Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 391: All digestive enzymes are?

A. Ligases
B. Transferases
C. Hydrolases (Correct Answer)
D. Lyases

Explanation: **Explanation:** **1. Why Hydrolases is correct:** Digestive enzymes belong to the **Hydrolase** class (Class 3) of enzymes. Their primary function is to catalyze the cleavage of chemical bonds (C-O, C-N, C-C) by the **addition of a water molecule** (hydrolysis). In the gastrointestinal tract, complex macromolecules are broken down into simpler units: * **Proteases/Peptidases** (e.g., Pepsin, Trypsin) hydrolyze peptide bonds. * **Glycosidases** (e.g., Salivary Amylase, Lactase) hydrolyze glycosidic bonds. * **Lipases** hydrolyze ester bonds in triglycerides. **2. Why other options are incorrect:** * **Ligases (Class 6):** These enzymes join two molecules together, usually coupled with the hydrolysis of ATP (e.g., Pyruvate carboxylase). Digestion is a catabolic (breakdown) process, not synthetic. * **Transferases (Class 2):** These transfer functional groups (like methyl or phosphate groups) from one substrate to another (e.g., Hexokinase). They do not break down food into absorbable units. * **Lyases (Class 4):** These catalyze the breakage of bonds by means other than hydrolysis or oxidation, often forming double bonds or adding groups to double bonds (e.g., Carbonic anhydrase, Fumarase). **3. NEET-PG High-Yield Pearls:** * **Classification Tip:** Remember the mnemonic **OTH LIL** (Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, Ligases) for the IUBMB classification. * **Zymogens:** Most digestive hydrolases are secreted as inactive precursors (zymogens) to prevent autolysis of the secretory organs (e.g., Trypsinogen). * **Location:** Most digestive enzymes are extracellular enzymes, unlike most other enzyme classes which function intracellularly.

Question 392: All of the following enzymes are regulated by calcium or calmodulin, except:

A. Adenylate cyclase
B. Glycogen synthase
C. Guanylyl cyclase
D. Hexokinase (Correct Answer)

Explanation: **Explanation:** The regulation of metabolic pathways often involves **Calcium ($Ca^{2+}$)** acting as a second messenger, either directly or by binding to the calcium-binding protein **Calmodulin**. This mechanism allows the cell to synchronize metabolic activity with physiological processes like muscle contraction or nerve signaling. **Why Hexokinase is the correct answer:** Hexokinase (the first enzyme of glycolysis) is primarily regulated by **allosteric inhibition by its product, Glucose-6-Phosphate**. In the liver, its isoenzyme Glucokinase is regulated by the Glucokinase Regulatory Protein (GKRP). Neither form is directly regulated by Calcium or Calmodulin. It serves as a "constitutive" or product-inhibited enzyme rather than one responsive to calcium signaling. **Analysis of incorrect options:** * **Adenylate cyclase:** Certain isoforms (especially in the brain) are stimulated by the $Ca^{2+}$-Calmodulin complex, linking neurotransmitter activity to cAMP production. * **Glycogen synthase:** Calcium-dependent kinases (like Calmodulin-dependent protein kinase) can phosphorylate Glycogen Synthase, converting it to its inactive (*b*) form. This ensures that when $Ca^{2+}$ levels rise (triggering contraction/energy need), glycogen synthesis is inhibited. * **Guanylyl cyclase:** Membrane-bound and soluble forms of Guanylyl cyclase are modulated by calcium-sensitive proteins (like GCAPs), crucial in visual phototransduction and smooth muscle relaxation. **High-Yield Clinical Pearls for NEET-PG:** * **Phosphorylase Kinase:** This is the classic "dual regulation" enzyme. It is fully activated only when it is both phosphorylated (by PKA) and bound to $Ca^{2+}$ (via its delta subunit, which is Calmodulin). * **TCA Cycle:** Three enzymes are stimulated by $Ca^{2+}$: Pyruvate Dehydrogenase, Isocitrate Dehydrogenase, and $\alpha$-Ketoglutarate Dehydrogenase. * **Calmodulin** contains four "EF-hand" motifs, each capable of binding one $Ca^{2+}$ ion.

Question 393: What is true about allosteric enzymes?

