# Enzymes

> CIE A-Level Biology · 9700
> Source: https://www.owlsprep.com/study/cie-9700-u3-overview/
> Weight: n/a

This unit explores the structure, mechanism, regulation and industrial applications of enzymes, the globular protein catalysts that control all biological metabolism. It provides a foundational understanding for all cellular processes covered later in the syllabus.

**Prerequisites:** [Structure of globular proteins (Unit 2: Biological Molecules)](https://www.owlsprep.com/study/cie-9700-u2-overview/)

## Learning objectives

- Describe the structure and function of enzymes as globular biological catalysts
- Explain how enzymes lower activation energy and bind substrates according to major models
- Analyse how different internal and external factors affect the rate of enzyme-controlled reactions
- Compare types of enzyme inhibitors and describe key industrial applications of immobilised enzymes

## Unit at a Glance

Enzymes are critical to life, as they allow metabolic reactions to proceed at the rates required for cell function at normal physiological temperatures. This unit builds sequentially: starting from core properties and mechanism of action, moving to factors that change enzyme activity, how inhibitors regulate function, and ending with real-world industrial applications of immobilised enzymes.

This unit is split into 4 core sub-topics:
- [Enzyme properties and mode of action](https://www.owlsprep.com/study/cie-9700-u3-enzyme-properties-and-mode-of/) — Covers enzyme structure as globular catalysts, activation energy, and the induced fit and lock-and-key models of substrate binding.
- [Factors affecting enzyme activity](https://www.owlsprep.com/study/cie-9700-u3-factors-affecting-enzyme-activity/) — Explores how temperature, pH, substrate concentration and enzyme concentration affect reaction rate and enzyme tertiary structure.
- [Enzyme inhibitors](https://www.owlsprep.com/study/cie-9700-u3-enzyme-inhibitors/) — Compares competitive and non-competitive inhibitors, including their effects on reaction kinetics and practical biological examples.
- [Immobilised enzymes](https://www.owlsprep.com/study/cie-9700-u3-immobilised-enzymes/) — Describes common methods of immobilisation and the advantages and industrial applications of immobilised enzymes.

## Common pitfalls

- **Wrong:** Confusing competitive and non-competitive inhibitors in terms of their effect on $V_{max}$ and $K_m$.
  - Why it fails: Students often mix up binding sites and resulting kinetic changes, leading to incorrect exam answers.
  - Correct: Competitive inhibitors bind the active site, so they do not change $V_{max}$ but increase $K_m$; non-competitive bind allosteric sites, lowering $V_{max}$ with no change to $K_m$.
- **Wrong:** Claiming enzymes are 'used up' in the reactions they catalyse.
  - Why it fails: This common misconception confuses enzymes with reactants in metabolic pathways.
  - Correct: Enzymes are biological catalysts, so they remain unchanged at the end of a reaction and can be reused multiple times.
- **Wrong:** Drawing pH/temperature enzyme activity curves starting from 0 rate and rising after the optimum point.
  - Why it fails: Students often forget that denaturation is irreversible after the optimum is passed.
  - Correct: For both temperature and pH, reaction rate falls to zero after the optimum as the enzyme's tertiary structure is permanently denatured.

## Cheatsheet

| Key Concept | Core Summary |
| --- | --- |
| Activation Energy | Minimum energy required for a reaction to proceed; enzymes lower this to increase reaction rate |
| Competitive Inhibitor | Binds enzyme active site, increases $K_m$, no change to maximum reaction rate $V_{max}$ |
| Non-competitive Inhibitor | Binds allosteric (non-active) site, decreases $V_{max}$, no change to $K_m$ |
| Effect of Temperature | Rate increases until optimum temperature; denaturation after optimum causes rate to drop to zero |
| Core Advantage of Immobilised Enzymes | Easily separated from products, can be reused, and more stable to changes in pH and temperature |

## What's next

Begin your study of this unit with the first sub-topic to build your foundational understanding of enzyme structure and catalytic mechanism. Once you complete all sub-topics in this unit on enzymes, you will move on to the next unit covering cell membranes and transport, another core topic for understanding cellular function.

- [Enzyme properties and mode of action](https://www.owlsprep.com/study/cie-9700-u3-enzyme-properties-and-mode-of/)
- [Factors affecting enzyme activity](https://www.owlsprep.com/study/cie-9700-u3-factors-affecting-enzyme-activity/)
- [Enzyme inhibitors](https://www.owlsprep.com/study/cie-9700-u3-enzyme-inhibitors/)

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