Enzymes — MCQs

Enzymes Indian Medical PG Practice Questions and MCQs

Question 511: Enzyme activity is expressed as?

A. Millimoles/lit
B. Milli gm/lit
C. Mg/dl
D. Micromoles/min (Correct Answer)

Explanation: ***Micromoles/min*** - Enzyme activity is typically measured by the rate at which an enzyme converts its **substrate into product**. - This rate is often expressed as the amount of product formed (e.g., **micromoles**) or substrate consumed per unit of time (e.g., **per minute**). *Millimoles/lit* - This unit expresses **concentration** (moles per liter) rather than a rate of reaction. - While enzyme reactions involve changes in substrate/product concentration, this unit alone does not describe the **activity or catalytic speed** of the enzyme. *Milli gm/lit* - This unit also expresses **concentration by mass** (milligrams per liter), not enzyme activity. - It does not account for the **time-dependent nature** of enzyme catalysis or the molar quantity of reactants/products. *Mg/dl* - This unit represents **concentration by mass** (milligrams per deciliter), commonly used for measuring substances like glucose or cholesterol in blood. - It is not appropriate for expressing the **catalytic rate or activity** of an enzyme.

Question 512: Citrate synthase is inhibited by -

A. Insulin
B. Glucagon
C. ADP
D. ATP (Correct Answer)

Explanation: ***ATP*** - **Citrate synthase**, a key enzyme in the Krebs cycle, is inhibited by **high levels of ATP**, indicating a high energy state in the cell. - This allosteric inhibition helps regulate the metabolic flux through the cycle, slowing it down when energy is abundant. *ADP* - **ADP** typically signifies a low energy state and would generally act as an **activator** rather than an inhibitor for metabolic pathways that produce ATP. - In this context, ADP would promote the activity of enzymes involved in energy generation, including those in the Krebs cycle. *Insulin* - **Insulin** is a hormone that promotes fuel storage and utilization, generally **activating** metabolic pathways rather than directly inhibiting enzymes like citrate synthase. - Its primary role is to regulate blood glucose levels and promote glucose uptake and utilization. *Glucagon* - **Glucagon** is a hormone that mobilizes fuel from storage and is typically associated with **catabolic processes**, often increasing metabolic activity in response to low blood glucose. - It does not directly inhibit citrate synthase; its main actions are on glucoregulation.

Question 513: How many isoenzymes does lactate dehydrogenase (LDH) have?

A. 5, based on H and M polypeptide subunits (Correct Answer)
B. 7, based on H and M polypeptide subunits
C. 9, based on H and M polypeptide subunits
D. 3, based on H and M polypeptide subunits

Explanation: **5, based on H and M polypeptide subunits** - **Lactate dehydrogenase (LDH)** is a tetrameric enzyme, meaning it is composed of four polypeptide subunits. - These subunits can be either **H (heart)** type or **M (muscle)** type, leading to five distinct isoenzymes (**LDH-1, LDH-2, LDH-3, LDH-4, LDH-5**) based on their combinations (HHHH, HHHM, HHMM, HMMM, MMMM). *7, based on H and M polypeptide subunits* - While LDH is composed of two types of subunits, H and M, the possible combinations of these four subunits result in **five distinct isoenzymes**, not seven. - Seven isoenzymes are not a recognized number for LDH. *9, based on H and M polypeptide subunits* - The combination of two types of subunits in a tetrameric structure cannot yield nine unique isoenzymes. - This number is incorrect and not supported by the biochemistry of LDH. *3, based on H and M polypeptide subunits* - Three isoenzymes would imply either fewer than four subunits or a more restricted combination, which is not the case for LDH's tetrameric structure with H and M subunits. - This number is insufficient to account for all possible combinations.

Question 514: Chymotrypsinogen is activated into chymotrypsin by:

