1. Unit 2 2.1 Metabolic Pathways & their Control
Higher Biology
Unit 2
2.1 Metabolic Pathways & their Control

2. National 5 Revision Think!
In your jotter make a quick mind map of all learning points on enzymes you can remember from National 5.
Now take a look at the mind map made by someone sitting next to you – can anything be added to yours?

3. National 5 Revision Enzymes;
are biological catalysts composed of protein
speed up cellular reactions
are unchanged by reactions
have active sites where they bind to their substrate
are specific because the shape of the active site is complementary to the shape of the substrate

4. National 5 Revision Enzymes; are affected by temperature and pH
are most active in their optimum conditions.
can denature in extremes of temperature and pH as their active site changes shape

5. Enzyme Action
The energy required to initiate a reaction is called the activation energy.
The activation energy is the energy required to break the bonds of the reactants.
When the bonds break, the reactants become unstable. They are now in the transition state and the reaction can occur.

6. Enzyme Action
Enzymes speed up the rates of chemical reactions by lowering the activation energy required to break bonds, to allow a reaction to occur.

7. Induced Fit
Enzymes are specific to their substrate because the shape of the enzyme’s active site is complementary to its substrate.
The substrate is said to have an affinity (chemical attraction) for the enzyme’s active site.

8. Induced Fit
The active site of an enzyme is not a rigid structure, it is flexible and dynamic. When a substrate binds to an active site, the shape of the enzyme changes slightly, allowing the active site to fit closely around the substrate. This is called induced fit. Induced fit increases reaction rate.

9. Orientation
If a reaction involves multiple substrates then the shape of the active site determines the orientation of the reactants. Orientation is the way substrates are held together in the active site. It ensures that they are held together in a way that the reaction between them can take place.

10. Orientation
Once the reaction has occurred, the products have a low affinity for the active site and are released. This leaves the enzyme free to repeat the process with new molecules of the substrate.

11. Key Points Metabolic pathways are controlled by different enzymes.
Enzymes often act in groups or as multi-enzyme complexes.
The energy required to initiate a chemical reaction is called the activation energy.
Enzymes lower the activation energy.

12. Key Points
Enzymes are flexible/dynamic and a substrate can induce the active site to change shape by induced fit.
The active site orientates substrate molecules so they fit more closely.
Substrates have an affinity for an enzyme’s active site and bind to it.
The products have a low affinity for the active site and are released.

13. Key Points
The concentration of substrate and end product affect the direction and rate of an enzyme-controlled reaction.
Most metabolic reactions are reversible and the presence of substrate or the removal of product drives a sequence of reactions in a particular direction.

14. Questions
Explain what is meant by the induced-fit model of enzyme action.
Describe the effect of increasing substrate concentration on the direction and rate of an enzyme-controlled reaction.
Describe 2 ways enzymes speed up the rate of reactions in metabolic pathways.

15. Question 1 Answer
An enzyme is flexible/dynamic and its active site can change shape.
A substrate induces the active site to change shape.
An active site can orientate the substrate molecules so that they fit more closely.

16. Question 2 Answer
An increase in substrate concentration drives the chemical reaction in the direction of the end product.
This increases the rate of reaction.

17. Question 3 Answer An enzyme lowers the activation energy.
An active site orientates the substrate molecules so that they fit more closely.