# Enzyme properties and mode of action

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

This subtopic introduces enzymes as globular protein biological catalysts, their core properties, and molecular mechanisms that underpin their activity. It forms the foundation for all further enzyme topics in the 9700 syllabus.

**Prerequisites:** [Globular protein structure and function](https://www.owlsprep.com/study/cie-9700-u2-globular-fibrous-proteins/)

## Learning objectives

- Describe the core properties of enzymes as biological catalysts
- Distinguish between intracellular and extracellular enzymes with examples
- Explain how enzymes lower activation energy to speed up reactions
- Compare the lock-and-key and induced fit models of enzyme action

## Core Properties of Enzymes

Enzymes are globular proteins that act as biological catalysts, speeding up the rate of metabolic reactions without being used up or permanently altered by the reaction. All enzymes are encoded by genes, and their function depends entirely on their complex 3D tertiary structure.

**Biological catalyst** — A protein-based molecule that increases the rate of a cellular reaction without being consumed or changed irreversibly during the reaction, so it can be reused multiple times.

*Example:* Catalase, which breaks down toxic hydrogen peroxide into water and oxygen in cells.

- Enzymes are highly specific for their substrate(s)
- Each enzyme can catalyse only one specific type of reaction
- Small amounts of enzyme can process large amounts of substrate due to reusability
- Enzyme activity depends on its 3D shape, and is affected by temperature and pH

**Worked example:** A student mixes 1 mmol of amylase with 100 mmol of starch, and waits until all starch is broken down into maltose. How much amylase is present at the end of the reaction? Explain your answer.

1. Recall the core definition of a catalyst: enzymes are not consumed or permanently altered during the reaction they catalyse.
2. Since amylase is not used up in the reaction, the amount of amylase remains unchanged.
3. The final amount of amylase at the end of the reaction is still 1 mmol, and it can be reused to catalyse the breakdown of more starch.

> **Exam tip:** CIE examiners always award a mark for stating that enzymes are unchanged and reusable after catalysis. Never forget to include this point in descriptive answers.

## Intracellular vs Extracellular Enzymes

Enzymes are grouped based on where they carry out their function, depending on the metabolic role they play. All enzymes are synthesised by ribosomes inside cells, but they may function inside or outside of their parent cell.

**Intracellular enzyme** — An enzyme that remains inside the cell that produces it and catalyses reactions there.

*Example:* Catalase, which breaks down toxic hydrogen peroxide, a by-product of metabolism, inside liver cells.

Extracellular enzymes are secreted out of the cells that produce them, and catalyse reactions outside of cells. Most extracellular enzymes are involved in digestion, breaking down large insoluble food molecules into small soluble molecules that can be absorbed into cells. A common example is amylase, produced by the pancreas and salivary glands, which breaks down starch into maltose in the gut.

**Worked example:** Name one example of an extracellular enzyme, state its role, and explain why it is classified as extracellular.

1. A common example of an extracellular enzyme is pancreatic amylase.
2. Amylase is produced by cells in the pancreas, and secreted into the lumen of the small intestine.
3. Its role is to break down starch into maltose during digestion.
4. It is classified as extracellular because it acts outside of the cells that produced it.

## Activation Energy and Enzyme Catalysis

For any chemical reaction to proceed, existing bonds in the reactants (substrates) must be broken before new bonds can form to make products. The minimum energy required to break these bonds is called activation energy ($E_a$). Enzymes speed up reactions by lowering the activation energy required for the reaction.

$$E_a(\text{without enzyme}) > E_a(\text{with enzyme})$$

> **info**
>
> Lowering activation energy allows reactions to proceed at the moderate temperatures (around 37°C in humans) that cells require, without needing high temperatures that would denature proteins and kill cells.

**Worked example:** Explain how enzymes allow reactions to proceed much faster than they would without enzymes.

1. Enzymes bind their substrate at the active site, and position the substrate molecules in the correct orientation for the reaction to occur.
2. Binding also distorts the bonds in the substrate that need to be broken, reducing the amount of energy required to break them.
3. This lowers the overall activation energy of the reaction, so more substrate molecules have enough energy to react at any given time.
4. More product is formed per unit time, so the rate of reaction increases.