A. They are single-unit enzymes.
B. They follow Michaelis-Menten saturation kinetics.
C. Allosteric activity is most prominent in the active site of the enzyme.
D. Hill's equation is used to study the kinetics of allosteric enzymes. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** Allosteric enzymes do not follow standard Michaelis-Menten kinetics; instead, they exhibit **cooperativity**. This means the binding of a substrate to one subunit increases the affinity of other subunits for the substrate. The **Hill equation** is used to quantify this degree of cooperativity. The **Hill coefficient ($n$H)** indicates the nature of binding: if $n$H > 1, it shows positive cooperativity (common in allosteric enzymes like Phosphofructokinase-1). **2. Why Other Options are Incorrect:** * **Option A:** Allosteric enzymes are typically **multi-subunit (oligomeric)** proteins. Their complex regulatory nature requires multiple polypeptide chains to allow for conformational changes between subunits. * **Option B:** They follow **Sigmoidal (S-shaped)** saturation kinetics, not the Hyperbolic curve characteristic of Michaelis-Menten kinetics. This sigmoidal shape allows for a "threshold effect," where small changes in substrate concentration lead to large changes in velocity. * **Option C:** Allosteric activity occurs at the **allosteric site (regulatory site)**, which is physically distinct from the active (catalytic) site. Effectors bind here to induce conformational changes that either increase or decrease the enzyme's affinity for the substrate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Steps:** Most allosteric enzymes catalyze the "committed step" or rate-limiting step of a metabolic pathway (e.g., **PFK-1** in glycolysis). * **K-series vs. V-series:** Allosteric inhibitors can either increase the $K_{0.5}$ (apparent $K_m$) or decrease the $V_{max}$. * **Feedback Inhibition:** This is a classic example of allosteric regulation where the end-product of a pathway inhibits the first committed enzyme. * **Example to Remember:** **Aspartate Transcarbamoylase (ATCase)** is the classic model for studying allosteric regulation.

Question 394: After adding Drug A to an enzyme-substrate reaction, the Km remains the same and Vmax decreases. Drug A is a:

A. Competitive inhibitor
B. Non-competitive inhibitor (Correct Answer)
C. Agonist
D. None of the above

Explanation: ### Explanation The correct answer is **Non-competitive inhibitor**. **1. Why Non-competitive inhibition is correct:** In non-competitive inhibition, the inhibitor binds to an **allosteric site** (a site other than the active site) on both the free enzyme and the enzyme-substrate (ES) complex. * **Effect on Vmax:** Because the inhibitor effectively "takes the enzyme out of commission" regardless of how much substrate is present, the maximum velocity (**Vmax**) of the reaction **decreases**. * **Effect on Km:** Since the inhibitor does not compete for the active site, the affinity of the remaining functional enzymes for the substrate remains unchanged. Therefore, the **Km remains the same**. **2. Why other options are incorrect:** * **Competitive inhibitor:** These drugs compete with the substrate for the **active site**. Increasing substrate concentration can overcome this inhibition. Thus, **Vmax remains constant**, but **Km increases** (lower affinity). * **Agonist:** This is a pharmacological term for a drug that binds to a receptor and activates it to produce a biological response. It is not a type of enzyme inhibition. * **Uncompetitive inhibitor (High-yield contrast):** Here, the inhibitor binds only to the ES complex. This results in a **decrease in both Vmax and Km**. **3. NEET-PG High-Yield Pearls:** * **Lineweaver-Burk Plot:** In non-competitive inhibition, the lines intersect on the **negative X-axis** ($-1/Km$ is constant). * **Classic Examples:** * **Non-competitive:** Cyanide (inhibits Cytochrome Oxidase), Heavy metals (Lead, Mercury). * **Competitive:** Statins (HMG-CoA Reductase), Methotrexate (Dihydrofolate Reductase). * **Memory Aid:** **C**ompetitive = **C**hanges Km; **N**on-competitive = **N**o change in Km.

Question 395: Lesch-Nyhan syndrome is due to a deficiency of which enzyme?

A. Hypoxanthine-guanine phosphoribosyltransferase (HGPT) deficiency (Correct Answer)
B. Phosphoribosyl pyrophosphate (PRPP) synthetase deficiency
C. Adenosine deaminase deficiency
D. Xanthine oxidase deficiency

Explanation: **Explanation:** **Lesch-Nyhan Syndrome (LNS)** is an X-linked recessive disorder characterized by a complete deficiency of the enzyme **Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. This enzyme is crucial for the **Purine Salvage Pathway**, where it converts hypoxanthine to IMP and guanine to GMP. 1. **Why Option A is correct:** In the absence of HGPRT, purine bases cannot be salvaged. This leads to two major consequences: an accumulation of PRPP (which stimulates *de novo* purine synthesis) and an overproduction of uric acid. The resulting hyperuricemia causes gout, while the metabolic imbalance in the basal ganglia leads to the characteristic neurological symptoms. 2. **Why other options are incorrect:** * **Option B:** PRPP Synthetase *overactivity* (not deficiency) leads to gout by increasing purine production. * **Option C:** Adenosine Deaminase (ADA) deficiency leads to **Severe Combined Immunodeficiency (SCID)** due to the toxic accumulation of dATP in lymphocytes. * **Option D:** Xanthine Oxidase deficiency leads to **Xanthinuria** and low serum uric acid levels; it is the target of the drug Allopurinol. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Hyperuricemia (orange sand in diapers/stones), Intellectual disability, and **Self-mutilation** (biting lips/fingers). * **Inheritance:** X-linked recessive (affects males). * **Metabolic Hallmark:** Increased *de novo* purine synthesis and increased PRPP levels. * **Treatment:** Allopurinol (manages uric acid but does not reverse neurological symptoms).

