7.5 Enzyme Inhibition

Short Introduction Sometimes enzymes are not able to perform their role due to the presence and interference of some chemicals at the reaction site. This phenomenon is called enzyme inhibition. The chemical substance which reacts with the enzyme in place of the substrate but does not convert to products, thus inhibiting the enzyme action, is called an inhibitor.

Main Idea

Enzyme inhibition occurs when a chemical substance (inhibitor) interferes with enzyme activity. Inhibitors can block or damage the active site temporarily or permanently. There are two major types of enzyme inhibition: competitive inhibition (inhibitor resembles the substrate and binds to the active site) and non-competitive inhibition (inhibitor binds to a different site, altering the enzyme's shape and damaging the active site). Inhibitors can be internal (e.g., accumulated products) or external (e.g., poisons, drugs, antibiotics).

Core Concept

Definition of Enzyme Inhibition Sometimes enzymes are not able to perform their role due to the presence and interference of some chemicals at the reaction site. This phenomenon is called enzyme inhibition. The chemical substance which reacts with the enzyme in place of the substrate but does not convert to products, thus inhibiting the enzyme action, is called an inhibitor. Inhibitors may block or damage the active site temporarily or permanently. Sources of Inhibitors Internal: Enzyme activity may be temporarily inhibited by accumulated products within the cell to regulate the rate of reaction. External: External factors responsible for enzyme inhibition include poisons, cyanide, antibiotics, some drugs, or accumulated products. Types of Enzyme Inhibition 1. Competitive Inhibition: If enzymes are inhibited due to the presence of molecules which occupy the active site due to their similarity in structure and shape to the substrate, it is called competitive inhibition. generated by DeepSeek to PDF

Such inhibitors are neither converted to products nor allow the actual substrate to bind, hence the reaction does not proceed. Competitive inhibition is temporary and can be reversed if the concentration of the actual substrate is increased than the concentration of inhibitor molecules. 2. Non-Competitive Inhibition: The structure and shape of these inhibitors do not resemble the substrate molecules, so they do not occupy the active site. They may attach to the enzyme surface at a point other than the active site, called the allosteric site. They alter the globular shape of the enzyme and damage the active site of the enzyme. Thus, the actual substrate will not be able to bind to the active site, and the reaction stops.

Detailed Explanation

Enzyme inhibition is a crucial regulatory mechanism in cells and is also the basis for many drugs and toxins. 1. Competitive Inhibition: In this type of inhibition, the inhibitor molecule is structurally similar to the substrate. It can bind to the active site of the enzyme, competing with the actual substrate for the same binding site. When the inhibitor is bound, the substrate cannot bind, and no reaction occurs. However, because the inhibitor is not converted to a product, it eventually dissociates. If the concentration of the actual substrate is increased, it can outcompete the inhibitor for the active site, reversing the inhibition. This is why competitive inhibition is reversible. An example is the drug methotrexate, which competitively inhibits the enzyme dihydrofolate reductase. 2. Non-Competitive Inhibition: In this type of inhibition, the inhibitor does not resemble the substrate and does not bind to the active site. Instead, it binds to a different site on the enzyme, called the allosteric site. This binding causes a conformational change in the enzyme's structure, which alters the shape of the active site. As a result, the substrate can no longer bind to the active site, and the reaction is inhibited. This type of inhibition is often irreversible or difficult to reverse because the enzyme's shape is changed. Examples include cyanide, which binds to the iron in cytochrome c oxidase (a non-competitive inhibitor), and heavy metal ions like mercury and lead. 3. Regulation by Inhibitors: Enzyme inhibition is not always harmful. Cells use inhibitors to regulate metabolic pathways. For example, the end product of a metabolic pathway can inhibit an enzyme early in the pathway (feedback inhibition), preventing the overproduction of the product. External inhibitors like antibiotics can be used to treat bacterial infections by inhibiting bacterial enzymes. 4. Temporary vs. Permanent: Inhibitors may block or damage the active site temporarily or permanently. Competitive inhibitors are usually temporary (reversible), while non-competitive inhibitors often cause permanent damage to the enzyme's structure.

