Enzymes and Enzyme Action
- Rachel Taylor
- Mar 31
- 3 min read
Updated: May 2
HSC Biology | Free Study Notes
This topic helps students understand how enzymes work, why they are specific, and how ideas such as the active site, substrate, and enzyme models explain biological processes.
In this lesson
what enzymes are
what biological catalysts do
the meaning of substrate and active site
why enzymes are specific
how the lock and key and induced fit models explain enzyme action
What are enzymes?
Enzymes are biological catalysts.
A catalyst is a substance that speeds up a chemical reaction without being used up in the reaction.
Why enzymes matter
Enzymes are important because many reactions in cells would happen too slowly without them.
Cells need enzymes for processes such as:
digestion
DNA replication
protein synthesis
Biological catalysts
What does “biological catalyst” mean?
A biological catalyst is a catalyst made by living organisms.
Most enzymes are proteins. Their shape is important because it determines what they can bind to and what reaction they can carry out.
What enzymes do
Enzymes:
speed up reactions
lower the amount of activation energy needed
allow reactions to happen fast enough for life
Important point
Enzymes do not change the final products of a reaction. They simply help the reaction happen more quickly.
Substrate
A substrate is the substance that an enzyme acts on.
Simple definition
The substrate is the reactant that fits into the enzyme’s active site.
Example idea
If an enzyme breaks down starch, then starch is the substrate.
Active site
The active site is the part of the enzyme where the substrate binds.
Why the active site matters
The active site:
has a particular shape
matches the substrate
is where the reaction takes place
When the substrate binds to the active site, an enzyme-substrate complex forms.
After the reaction:
the products are released
the enzyme can be used again
Specificity
Enzymes are specific, which means each enzyme only acts on one substrate or a small group of very similar substrates.
Why enzymes are specific
Enzyme specificity depends on:
the shape of the active site
the chemical properties of the active site
Only substrates with the correct shape and properties can bind properly.
Why specificity is important
Specificity ensures that:
the correct reactions happen in cells
cell processes stay controlled
substances are broken down or built up accurately
Lock and key model
The lock and key model explains enzyme action by suggesting that the substrate fits exactly into the active site.
Main idea
the enzyme is the lock
the substrate is the key
only the correct key fits the lock
What this model explains well
This model helps explain:
enzyme specificity
why only certain substrates can bind
Limitation
It makes the active site seem completely rigid, which is a bit too simple.
Induced fit model
The induced fit model is a more accurate explanation of enzyme action.
Main idea
In this model:
the active site is not completely rigid
the enzyme changes shape slightly as the substrate binds
this improves the fit between enzyme and substrate
Why this model is useful
The induced fit model explains:
why enzymes are still specific
how the active site can adjust slightly
why enzyme action is more flexible than the lock and key model suggests
Lock and key vs induced fit
Similarity
Both models explain that:
the substrate binds to the active site
enzymes are specific
Difference
Lock and key suggests an exact fit from the start
Induced fit suggests the enzyme changes shape slightly to fit the substrate better
For exam questions, it is often best to say that the lock and key model is a simple model, while induced fit is a more accurate one.
Step-by-step enzyme action
Basic sequence
The substrate approaches the enzyme.
The substrate binds to the active site.
An enzyme-substrate complex forms.
The reaction occurs.
Products are released.
The enzyme remains unchanged and can be reused.
Worked example
Exam-style question
Explain why enzymes are specific.
Worked answer
Enzymes are specific because each enzyme has an active site with a particular shape and chemical properties. Only a substrate with a complementary shape can bind to the active site and form an enzyme-substrate complex.
Why this works
This answer:
uses the terms active site and substrate
explains specificity clearly
links structure to function
Common mistakes
Saying enzymes are used up in reactions. They are not used up.
Mixing up the enzyme and the substrate.
Saying the active site is the whole enzyme. It is only one part of the enzyme.
Thinking one enzyme can work on any substrate.
Describing lock and key and induced fit as completely different processes, rather than two models explaining the same idea.
Quick quiz
What is an enzyme?
What is meant by a biological catalyst?
What is a substrate?
What is the active site?
Which model suggests the enzyme changes shape slightly when the substrate binds?
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