Skip to main content
SingaporeBiologySyllabus dot point

How do enzymes catalyse biochemical reactions, and what does the induced fit model add to the lock and key idea?

Explain how enzymes act as biological catalysts by lowering activation energy, and describe the lock and key and induced fit models

A focused answer to the H2 Biology Energy and Equilibrium outcome on enzymes. How enzymes lower activation energy, the enzyme-substrate complex, the lock and key and induced fit models, and the meaning of specificity.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to explain how enzymes work as biological catalysts by lowering activation energy through forming an enzyme-substrate complex, and to describe and compare the lock and key and induced fit models. You also need to explain specificity. This rests on protein structure and underpins respiration and photosynthesis.

The answer

Enzymes lower activation energy

Every reaction needs a minimum input of energy, the activation energy, to get started. An enzyme is a biological catalyst that lowers the activation energy, so the reaction proceeds faster at the temperatures found in living cells. The enzyme is not used up and can be used again.

It works by binding its substrate at the active site to form an enzyme-substrate complex. This holds the substrate in the right orientation and strains its bonds, making the reaction easier. The products then leave and the enzyme is free again.

The lock and key model

The earliest model pictures the active site as a fixed, rigid shape exactly complementary to the substrate, like a key fitting a lock. This explains specificity but treats the active site as unchanging.

The induced fit model

The accepted model proposes that the active site is not a perfect fit initially. When the substrate binds, the active site changes shape to mould around it. This induced conformational change places strain on the substrate's bonds and is what drives catalysis, explaining enzyme action better than a rigid fit.

Specificity

An enzyme is specific because the shape of its active site, set by its tertiary structure, is complementary to only one substrate (or a narrow group). Only that substrate can bind, so only that reaction is catalysed.

Examples in context

Example 1. Digestive enzymes. Amylase, protease and lipase each act only on their specific substrate (starch, protein, lipid). Their specificity, set by their active site shapes, ensures each food molecule is broken down by the right enzyme, illustrating why specificity matters physiologically.

Example 2. Metabolic pathways. Respiration and photosynthesis are sequences of reactions, each catalysed by a specific enzyme. Because every step has its own enzyme, the cell can control the pathway precisely by regulating individual enzymes, which is the foundation for the later energetics dot points.

Try this

Q1. State what is meant by the active site of an enzyme. [1 mark]

  • Cue. The region of the enzyme with a specific shape to which the substrate binds, forming an enzyme-substrate complex.

Q2. Explain how an enzyme increases the rate of a reaction. [2 marks]

  • Cue. It lowers the activation energy by binding the substrate and straining its bonds, so a greater proportion of molecules can react at a given temperature.

Q3. Give one way the induced fit model differs from the lock and key model. [1 mark]

  • Cue. In induced fit the active site changes shape when the substrate binds, whereas in lock and key the active site is a fixed, rigid complementary shape.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksExplain how an enzyme speeds up a reaction, and describe how the induced fit model accounts for enzyme action better than the lock and key model.
Show worked answer →

The answer needs the mechanism of catalysis and then the comparison of models.

An enzyme speeds up a reaction by lowering the activation energy, the minimum energy needed for the reaction to proceed. It does this by binding the substrate at its active site to form an enzyme-substrate complex, which holds the substrate in a favourable orientation and strains its bonds, so the reaction proceeds more readily. The enzyme is not used up and emerges unchanged.

In the lock and key model the active site is a fixed, rigid shape exactly complementary to the substrate. The induced fit model proposes instead that the active site is initially not perfectly complementary; when the substrate binds, the active site changes shape slightly to mould around it. This induced change places strain on the substrate's bonds, which better explains how the enzyme actually catalyses the reaction rather than merely holding the substrate.

Markers reward the lowering of activation energy, the enzyme-substrate complex, the description of both models, and the point that the conformational change in induced fit explains catalysis.

Original3 marksExplain what is meant by the specificity of an enzyme and why it occurs.
Show worked answer →

The answer should define specificity and explain its structural basis.

Specificity means that an enzyme catalyses only one type of reaction, or acts on only one substrate or a narrow group of similar substrates.

It occurs because the active site has a particular three-dimensional shape, determined by the enzyme's tertiary structure, that is complementary only to a specific substrate. Only that substrate can bind to form an enzyme-substrate complex, so only that reaction is catalysed.

Markers reward the definition of specificity, the role of the active site shape, and the link to the enzyme's tertiary structure determining that shape.

Related dot points