Enzyme Inhibition: Comprehensive Quiz Bee Reviewer
Overview
Enzyme inhibition refers to processes in which chemicals (inhibitors) decrease or abolish
enzyme activity, reducing the rate of enzyme-catalyzed reactions. Inhibitors can be small
inorganic ions or organic substances, and are classified as reversible or irreversible based on
bonding types.
Key Concepts and Definitions
Definitions
• Enzyme: Protein catalyst that accelerates biochemical reactions.
• Inhibitor: Substance which decreases enzyme activity; sometimes called a negative
modifier.
• Enzyme Inhibition: The prevention or reduction of normal enzyme function by
inhibitors.
Classification of Enzyme Inhibition
Example
Type Description Reversibility
Bonds
Reversible Non-covalent Inhibitor can dissociate from enzyme Yes
Inhibitor forms permanent bond, inactivating
Irreversible Covalent No
enzyme
Major Types of Enzyme Inhibition
1. Competitive Inhibition
Key Facts:
• Inhibitor resembles substrate (structural analogue) and competes for active site.
• Effect is reversible by increasing substrate concentration.
• Vmax: Unchanged
• Km: Increased (lower affinity for substrate)
• Example drugs: Sulfamethoxazole, Trimethoprim, Methotrexate, Statins, Ethanol (as
antidote for methanol poisoning)
Mnemonic:
"Compete for the KEY (Km increases, Enzyme, You can outcompete with more
substrate)"
Quick Tips:
• Think "musical chairs": more substrate or inhibitor, more competition for the seat (active
site).
• Used clinically for antibiotic action, chemotherapy, and poisoning antidotes.
2. Non-Competitive Inhibition
Key Facts:
• Inhibitor binds to a site other than the active site (allosteric).
• Substrate can still bind, but enzyme can't catalyze the reaction.
• Vmax: Decreased
• Km: Unchanged
• Increasing substrate does not reverse inhibition.
• Clinical example: EDTA chelation, cyanide poisoning (inhibits cytochrome c oxidase).
Mnemonic:
"Non-Compete, Vmax Retreat (Non-competitive = Vmax ↓, Km the same)"
Strategic Tip:
• Focus on the reduction of functional enzymes, not substrate-affinity changes.
3. Uncompetitive Inhibition
Key Facts:
• Inhibitor binds only to enzyme-substrate (ES) complex.
• Vmax: Decreased
• Km: Decreased (apparent affinity increases)
• Not overcome by adding substrate.
• Example drug: Mycophenolic acid (used in transplant medicine).
Mnemonic:
"Unique Uncompetitive: ES only, Both values Down!"
Quick Note:
• Rare, but crucial in specific drug actions.
4. Irreversible Inhibition
Key Facts:
• Inhibitor binds covalently, permanently deactivating enzyme.
• Only solution is synthesis of new enzyme.
• Vmax: Decreased (permanent loss)
• Example: Penicillin, organophosphates (nerve agents), aspirin, disulfiram, 5-fluorouracil.
Mnemonic:
"Irreversible = Inactive Forever"
Reference Note:
• Important in drug targeting (aspirin for COX), antidotes, and herbicide mechanisms.
5. Allosteric Regulation (Not Inhibition but Modulation)
Key Facts:
• Effectors bind at sites other than the active site.
• Can accelerate (positive effector) or inhibit (negative effector) enzyme activity.
• Alters Km, Vmax or both.
• Common in rate-limiting enzymes (e.g., phosphofructokinase in glycolysis).
Mnemonic:
"Allos-teric = Alternate Site Effects"
Strategic Tip:
• Allosteric enzymes show sigmoid (S-shaped) activity curves, unlike standard Michaelis-
Menten kinetics.
Kinetic Effects: Quick Reference Table
Effect on Effect on Reversed by
Inhibition Type Example Drugs
Vmax Km ↑[substrate]?
Sulfa drugs,
Competitive No change Increases Yes
Trimethoprim, Statins
Non-
Decreases No change No EDTA, Cyanide
Competitive
Effect on Effect on Reversed by
Inhibition Type Example Drugs
Vmax Km ↑[substrate]?
Uncompetitive Decreases Decreases No Mycophenolic acid
Penicillin, Aspirin,
Irreversible Decreases No change No
Disulfiram
Allosteric Varies Varies N/A ATP, AMP (regulators)
Clinical Examples and Mechanisms
• Sulfonamides: Competitive, block PABA in folate synthesis, antibacterial.
• Methotrexate: Competitive, inhibits dihydrofolate reductase, anticancer.
• Ethanol: Competitive, antidote for methanol poisoning via alcohol dehydrogenase
competition.
• Statins: Competitive, inhibit HMG-CoA reductase, lower cholesterol.
• EDTA: Non-competitive, chelates metal ions, anticoagulant.
• Cyanide: Non-competitive, blocks electron transport, fatal inhibition.
• Penicillin: Irreversible, inhibits cell wall synthesis in bacteria.
• Aspirin: Irreversible, inhibits cyclooxygenase, anti-inflammatory.
• Disulfiram: Irreversible, inhibits acetaldehyde dehydrogenase, used to treat alcoholism.
• Mycophenolic acid: Uncompetitive, inhibits inosine monophosphate dehydrogenase,
used in transplant rejection prevention.
Quiz Bee-Style Questions
Multiple Choice
1. Which type of inhibition can be overcome by adding more substrate?
o a) Non-competitive
o b) Competitive
o c) Irreversible
o d) Uncompetitive
2. Which clinical drug is an irreversible inhibitor of cyclooxygenase?
o a) Statin
o b) Aspirin
o c) Sulfonamide
o d) Disulfiram
3. Non-competitive inhibitors affect which kinetic parameter?
o a) Only Km
o b) Vmax only
o c) Both
o d) Neither
True or False
1. "Vmax decreases in uncompetitive and non-competitive inhibition." (True)
2. "Competitive inhibitors bind to an allosteric site." (False)
3. "Sulfonamides act as competitive inhibitors in bacteria." (True)
Short Answer
1. Name a clinical use of ethanol as a competitive inhibitor.
Antidote for methanol poisoning.
2. Which type of inhibitor forms covalent bonds with enzymes?
Irreversible inhibitors.
3. Name an enzyme regulated via allosteric inhibition in glycolysis.
Phosphofructokinase (PFK-1).
Mnemonics and Strategies
• Competitive: "Competing for the Chair"—More substrate, more chance to sit (active
site); can out-compete inhibitor.
• Non-Competitive: "No Competition—Knocked Out"—Enzyme can't work even if
substrate is present.
• Uncompetitive: "Unique and Down"—Both Km and Vmax decrease, unique to ES
complex.
• Irreversible: "Inactivated Irrevocably"—Permanent, only new enzymes can restore
function.
Quick Tips
• Always link inhibitor type to clinical drug examples.
• Remember kinetic patterns: which parameter(s) change for each inhibition type.
• Focus on whether added substrate rescues enzyme activity—easy way to differentiate
inhibition types.
• For non-competitive and uncompetitive inhibition, think site of binding (not the active
site).
Final Notes
• Draw and review graphs (Michaelis-Menten and Lineweaver-Burk plots) for each
inhibition type.
• Practice associating each clinical drug with its mechanism of enzyme inhibition for faster
recall.
• Use memory aids for differentiating inhibition types and their kinetic characteristics.
This reviewer covers the essential points on enzyme inhibition, integrating mnemonics, example
drugs, and strategic tips for quick recall and efficient quiz bee preparation.
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