A. Trypsin (Correct Answer)
B. Pepsin
C. Renin
D. HCl

Explanation: ***Activation of Chymotrypsinogen by Trypsin*** - **Trypsin** is the primary enzyme responsible for the activation of **chymotrypsinogen** into its active form, **chymotrypsin**, by cleaving a specific peptide bond. - This activation is part of a cascade of proteolytic enzyme activations in the **pancreatic juice**, crucial for protein digestion in the small intestine. *Pepsin* - **Pepsin** is a protease active in the **stomach**, requiring an acidic environment for its activity, and is involved in the initial breakdown of proteins. - It does not play a role in the activation of pancreatic zymogens like chymotrypsinogen; its primary function is protein digestion in the gastric lumen. *Renin* - **Renin** is an enzyme primarily involved in the **renin-angiotensin-aldosterone system** (RAAS), which regulates blood pressure and fluid balance. - Its action involves cleaving **angiotensinogen** to form angiotensin I, and it has no role in the activation of digestive enzymes like chymotrypsinogen. *HCl* - **Hydrochloric acid (HCl)** is produced in the stomach and is essential for providing the acidic environment required for **pepsin's activity** and for denaturing proteins. - While HCl is crucial for digestion, it does not directly activate chymotrypsinogen; this activation is an enzymatic process carried out by another protease.

Question 515: Which of the following is an example of an exopeptidase?

A. Trypsin
B. Chymotrypsin
C. Elastase
D. Carboxypeptidases (Correct Answer)

Explanation: ***Carboxypeptidases*** - **Carboxypeptidases** are enzymes that cleave the **C-terminal** (carboxyl end) amino acid from a polypeptide chain, making them a type of exopeptidase. - They are crucial in protein digestion, releasing individual amino acids from the end of protein chains. *Trypsin* - **Trypsin** is an **endopeptidase** that cleaves peptide bonds within protein chains, specifically at the carboxyl side of **lysine** or **arginine** residues. - It does not cleave amino acids from the ends of polypeptide chains. *Chymotrypsin* - **Chymotrypsin** is an **endopeptidase** that cleaves peptide bonds within a polypeptide chain, primarily at the carboxyl side of **tyrosine**, **tryptophan**, or **phenylalanine**. - Its action is internal to the protein sequence, not at the termini. *Elastase* - **Elastase** is also an **endopeptidase** that cleaves peptide bonds internally, specifically targeting small, uncharged amino acid residues like **alanine**, **glycine**, and **valine**. - Its primary role is to break down elastin, an elastic protein in connective tissues, but it does so by internal cleavage.

Question 516: Which kinetic parameter is primarily associated with enzyme specificity?

A. Both
B. Km
C. Vmax
D. None of the options (Correct Answer)

Explanation: ***None of the options*** - **Enzyme specificity** is primarily determined by the unique three-dimensional **active site structure** of the enzyme, which allows it to bind only to specific substrates through complementary shape and chemical interactions. - This structural complementarity involves steric fit and specific non-covalent interactions (hydrogen bonds, van der Waals forces, electrostatic interactions) between the enzyme and its substrate. - **Neither Km nor Vmax are determinants of enzyme specificity**—they are kinetic parameters that describe enzyme behavior, not structural selectivity. *Km (Michaelis constant)* - Represents the substrate concentration at which the reaction rate is half of Vmax. - Indicates the **affinity** of an enzyme for its substrate (lower Km = higher affinity). - While enzymes may show different Km values for different substrates, **Km reflects binding affinity, not the structural basis of specificity**. *Vmax (Maximum velocity)* - The maximum rate of reaction when the enzyme is saturated with substrate. - Reflects **catalytic efficiency** and the amount of active enzyme present. - Does not relate to the enzyme's ability to discriminate between different substrate molecules. *Both* - Incorrect because neither Km nor Vmax determines which substrates an enzyme can recognize and bind. - Enzyme specificity is a **structural property** of the active site, while Km and Vmax are **kinetic properties** that describe reaction rates.

Question 517: Carbonic anhydrase activity is found in all of the following except?

A. Brain
B. Kidney
C. RBC
D. Plasma (Correct Answer)

Explanation: ***Plasma*** - **Carbonic anhydrase** is an intracellular enzyme that catalyzes the rapid interconversion of carbon dioxide and water to carbonic acid, **bicarbonate**, and protons. - It is notably **absent in plasma** in healthy individuals, as it is primarily found within cells where its function is crucial for pH regulation and CO2 transport. *Brain* - Carbonic anhydrase is found in various brain cells, including **neurons**, **oligodendrocytes**, and **astrocytes**. - It plays a vital role in pH regulation, fluid balance, and the production of cerebrospinal fluid (CSF) within the **central nervous system**. *Kidney* - The kidney is rich in carbonic anhydrase, particularly in the **proximal tubules** and collecting ducts. - It is critical for **bicarbonate reabsorption** and proton excretion, essential processes for maintaining acid-base balance. *RBC* - **Red blood cells (RBCs)** contain a high concentration of carbonic anhydrase (specifically CA-I and CA-II isoforms). - This enzyme facilitates the rapid conversion of CO2 to bicarbonate for transport to the lungs and the reverse reaction for **CO2 exhalation**.

