Enzyme Inhibitors: Kinetics, design of reversible and covalent inhibitors MCQs With Answer

Introduction: This quiz set focuses on enzyme inhibitors — their kinetic behavior and the principles behind designing both reversible and covalent inhibitors — tailored for M.Pharm (MPC 103T) students. It reinforces core concepts such as Michaelis–Menten kinetics, Lineweaver–Burk interpretations, distinctions among competitive, non‑competitive, uncompetitive and mixed inhibition, and practical methods for measuring inhibition constants (Ki, IC50) and time‑dependent effects. The collection also explores medicinal chemistry strategies: transition‑state analogues, fragment‑based design, warhead selection for covalent targeting, and kinetic parameters for covalent inactivators (Kinact, KI). Use these MCQs to test mechanistic understanding and drug design reasoning essential for advanced medicinal chemistry.

Q1. Which Lineweaver–Burk plot change is characteristic of a classic competitive inhibitor?

• Increase in slope with same y‑intercept
• Increase in y‑intercept with no change in slope
• Parallel lines shifted up
• Decrease in slope and increase in y‑intercept

Correct Answer: Increase in slope with same y‑intercept

Q2. The Cheng–Prusoff equation relates IC50 to Ki for a competitive inhibitor. Which variable must be known in addition to IC50 and Ki?

• Substrate concentration [S]
• Maximum velocity Vmax
• Turnover number kcat
• Enzyme molecular weight

Correct Answer: Substrate concentration [S]

Q3. In mixed (non‑pure) inhibition, what effect does the inhibitor have on Km and Vmax?

• Km can increase or decrease; Vmax decreases
• Km decreases; Vmax increases
• Km unchanged; Vmax decreases
• Km increases; Vmax unchanged

Correct Answer: Km can increase or decrease; Vmax decreases

Q4. For a tight‑binding reversible inhibitor where [I] is comparable to [E], which equation is most appropriate to determine Ki?

• Morrison equation (tight‑binding equation)
• Michaelis–Menten equation
• Lineweaver–Burk linearization
• Cheng–Prusoff equation

Correct Answer: Morrison equation (tight‑binding equation)

Q5. Time‑dependent inhibition (TDI) often indicates what type of interaction between inhibitor and enzyme?

• Covalent or slowly reversible binding resulting in progressive loss of activity
• Instantaneous reversible competitive binding only
• Noncompetitive binding with no change over time
• Uncompetitive binding that is independent of time

Correct Answer: Covalent or slowly reversible binding resulting in progressive loss of activity

Q6. Which parameter pair describes the kinetic efficiency of many covalent (irreversible) inhibitors?

• Kinact and KI
• kcat and Km
• Vmax and Ki
• IC50 and EC50

Correct Answer: Kinact and KI

Q7. A suicide (mechanism‑based) inhibitor typically requires what for inactivation to occur?

• Enzyme catalytic turnover that converts inhibitor to a reactive species
• Presence of exogenous reducing agents
• High inhibitor concentration but no enzymatic activity
• Competitive displacement by substrate

Correct Answer: Enzyme catalytic turnover that converts inhibitor to a reactive species

Q8. Which warhead is commonly used to target active‑site cysteine residues in proteases?

• Acrylamide (Michael acceptor)
• Carboxylic acid
• Ether linkage
• Alkane chain

Correct Answer: Acrylamide (Michael acceptor)

Q9. Boronic acids inhibit serine proteases by what primary interaction?

• Reversible covalent bonding to the active‑site serine hydroxyl forming a tetrahedral adduct
• Formation of a permanent peptide bond with the enzyme
• Metal chelation of a catalytic zinc ion
• Hydrophobic stacking in the S1 pocket

Correct Answer: Reversible covalent bonding to the active‑site serine hydroxyl forming a tetrahedral adduct

Q10. What is the expected effect on IC50 measured for a competitive inhibitor if substrate concentration is increased above Km?