Intermediate Level

Enzyme inhibition occurs when a chemical substance (inhibitor) interferes with enzyme activity, blocking or damaging the active site. Inhibitors can be internal (accumulated products) or external (poisons, cyanide, antibiotics, drugs). Competitive inhibition: The inhibitor resembles the substrate and binds to the active site, preventing the substrate from binding. It is temporary and reversible; increasing substrate concentration can overcome it. Non-competitive inhibition: The inhibitor does not resemble the substrate and binds to a site other than the active site (allosteric site). This changes the enzyme's shape and damages the active site, preventing substrate binding. This type is often permanent.

Important Points

• 1. Enzyme inhibition occurs when chemicals interfere with enzyme activity at the reaction site.
• 2. The chemical substance that inhibits the enzyme is called an inhibitor.
• 3. Inhibitors may block or damage the active site temporarily or permanently.
• 4. Inhibitors can be internal (accumulated products) or external (poisons, cyanide, antibiotics, some drugs).
• 5. Competitive inhibition: Inhibitor resembles the substrate in structure and shape, occupies the active
• site, and prevents the actual substrate from binding. It is temporary and reversible; increasing substrate
• concentration can overcome it.
• 6. Non-competitive inhibition: Inhibitor does not resemble the substrate, binds to a site other than the
• active site (the allosteric site), alters the enzyme's globular shape, and damages the active site. The
• substrate cannot bind, and the reaction stops.

Remember

• Enzyme inhibition = chemicals interfere with enzyme activity.
• Inhibitor = chemical that inhibits the enzyme.
• Internal inhibitors = accumulated products.
• External inhibitors = poisons, cyanide, antibiotics, some drugs.
• generated by DeepSeek to PDF
• Competitive inhibition: Inhibitor resembles substrate → binds to active site → reversible (increase
• substrate to overcome).
• Non-competitive inhibition: Inhibitor does not resemble substrate → binds to allosteric site → changes
• enzyme shape → damages active site → reaction stops.

Common Mistakes

• 1. Confusing competitive and non-competitive inhibition: Competitive inhibitors bind to the active site
• and resemble the substrate; non-competitive inhibitors bind to a different site and do not resemble the
• substrate.
• 2. Thinking all inhibition is permanent: Competitive inhibition is temporary and reversible; non-
• competitive inhibition may be permanent.
• 3. Forgetting the allosteric site: Non-competitive inhibitors bind to the allosteric site, not the active site.
• 4. Thinking inhibitors are always harmful: Inhibitors can be internal regulators (e.g., accumulated
• products) that help control metabolic rates.
• 5. Thinking increasing substrate concentration can overcome non-competitive inhibition: Increasing
• substrate concentration only overcomes competitive inhibition, not non-competitive inhibition.

Quick Revision

• Enzyme inhibition occurs when a chemical substance (inhibitor) interferes with enzyme activity, blocking or
• damaging the active site. Inhibitors can be internal (accumulated products) or external (poisons, cyanide, antibiotics,
• drugs). There are two main types: competitive inhibition, where the inhibitor resembles the substrate and binds to
• the active site (temporary and reversible; increasing substrate concentration can overcome it), and non-competitive
• inhibition, where the inhibitor does not resemble the substrate and binds to the allosteric site, altering the
• enzyme's shape and damaging the active site (often permanent; increasing substrate concentration does not
• overcome it).

60-Second Revision

• Enzyme inhibition = chemicals interfere with enzyme activity.
• Inhibitor = chemical that inhibits the enzyme.
• Internal inhibitors = accumulated products.
• External inhibitors = poisons, cyanide, antibiotics, some drugs.
• Competitive inhibition: Inhibitor resembles substrate → binds to active site → reversible (increase
• substrate to overcome).
• Non-competitive inhibition: Inhibitor does not resemble substrate → binds to allosteric site → changes
• enzyme shape → damages active site → reaction stops.
• generated by DeepSeek to PDF