## Models of Enzyme-Substrate Binding

Two models have been proposed to describe how enzymes interact with their substrates at the active site. The older lock-and-key model explains specificity, but the more modern induced fit model is the currently accepted model, supported by experimental evidence.

**Active site** — A cleft or pocket on the surface of the enzyme, with a unique 3D shape formed by the R-groups of amino acids in the enzyme's tertiary structure. This is where the substrate binds and catalysis occurs.

**Comparing methods**

The two models differ in their description of active site flexibility:

- **Lock-and-Key Model** — Proposes the active site has a fixed, rigid shape that is exactly complementary to the shape of the specific substrate. The substrate fits into the active site like a key fits into a lock.
  - Pros: Simple model that clearly explains enzyme specificity
  - Cons: Does not explain how enzymes lower activation energy or change shape during catalysis

- **Induced Fit Model** — Proposes the active site is not fully complementary to the substrate until binding occurs. Substrate binding triggers a conformational (shape) change in the enzyme that makes the active site fit tightly around the substrate.
  - Pros: Correctly explains how enzymes distort substrate bonds to lower activation energy, supported by experimental evidence
  - Cons: More complex than the lock-and-key model

**Worked example:** Explain why the induced fit model is the currently accepted model of enzyme action.

1. The lock-and-key model assumes the active site has a fixed shape, which does not match experimental observations of enzyme structure during catalysis.
2. When a substrate binds to an enzyme, experiments show the enzyme changes shape to wrap tightly around the substrate.
3. This shape change distorts the bonds in the substrate that need to be broken, which lowers the activation energy of the reaction, a mechanism that cannot be explained by the lock-and-key model.
4. For these reasons, the induced fit model is the accepted description of enzyme action.

> **Exam tip:** Always mention the shape change of the active site when describing induced fit — this is the key marking point in CIE exams.

## Common pitfalls

- **Wrong:** Stating that enzymes provide activation energy for reactions
  - Why it fails: Enzymes do not supply energy to reactions, they only reduce the amount of activation energy that is required
  - Correct: Always state that enzymes lower the activation energy of the reaction they catalyse
- **Wrong:** Claiming enzymes are changed irreversibly after every reaction
  - Why it fails: Enzymes are catalysts, so they are released unchanged after the reaction and can be reused
  - Correct: State that enzymes are not consumed or permanently altered during catalysis, and can be reused
- **Wrong:** Calling catalase an extracellular enzyme
  - Why it fails: Catalase functions inside liver cells to break down toxic hydrogen peroxide, so it is intracellular
  - Correct: Classify enzymes by where they act: intracellular act inside producing cells, extracellular act outside
- **Wrong:** Describing lock-and-key as the currently accepted model
  - Why it fails: Lock-and-key is an older, simplified model that does not match experimental evidence
  - Correct: State that induced fit is the currently accepted model of enzyme action

## Cheatsheet

| Category | Description | Example |
| --- | --- | --- |
| Core property | Biological globular protein catalyst | Any enzyme |
| Core property | Unchanged after reaction, reusable | --- |
| Core property | Specific to substrate due to active site shape | --- |
| Intracellular enzyme | Acts inside producing cells | Catalase |
| Extracellular enzyme | Acts outside producing cells | Amylase |
| Lock-and-key model | Fixed active site shape, complementary to substrate | Older specificity model |
| Induced fit model | Active site changes shape after substrate binding | Accepted modern model |

## What's next

This subtopic provides the foundational knowledge for all further enzyme topics in the CIE 9700 syllabus. Understanding enzyme structure and mode of action is critical for explaining how factors like temperature, pH, and inhibitors alter enzyme activity, which is one of the most frequently assessed topics in both paper 1 and paper 2. It also underpins all practical work involving enzymes, which is assessed in paper 3. Building a solid understanding of this subtopic will make all subsequent enzyme topics much easier to master.

- [Factors affecting enzyme activity](https://www.owlsprep.com/study/cie-9700-u3-factors-affecting-enzyme-activity/)
- [Protein structure](https://www.owlsprep.com/study/cie-9700-u2-protein-structure/)
- [Enzyme inhibitors](https://www.owlsprep.com/study/cie-9700-u3-enzyme-inhibitors/)

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