# Enzyme inhibitors

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u3-enzyme-inhibitors/

This sub-topic covers reversible and irreversible enzyme inhibitors, their binding mechanisms, effects on enzyme kinetics, and biological applications, aligned to CIE A-Level Biology 9700 assessment objectives.

**Prerequisites:** [Enzyme mechanism and active site structure](https://www.owlsprep.com/study/cie-9700-u3-enzyme-mechanism/)

## Learning objectives

- Distinguish between competitive, non-competitive and irreversible enzyme inhibitors
- Explain the effects of different inhibitor types on $V_{max}$ and $K_m$
- Interpret kinetic graphs of enzyme activity to identify inhibitor type
- Relate enzyme inhibition to biological and real-world examples

## Types of Reversible Inhibitors

**Competitive inhibitor** — A reversible inhibitor structurally similar to the substrate that competes with substrate for binding to the enzyme's active site

*Example:* Oxygen competes with carbon dioxide for the active site of RuBisCO in photosynthesis

**Non-competitive inhibitor** — A reversible inhibitor that binds to an allosteric site (not the active site) on an enzyme, altering the enzyme's tertiary structure to disable active site function

*Example:* Cyanide inhibits cytochrome oxidase in aerobic respiration

**Worked example:** Explain why increasing substrate concentration reverses the effect of competitive inhibition but not non-competitive inhibition

1. For competitive inhibitors, inhibitor and substrate compete for the same active site. If substrate concentration is increased:
2. There are far more substrate molecules than inhibitor molecules, so substrate is much more likely to bind to free active sites than inhibitor.
3. At very high substrate concentration, nearly all active sites will be occupied by substrate, so reaction rate approaches the original uninhibited $V_{max}$.
4. For non-competitive inhibitors, inhibitor does not bind to the active site, so it can bind to the enzyme regardless of substrate concentration.
5. Binding permanently inactivates the enzyme for as long as the inhibitor is bound, so a proportion of enzyme molecules are always non-functional, even at high substrate concentration. This means $V_{max}$ can never reach the original uninhibited value.

> **Exam tip:** Always link the location of inhibitor binding to its effect on reaction rate to gain full marks in CIE written papers

## Effects on $V_{max}$ and $K_m$

$K_m$ (the Michaelis constant) is the substrate concentration at half $V_{max}$, and measures the affinity of an enzyme for its substrate: a lower $K_m$ means higher affinity. Inhibitors change these parameters in predictable ways that depend on their binding mechanism.

**Worked example:** Compare the effect of reversible competitive and non-competitive inhibitors on $V_{max}$ and $K_m$, and explain each outcome

1. Competitive inhibitor: $V_{max}$ is unchanged
2. Explanation: At high enough substrate concentration, substrate outcompetes inhibitor for all active sites, so the maximum possible rate remains the same as the uninhibited reaction.
3. Competitive inhibitor: $K_m$ increases
4. Explanation: Inhibitor competes for active sites, so a higher substrate concentration is required to reach half $V_{max}$, indicating reduced affinity of the enzyme for substrate.
5. Non-competitive inhibitor: $V_{max}$ decreases
6. Explanation: Inhibitor binding permanently inactivates a proportion of enzyme molecules, so fewer functional active sites are available, reducing the maximum possible reaction rate.
7. Non-competitive inhibitor: $K_m$ is unchanged
8. Explanation: The active sites of still-functional enzymes have the same affinity for substrate as before, so the substrate concentration required to reach half $V_{max}$ does not change.

## Irreversible Inhibitors

Irreversible inhibitors bind permanently to enzymes, usually via strong covalent bonds, that permanently disable enzyme function. They are not in dynamic equilibrium like reversible inhibitors, so their effect cannot be reversed by increasing substrate concentration.

Most irreversible inhibitors are toxic, as they block essential metabolic reactions. For example, nerve gas sarin binds covalently to the active site of acetylcholinesterase, the enzyme that breaks down the neurotransmitter acetylcholine, leading to paralysis and death.

**Worked example:** Explain why irreversible inhibitors always reduce $V_{max}$, regardless of their binding site

1. Irreversible inhibitors bind covalently to the enzyme, permanently inactivating every molecule they bind.
2. This reduces the total concentration of functional enzyme molecules available for the reaction.
3. $V_{max}$ is directly proportional to the concentration of functional enzyme, so $V_{max}$ is always reduced.
4. Increasing substrate concentration cannot displace the permanently bound inhibitor or restore enzyme activity, so no recovery of $V_{max}$ occurs.

