NEET-PG:** * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the BCKDH complex. It leads to the accumulation of branched-chain amino acids and their keto-acids, giving urine a characteristic burnt-sugar smell. * **Thiamine-responsive MSUD:** Some variants of MSUD improve with high doses of Thiamine (Vitamin B1), as it stabilizes the E1 subunit. * All three complexes (PDH, α-KGDH, BCKDH) are inhibited by **Arsenite**, which binds to the SH groups of Lipoic acid.

Q: What is true about isoenzymes?

Different structure and similar function. **Explanation:** **Isoenzymes (or Isozymes)** are multiple forms of an enzyme that catalyze the **same chemical reaction** but differ in their physical and chemical properties. 1. **Why Option B is Correct:** Isoenzymes are products of different genes or different alleles. Consequently, they possess **different primary structures** (amino acid sequences). Despite these structural differences, they retain the same active site configuration required to catalyze the **same reaction** (similar function). This allows for fine-tuned metabolic regulation in different tissues. 2. **Why Other Options are Incorrect:** * **Option A:** If the structure were the same, they would be the same enzyme, not isoforms. * **Option C:** By definition, isoenzymes must catalyze the same reaction. Enzymes catalyzing different reactions are simply different enzymes. * **Option D:** Isoenzymes typically have **different kinetic properties** (different Km and Vmax) and different regulatory properties, allowing them to function optimally under the specific metabolic needs of different organs. **High-Yield Clinical Pearls for NEET-PG:** * **Lactate Dehydrogenase (LDH):** A tetramer with 5 isoenzymes. **LDH-1 (H4)** is predominant in the heart, while **LDH-5 (M4)** is found in skeletal muscle and liver. * **Creatine Kinase (CK):** A dimer with 3 isoenzymes. **CK-MB** is a specific marker for Myocardial Infarction; **CK-MM** is found in skeletal muscle; **CK-BB** is found in the brain. * **Hexokinase vs. Glucokinase:** These are functional isoenzymes. Glucokinase (Liver/Pancreas) has a **high Km** (low affinity) for glucose, whereas Hexokinase (Extrahepatic) has a **low Km** (high affinity).

Q: Which of the following is NOT a coenzyme of pyruvate dehydrogenase complex?

Biotin. The **Pyruvate Dehydrogenase (PDH) Complex** is a multi-enzyme system that catalyzes the oxidative decarboxylation of pyruvate into Acetyl-CoA, linking glycolysis to the TCA cycle. ### Why Biotin is the Correct Answer **Biotin (Vitamin B7)** is not a part of the PDH complex. Biotin functions as a coenzyme for **carboxylation** reactions (adding $CO_2$). It is required by **Pyruvate Carboxylase**, which converts pyruvate into oxaloacetate. In contrast, the PDH complex performs **decarboxylation** (removing $CO_2$). ### Why Other Options are Incorrect The PDH complex requires five specific cofactors, often remembered by the mnemonic **"Tender Loving Care For No-one"**: * **T: Thiamine pyrophosphate (TPP/B1)** – Required by E1 (Pyruvate dehydrogenase). * **L: Lipoic acid / Lipoamide** – Required by E2 (Dihydrolipoyl transacetylase). * **C: CoA (B5/Pantothenic acid):** (Option D) Acts as the carrier for the acetyl group, forming Acetyl-CoA. * **F: FAD (B2/Riboflavin):** (Option C) Required by E3 (Dihydrolipoyl dehydrogenase) to accept electrons. * **N: NAD+ (B3/Niacin):** (Option B) The final electron acceptor that produces NADH for the electron transport chain. ### High-Yield 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. * **Thiamine Deficiency:** Leads to Beriberi and Wernicke-Korsakoff syndrome because PDH and Alpha-ketoglutarate dehydrogenase cannot function without TPP. * **Location:** The PDH complex is located in the **mitochondrial matrix**. * **Regulation:** PDH is inhibited by its products (Acetyl-CoA, NADH) and by phosphorylation via PDH kinase.

Question 1

Alkaline phosphatase (ALP) is not found in which of the following?

Accepted Answer

Red blood cells

Answer

Liver

Answer

Bone

Answer

Prostate

Question 2

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 3

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 4

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 5

Which of the following acts as a free radical scavenger?

Accepted Answer

Catalase

Answer

Peroxidase

Answer

Dehydrogenase

Answer

All of the above

Question 6

Fischer's lock and key theory postulates that:

Accepted Answer

The active site of an enzyme exists in a proper conformation to the substrate molecule even in the absence of the substrate.

Answer

The active site of an enzyme undergoes conformational changes to fit the substrate during binding.

Answer

There is absolute specificity in the reaction between an enzyme and a substrate.

Answer

Enzymes possess catalytic power and accelerate reactions by reducing the energy of activation.

Question 7

All of the following are cofactors for Branched chain ketoacid dehydrogenase, EXCEPT?

Accepted Answer

NADP

Answer

FAD

Answer

Lipoamide

Answer

Thiamin pyrophosphate

Question 8

What is true about isoenzymes?

Accepted Answer

Different structure and similar function

Answer

Same structure and similar function

Answer

Catalyze different reactions

Answer

Have the same Km

Question 9

What is allosteric inhibition of an enzyme?

Accepted Answer

Binding of an inhibitor to a site other than the catalytic site, causing enzyme inhibition.

Answer

Binding of an inhibitor to the catalytic site, causing enzyme inhibition.

Answer

Enzyme inhibition by molecules that do not bind to the enzyme.

Answer

Enzyme inactivation through phosphorylation or dephosphorylation.

Question 10

Which of the following is NOT a coenzyme of pyruvate dehydrogenase complex?

Accepted Answer

Biotin

Answer

NAD

Answer

FAD

Answer

CoA

Enzymes — MCQs

Enzymes — MCQs

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