• Measured IC50 will increase
• Measured IC50 will decrease
• IC50 remains unchanged
• IC50 becomes equal to Ki

Correct Answer: Measured IC50 will increase

Q11. Which design strategy most directly aims to mimic the transition state of an enzymatic reaction?

• Transition‑state analogues
• Prodrug approach
• Increasing lipophilicity to enhance permeability
• Adding bulky substituents to reduce clearance

Correct Answer: Transition‑state analogues

Q12. In enzyme kinetics, an uncompetitive inhibitor shows which effect on double reciprocal (Lineweaver–Burk) plot?

• Parallel lines with increased slope and y‑intercept
• Lines intersecting on the x‑axis
• Same slope, increased y‑intercept (parallel lines)
• Converging lines at left of the y‑axis

Correct Answer: Parallel lines with increased slope and y‑intercept

Q13. Which analytical technique is most useful to directly confirm covalent modification of an enzyme by an inhibitor?

• Mass spectrometry of the enzyme‑inhibitor adduct
• UV–Vis absorption at 280 nm only
• Partition coefficient measurement (LogP)
• Light scattering to measure aggregation

Correct Answer: Mass spectrometry of the enzyme‑inhibitor adduct

Q14. For reversible covalent inhibitors, what characteristic differentiates them from irreversible covalent inhibitors?

• The covalent bond can equilibrate and be reversed under physiological conditions
• They never form any covalent bond with the enzyme
• They always react nonspecifically with all proteins
• They are activated only by light

Correct Answer: The covalent bond can equilibrate and be reversed under physiological conditions

Q15. Which property is most important to minimize off‑target reactivity when designing a covalent inhibitor?

• Appropriate electrophile reactivity tuned by local environment and warhead selection
• Maximizing intrinsic electrophile reactivity regardless of targeting
• Adding multiple electrophilic centers to increase binding probability
• Using only positively charged warheads

Correct Answer: Appropriate electrophile reactivity tuned by local environment and warhead selection

Q16. The apparent Ki for a competitive inhibitor can be calculated from the slopes of Lineweaver–Burk plots. Which two conditions are compared?

• Slope in presence of inhibitor versus slope in absence of inhibitor
• Y‑intercept in presence versus absence of inhibitor
• Maximum velocity with two different inhibitors
• Direct IC50 values at two enzyme concentrations

Correct Answer: Slope in presence of inhibitor versus slope in absence of inhibitor

Q17. Fragment‑based drug design (FBDD) for enzyme inhibitors primarily relies on what advantage?

• High ligand efficiency of small fragments that can be elaborated to potent inhibitors
• Fragments always bind with high affinity without optimization
• Fragments are nonpolar so they avoid solubility issues
• FBDD removes the need to validate target binding

Correct Answer: High ligand efficiency of small fragments that can be elaborated to potent inhibitors

Q18. Which experimental observation best indicates mechanism‑based inactivation rather than simple reversible inhibition?

• Irreversible loss of catalytic activity after removal/dialysis of free inhibitor
• Immediate recovery of activity upon dilution of inhibitor
• Competitive displacement by high substrate concentration
• No time‑dependence in inhibition potency

Correct Answer: Irreversible loss of catalytic activity after removal/dialysis of free inhibitor

Q19. A non‑competitive inhibitor shows which effect on Vmax and Km?

• Vmax decreases; Km unchanged
• Vmax unchanged; Km increases
• Both Vmax and Km increase
• Both Vmax and Km decrease equally

Correct Answer: Vmax decreases; Km unchanged

Q20. When optimizing selectivity of a covalent inhibitor for a single cysteine in a protein family, which combined approach is most effective?

• Using structure‑based design to align a low‑reactivity warhead with unique noncatalytic cysteine microenvironment
• Maximizing warhead reactivity and adding bulky hydrophobic groups indiscriminately
• Targeting highly conserved active‑site residues shared across the family
• Relying solely on high dosing to outcompete off‑targets

Correct Answer: Using structure‑based design to align a low‑reactivity warhead with unique noncatalytic cysteine microenvironment

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