Question 396: Glycogen phosphorylase requires which of the following as a cofactor?

A. Thiamine pyrophosphate
B. Pyridoxal phosphate (Correct Answer)
C. Citrate
D. FAD

Explanation: **Explanation:** **Glycogen phosphorylase** is the rate-limiting enzyme of glycogenolysis, responsible for cleaving $\alpha$-1,4-glycosidic bonds to release glucose-1-phosphate. It requires **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6, as an essential cofactor. Unlike most PLP-dependent enzymes (which typically involve amino acid metabolism like transamination), glycogen phosphorylase utilizes the **phosphate group** of PLP as a general acid-base catalyst. This phosphate group promotes the phosphorolysis of the glycosidic bond. Interestingly, the aldehyde group of PLP—usually the reactive site in other enzymes—is covalently linked to a lysine residue in glycogen phosphorylase via a Schiff base but does not participate directly in the catalysis. **Analysis of Incorrect Options:** * **A. Thiamine pyrophosphate (TPP):** A derivative of Vitamin B1, TPP is a cofactor for oxidative decarboxylation reactions (e.g., Pyruvate Dehydrogenase, $\alpha$-ketoglutarate dehydrogenase) and the Transketolase enzyme in the HMP shunt. * **C. Citrate:** This is an intermediate of the TCA cycle and acts as an allosteric regulator (inhibitor of PFK-1 and activator of Acetyl-CoA Carboxylase), not a cofactor for phosphorylase. * **D. FAD:** A derivative of Vitamin B2 (Riboflavin), FAD acts as an electron carrier in redox reactions (e.g., Succinate dehydrogenase). **High-Yield Clinical Pearls for NEET-PG:** * **Muscle vs. Liver:** Glycogen phosphorylase is deficient in **McArdle Disease (Type V GSD)** in muscles and **Hers Disease (Type VI GSD)** in the liver. * **Unique Role:** Over 80% of the body's Vitamin B6 is stored in skeletal muscle, primarily bound to glycogen phosphorylase. * **Regulation:** The enzyme is activated by phosphorylation (via Phosphorylase Kinase) and allosterically by **AMP** in the muscle.

Question 397: What is the other name for AST?

A. SGOT (Correct Answer)
B. Alkaline phosphatase
C. Acid phosphatase
D. SGPT

Explanation: **Explanation:** **AST (Aspartate Aminotransferase)** is a key enzyme involved in amino acid metabolism. Its alternative name is **SGOT (Serum Glutamic Oxaloacetic Transaminase)**. This name reflects the biochemical reaction it catalyzes: the transfer of an amino group from aspartate to alpha-ketoglutarate, resulting in the formation of **Glutamate** and **Oxaloacetate**. **Analysis of Options:** * **SGOT (Correct):** As mentioned, AST and SGOT are synonymous. AST is primarily found in the heart, liver, and skeletal muscle. * **SGPT (Incorrect):** This is the alternative name for **ALT (Alanine Aminotransferase)**. ALT catalyzes the transfer of an amino group from alanine to alpha-ketoglutarate, forming Glutamate and Pyruvate. * **Alkaline Phosphatase (ALP) (Incorrect):** This enzyme is a marker for cholestasis (bile duct obstruction) and bone turnover. It is not a transaminase. * **Acid Phosphatase (ACP) (Incorrect):** Historically used as a marker for prostate cancer (specifically the prostatic isoenzyme), it is also found in lysosomes and erythrocytes. **High-Yield Clinical Pearls for NEET-PG:** 1. **Tissue Specificity:** ALT (SGPT) is more **liver-specific** than AST. AST is also found in cardiac and skeletal muscles. 2. **De Ritis Ratio (AST/ALT):** * A ratio **> 2:1** is highly suggestive of **Alcoholic Liver Disease** (Alcohol suppresses pyridoxal phosphate, which ALT requires more than AST). * A ratio **< 1** is typically seen in acute viral hepatitis. 3. **Myocardial Infarction:** AST was historically used as a cardiac marker (rising 6–8 hours after MI), though it has been replaced by Troponins. 4. **Cofactor:** All transaminases require **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor.