## Interpreting Inhibitor Kinetics Graphs

CIE exams frequently ask to identify inhibitor types from plots of reaction rate against substrate concentration. The shape of the curve matches the kinetic properties of each inhibitor type.

**Worked example:** Match the following curve descriptions to the correct reversible inhibitor type: (1) Rate approaches the same $V_{max}$ as uninhibited at high substrate concentration, shifted right; (2) Rate plateaus at a lower $V_{max}$ than uninhibited, no recovery at high substrate concentration

1. Description 1 matches competitive inhibition.
2. Reason: Competitive inhibitors do not change $V_{max}$, so the curve plateaus at the same maximum rate as uninhibited reaction. The shift right occurs because higher substrate concentration is needed to reach half $V_{max}$, reflecting increased $K_m$.
3. Description 2 matches non-competitive inhibition.
4. Reason: Non-competitive inhibitors reduce the concentration of functional enzyme, so $V_{max}$ is always lower than the uninhibited reaction, so the curve plateaus early. Functional active sites retain the same affinity for substrate, so half $V_{max}$ occurs at the same substrate concentration as the uninhibited reaction.

**Check your understanding**

Test your understanding of inhibitor kinetics

1. What change to $K_m$ occurs in reversible competitive inhibition?

   - $K_m$ increases
   - $K_m$ decreases
   - $K_m$ unchanged
   - $K_m$ equals $V_{max}$

   *Why:* Correct! Competitive inhibition reduces enzyme affinity for substrate, so higher substrate concentration is needed for half $V_{max}$, increasing $K_m$.

2. What change to $V_{max}$ occurs in reversible non-competitive inhibition?

   - $V_{max}$ increases
   - $V_{max}$ decreases
   - $V_{max}$ unchanged
   - $V_{max}$ equals $K_m$

   *Why:* Correct! Non-competitive inhibition inactivates a proportion of enzyme molecules, reducing the maximum possible reaction rate.

## Common pitfalls

- **Wrong:** Claiming non-competitive inhibitors bind to the enzyme active site
  - Why it fails: Confusion between binding locations for the two main reversible inhibitor types
  - Correct: Non-competitive inhibitors bind to an allosteric site separate from the active site
- **Wrong:** Stating competitive inhibitors reduce $V_{max}$
  - Why it fails: Mixing up the effects of inhibitors on $V_{max}$ and $K_m$
  - Correct: Competitive inhibitors do not change $V_{max}$, because high substrate concentration can outcompete inhibitor for all active sites
- **Wrong:** Claiming non-competitive inhibitors change $K_m$
  - Why it fails: Forgetting that $K_m$ only measures affinity of functional active sites
  - Correct: Non-competitive inhibitors leave affinity of unbound active sites unchanged, so $K_m$ stays the same
- **Wrong:** Thinking all inhibitors are reversible
  - Why it fails: Overgeneralization from the two main reversible types tested in this topic
  - Correct: Irreversible inhibitors bind covalently and permanently inactivate enzymes, with effects that cannot be reversed by increasing substrate concentration
- **Wrong:** Claiming all inhibitors that reduce $V_{max}$ are non-competitive
  - Why it fails: Ignoring irreversible inhibitors and mixed inhibitor types
  - Correct: Irreversible inhibitors of any binding type always reduce $V_{max}$ by lowering the concentration of functional enzyme

## Cheatsheet

| Inhibitor Type | Binding Site | Reversible | $V_{max}$ change | $K_m$ change | Example |
| --- | --- | --- | --- | --- | --- |
| Competitive | Active site | Yes | Unchanged | Increased | O₂ vs CO₂ on RuBisCO |
| Non-competitive | Allosteric site | Yes | Decreased | Unchanged | Cyanide on cytochrome oxidase |
| Irreversible | Any site (often active) | No | Decreased | Variable | Sarin on acetylcholinesterase |

## What's next

Enzyme inhibition is the core mechanism for regulating metabolic pathways in cells, and is the basis of many common pharmaceutical drugs, poisons, and agricultural pesticides. Mastering inhibitor types and their kinetic effects is essential for understanding how cells control their internal chemistry and respond to changes in the environment. This topic also underpins understanding of drug action, immune system function, and photosynthetic carbon fixation in later units of CIE A-Level Biology.

- [Immobilised enzymes](https://www.owlsprep.com/study/cie-9700-u3-immobilised-enzymes/)
- [Cell Membranes and Transport](https://www.owlsprep.com/study/cie-9700-u4-overview/)
- [Fluid mosaic membrane structure](https://www.owlsprep.com/study/cie-9700-u4-fluid-mosaic-membrane-structure/)

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