1. Metabolic Pathways (c)
Identify the active site of an enzyme
Define the term ‘induced fit’
Describe the sequence of events which occur at the active site using the terms reactants, orientation, activation energy, products and affinity

2. What happens when enzymes bind to their substrate?
Why can enzymes only bind to one substrate?

3. What happens when enzymes bind to their substrate?
Why can enzymes only bind to one substrate?
You may describe the lock and key theory (the shape of an enzyme’s active site is complementary and specific to that of its substrate)
Although this is true, it is not the whole story.

4. What happens when enzymes bind to their substrate?
Look at the diagrams… what do you think is meant by INDUCED FIT?

5. TASK: Make an enzyme
Use the play dough to create an enzyme with an active site which will fit the shape of the ‘substrate’ given to you by your teacher
Will the active site be an exact match?
What will you have to do to the enzyme when the substrates enter the active site?
DO NOT GIVE PUPILS THE ‘SUBSTRATE’ UNTIL THEY HAVE BUILT THEIR ENZYME.
SIMPLY SHOW THEM-THIS IS IMPORTANT BECAUSE THEY MIGHT USE THE SUBSTRATE TO SHAPE THE ACTIVE SITE SO IT IS AN EXACT FIT!

6. What happens when enzymes bind to their substrate?
The active site is where the substrate will bind to, on the enzyme.
Substrates show a high affinity (chemical attraction) for the enzyme’s active site.
The active site is not a rigid structure, instead it is flexible and dynamic.
Therefore, when a substrate molecule enters the active site the shape of the enzyme and the active site change slightly, ensuring the active site fits very closely around the substrate. This is called induced fit.

7. What happens when enzymes bind to their substrate?
If there are 2 or more substrates, then the shape of the active site will also play a role in orientating reactants and ensuring the reaction can take place:
The active site holds the reactants closely together in an induced fit
The enzyme then weakens the chemical bonds in the reactants and promotes the chemical reaction by lowering the activation energy of the transition state
Once the reaction has occurred, the new product has a low affinity for the active site and is therefore released
This leaves the enzyme free to repeat the process again.

8. Review of the work so far…
Stop at 4 mins 47 secs
The Induced Fit Model of Enzyme Catalysis