Question 398: Which of the following enzymes exhibits absolute specificity for its substrate?

A. Lactate dehydrogenase
B. L-amino acid oxidase
C. Urease (Correct Answer)
D. Hexokinase

Explanation: ### Explanation **Concept: Enzyme Specificity** Enzyme specificity refers to the ability of an enzyme to choose a specific substrate from a group of similar chemical compounds. **Absolute specificity** is the highest level of selectivity, where an enzyme acts on **only one specific substrate** to catalyze a single reaction. **Why Urease is Correct:** **Urease** is the classic example of absolute specificity. It acts exclusively on **urea** to produce ammonia and carbon dioxide. It will not catalyze the hydrolysis of any other substituted ureas or related compounds, regardless of structural similarity. **Analysis of Incorrect Options:** * **Lactate Dehydrogenase (LDH):** Exhibits **optical/stereo-specificity**. It acts specifically on the L-isomer of lactate but not the D-isomer. However, it is not "absolute" in the strictest sense as it can interact with other α-keto acids. * **L-amino acid oxidase:** Exhibits **group specificity**. It acts on a group of structurally related compounds—specifically, various L-amino acids—rather than a single molecule. * **Hexokinase:** Exhibits **group specificity**. It catalyzes the phosphorylation of several six-carbon sugars (hexoses), including glucose, fructose, and mannose. (Note: *Glucokinase* is more specific to glucose but still lacks the absolute exclusivity of Urease). **NEET-PG High-Yield Pearls:** 1. **Bond Specificity:** Enzymes like **pepsin** or **trypsin** act on specific types of chemical bonds (e.g., peptide bonds) regardless of the surrounding structure. 2. **Stereospecificity:** Most enzymes in the human body are specific to **L-amino acids** and **D-sugars**. 3. **Clinical Correlation:** Urease is clinically significant in the **Urea Breath Test** used to diagnose *H. pylori* infections, as the bacteria produce urease to neutralize gastric acid.

Question 399: Which of the following biomolecules exhibits catalytic activity?

A. Phospholipids
B. DNA
C. RNA (Correct Answer)
D. Heteropolysaccharides

Explanation: **Explanation:** The correct answer is **RNA**. While the vast majority of biological catalysts are proteins (enzymes), certain RNA molecules possess the ability to catalyze biochemical reactions. These catalytic RNA molecules are known as **Ribozymes**. 1. **Why RNA is correct:** RNA can fold into complex three-dimensional structures, creating active sites similar to protein enzymes. The most clinically significant example is the **23S rRNA** (in prokaryotes) or **28S rRNA** (in eukaryotes) of the large ribosomal subunit, which acts as a **peptidyl transferase** to catalyze peptide bond formation during translation. Other examples include RNase P and self-splicing introns. 2. **Why other options are incorrect:** * **Phospholipids:** These are structural components of cell membranes and signaling molecules (e.g., Phosphatidylinositol); they do not possess intrinsic catalytic activity. * **DNA:** DNA serves as the stable repository of genetic information. While "Deoxyribozymes" have been created synthetically in labs, DNA does not naturally exhibit catalytic activity in vivo due to its stable double-helical structure and lack of a 2'-OH group. * **Heteropolysaccharides:** These are complex carbohydrates (like Heparin or Hyaluronic acid) used for structural support or anticoagulation; they lack the structural complexity required for catalysis. **NEET-PG High-Yield Pearls:** * **Ribozyme Examples:** Peptidyl transferase (the most important), RNase P (cleaves tRNA precursors), and Mitophagy-related RNA. * **Abzymes:** These are antibodies with catalytic activity (often seen in autoimmune diseases). * **RNA World Hypothesis:** Suggests that early life relied on RNA for both genetic storage and catalysis before the evolution of DNA and proteins. * **Cofactor vs. Coenzyme:** Remember that many enzymes require non-protein components (like B-complex vitamins) to function, but the ribozyme is unique because its "apoenzyme" part is RNA, not protein.

Question 400: Which of the following methods is used for regulating the quantity of an enzyme?

A. Phosphorylation
B. Induction (Correct Answer)
C. Acetylation
D. Glycosylation

Explanation: **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.

Practice by Chapter

Enzyme Classification and Nomenclature

Practice Questions

Enzyme Kinetics and Michaelis-Menten Equation

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Enzyme Inhibition: Competitive and Non-competitive

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Allosteric Regulation

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Coenzymes and Cofactors

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Isoenzymes and Clinical Significance

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Enzyme Regulation: Covalent Modification

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Enzyme Regulation: Zymogen Activation

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Enzyme Induction and Repression

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Ribozymes and Catalytic RNA

Practice Questions

Enzyme Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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