• Frequently Asked Questions (FAQs)
• Q1. What is enzyme inhibition? Answer: Enzyme inhibition is the phenomenon where enzymes are not able to
• perform their role due to the presence and interference of some chemicals at the reaction site.
• Q2. What is an inhibitor? Answer: An inhibitor is a chemical substance which reacts with the enzyme in place of the
• substrate but does not convert to products, thus inhibiting the enzyme action.
• Q3. What are the sources of inhibitors? Answer: Inhibitors can be internal (accumulated products within the cell)
• or external (poisons, cyanide, antibiotics, some drugs, or accumulated products).
• Q4. What is competitive inhibition? Answer: Competitive inhibition occurs when enzymes are inhibited by
• molecules that occupy the active site due to their similarity in structure and shape to the substrate. These inhibitors
• are neither converted to products nor allow the actual substrate to bind.
• Q5. Is competitive inhibition reversible? Answer: Yes, competitive inhibition is temporary and can be reversed if
• the concentration of the actual substrate is increased than the concentration of inhibitor molecules.
• Q6. What is non-competitive inhibition? Answer: Non-competitive inhibition occurs when inhibitors do not
• resemble the substrate and do not occupy the active site. They attach to the enzyme at a point other than the active
• site (allosteric site), alter the enzyme's globular shape, and damage the active site, preventing the substrate from
• binding.
• Q7. What is an allosteric site? Answer: The allosteric site is a point on the enzyme surface other than the active site
• where non-competitive inhibitors can bind.
• Q8. What is the difference between competitive and non-competitive inhibition? Answer: In competitive
• inhibition, the inhibitor resembles the substrate and binds to the active site; it is reversible by increasing substrate
• concentration. In non-competitive inhibition, the inhibitor does not resemble the substrate and binds to the
• allosteric site, altering the enzyme's shape; increasing substrate concentration does not overcome it.
• Q9. Can inhibitors be beneficial? Answer: Yes, inhibitors can be beneficial. For example, internal inhibitors
• (accumulated products) help regulate the rate of metabolic reactions, and external inhibitors like antibiotics can be
• used to treat bacterial infections.
• Q10. Do inhibitors permanently damage the enzyme? Answer: Inhibitors may block or damage the active site
• temporarily or permanently. Competitive inhibition is usually temporary, while non-competitive inhibition can cause
• permanent damage.
• Multiple Choice Questions (MCQs)
• Q1. The phenomenon where enzymes are not able to perform their role due to interfering chemicals is called: A.
• Enzyme activation B. Enzyme inhibition C. Enzyme denaturation D. Enzyme synthesis Correct Answer: B Explanation:
• Enzyme inhibition occurs when chemicals interfere with enzyme activity.
• Q2. The chemical substance that inhibits an enzyme is called an: A. Activator B. Cofactor C. Inhibitor D. Substrate
• Correct Answer: C Explanation: An inhibitor is a chemical substance that inhibits enzyme action.
• Q3. In competitive inhibition, the inhibitor: A. Binds to the allosteric site B. Resembles the substrate and binds to the
• active site C. Denatures the enzyme permanently D. Increases the reaction rate Correct Answer: B Explanation: In
• competitive inhibition, the inhibitor resembles the substrate and occupies the active site.
• generated by DeepSeek to PDF

• Q4. Competitive inhibition can be reversed by: A. Decreasing substrate concentration B. Increasing substrate
• concentration C. Adding more inhibitor D. Changing the pH Correct Answer: B Explanation: Competitive inhibition
• can be reversed if the concentration of the actual substrate is increased.
• Q5. In non-competitive inhibition, the inhibitor binds to the: A. Active site B. Allosteric site C. Substrate D. Product
• Correct Answer: B Explanation: Non-competitive inhibitors bind to the allosteric site (a point other than the active
• site).
• Q6. What happens to the enzyme's shape in non-competitive inhibition? A. It remains unchanged B. It is altered,
• damaging the active site C. It becomes more active D. It becomes a substrate Correct Answer: B Explanation: Non-
• competitive inhibitors alter the globular shape of the enzyme and damage the active site.
• Q7. Which of the following is an internal source of enzyme inhibition? A. Cyanide B. Antibiotics C. Accumulated
• products D. Poisons Correct Answer: C Explanation: Internal inhibitors include accumulated products within the cell.
• Q8. Which of the following is an external source of enzyme inhibition? A. Accumulated products B. Cyanide C.
• Substrate D. Cofactor Correct Answer: B Explanation: External inhibitors include poisons, cyanide, antibiotics, and
• some drugs.
• Q9. Competitive inhibition is usually: A. Permanent B. Temporary and reversible C. Irreversible D. Always harmful
• Correct Answer: B Explanation: Competitive inhibition is temporary and can be reversed.
• Q10. Non-competitive inhibitors do not resemble: A. The enzyme B. The product C. The substrate D. The cofactor
• Correct Answer: C Explanation: Non-competitive inhibitors do not resemble the substrate molecules.
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• Currency of Cell.