Question 518: Trypsinogen is converted to trypsin by?

A. Combination of 2 molecules of trypsinogen
B. Phosphorylation
C. Addition of alkyl group
D. Removal of specific amino acids from trypsinogen (Correct Answer)

Explanation: ***Removal of specific amino acids from trypsinogen*** - Trypsinogen is an **inactive zymogen** that is activated by the enzymatic cleavage of a **short N-terminal peptide**. - This cleavage event, primarily catalyzed by **enteropeptidase** (or trypsin itself), transforms trypsinogen into active **trypsin**, a process known as **proteolytic activation**. *Combination of 2 molecules of trypsinogen* - The activation of trypsinogen to trypsin is a **unimolecular conformational change** followed by proteolytic cleavage, not a combination reaction between two zymogen molecules. - While trypsin can activate other trypsinogen molecules, the initial activation does not involve the physical combination of two zymogen molecules. *Phosphorylation* - **Phosphorylation** is a common regulatory mechanism in proteins but is not the primary method for activating inactive trypsinogen. - Trypsinogen activation relies on a **proteolytic cleavage event**, rather than the addition of a phosphate group. *Addition of alkyl group* - The addition of an **alkyl group** is not a known mechanism for the physiological activation of trypsinogen. - Enzymatic activation typically involves **hydrolysis of peptide bonds** or other specific post-translational modifications.

Question 519: What is the mechanism by which mercury causes damage?

A. Causes toxicity through various mechanisms
B. Binds to -SH groups of enzymes (Correct Answer)
C. Inhibits electron transport chain
D. Inhibits protein synthesis

Explanation: ***Binds to -SH groups of enzymes*** - Mercury, particularly its inorganic and organic forms, has a high affinity for **sulfhydryl (-SH) groups** found in **cysteine residues** of proteins and enzymes. - This binding disrupts the **tertiary structure** and **catalytic activity** of vital enzymes, leading to widespread cellular dysfunction and toxicity. *Causes toxicity through various mechanisms (not specific to -SH binding)* - While mercury can indeed cause toxicity through various mechanisms, the **most prominent and fundamental mechanism** underpins many of these downstream effects. - This option is too general and does not pinpoint the primary molecular interaction responsible for mercury's widespread cellular damage. *Indirectly inhibits the electron transport chain (ETC) by enzyme disruption* - This statement is partially true in that mercury's enzyme disruption can affect the ETC, but it's an **indirect consequence** rather than the primary mechanism itself. - The direct mechanism involves the initial binding to -SH groups, which then leads to the dysfunction of enzymes, including those involved in the ETC. *Indirectly inhibits protein synthesis by disrupting enzyme function* - Similar to ETC inhibition, mercury's disruption of enzyme function can ultimately impair protein synthesis, but this is an **effect down the causal chain**. - The initial and direct molecular interaction is the binding to sulfhydryl groups of key enzymes involved in various cellular processes, including protein synthesis.

Question 520: What type of enzyme is hexokinase?

A. Ligase
B. Transferase (Correct Answer)
C. Oxidoreductase
D. Reductase

Explanation: ***Transferase*** - Hexokinase catalyzes the transfer of a **phosphate group** from **ATP** to glucose, forming glucose-6-phosphate. - Enzymes that catalyze the transfer of functional groups from one molecule to another are classified as **transferases**. *Ligase* - **Ligases** are enzymes that catalyze the joining of two large molecules by forming a new chemical bond, usually accompanied by the hydrolysis of a small pendant chemical group on one of the larger molecules or the less-stable of the two products. - This activity usually involves reactions like **DNA ligation**, not phosphate group transfer to a sugar. *Oxidoreductase* - **Oxidoreductases** catalyze **oxidation-reduction reactions**, involving the transfer of electrons from one molecule to another. - Hexokinase does not perform redox reactions; it transfers a phosphate group. *Reductase* - **Reductases** are a specific type of **oxidoreductase** that catalyze reactions where a molecule is reduced (gains electrons). - This is a subset of oxidation-reduction chemistry and is not the function of hexokinase.

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Enzyme Classification and Nomenclature

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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

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Enzyme Diagnostic Applications

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Enzyme Therapy and Inhibitors as Drugs

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Enzymes — MCQs

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