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

2. National 5 Revision Think! What factors affect enzyme activity?
How do they affect the enzyme?

3. pH, temperature and substrate concentration all affect enzyme activity
If the pH or temperature is too far outwith the optimum then the enzyme will be denatures (its active site will change shape and it can no longer react with its substrate)

4. As substrate concentration increases more active sites are involved in reactions
When all active sites are occupied the reaction rate can’t increase any more

5. Direction of enzyme action
As substrate W enters this pathway enzyme 1 becomes active and converts W to X. In the presence of X enzyme 2 becomes active and converts X to Y and so on.
The presence of subtrate W drives the sequence of reactions from W to Z.
The product of one reaction acts as the substrate of the next.

6. Multi-enzyme complexes
Sometimes enzymes are grouped together.
The product of one enzyme can be passed onto the next enzyme in the pathway.
Sometimes called metabolic or substrate channelling.
Enzyme 1 + substrate
Enzyme 2 + substrate

7. Multi-enzyme complexes
Most enzymes are reversible, this means they will convert products back into substrates.
If the substrate is quickly converted into another substance the enzyme cannot catalyse the reverse reaction.
Enzyme + substrate
Enzyme + products

8. Control of metabolic pathways
There are 3 ways of controlling rates of reaction by altering the activity of enzymes.
Competitive Inhibitors
Non-competitive Inhibitors
Feedback control

9. Inhibitors
An inhibitor is a substance which decreases the rate of an enzyme controlled reaction

10. Competitive Inhibition
These are molecules that have a similar structure to the normal substrate.
They bind to the active site.
They compete with the substrate.
Substrate
Enzyme /Inhibitor
Enzyme
Unused
substrate
Competitive
Inhibitor

11. Competitive Inhibition
No inhibitor
If the concentration of substrate is increased, the substrate can bind with the active site more often than the inhibitor. The rate of reaction can be returned to normal.
Inhibitor
Substrate concentration

12. Non-competitive Inhibition
These are molecules that have a different structure to the normal substrate.
They bind to another part of the enzyme (allosteric site) and alter the shape of the active site.
Substrate
Enzyme /Inhibitor
Enzyme
Non-competitive
Inhibitor
Unused
substrate

13. Non-competitive Inhibition
The presence of non-competitive inhibitor is permanent and is not affected by changes in the substrate concentration.

14. Feedback Inhibition
In a metabolic pathway, a product is produced that can inhibit the activity of an enzyme at the start of the pathway.

15. Key Points
Concentration of substrate and end product affect the direction and rate of enzyme controlled reactions.
Competitive inhibition involves competition for the active site of the enzyme by molecules similar in shape to the substrate molecule.
Competitive inhibition can be reduced by increasing substrate concentration.

16. Key Points
Non-competitive inhibition involves molecules binding to the allosteric site. This changes the shape of the enzyme’s active site and the substrate can no longer bind to it.
Feedback inhibition is a method of metabolic control. An end product binds to an enzyme that catalyses a reaction early in the pathway and inhibits it.

17. Questions
What property of a competitive inhibitor enables it to compete with a substrate?
What effect does an increase in the concentration of a substrate have on the rate of a reaction when a competitive inhibitor and enzyme are present? Explain why

18. Answers
The inhibitor and substrate are a similar shape allowing the inhibitor to bind to the active site of the enzyme
Increasing the concentration of substrate will increase the rate of reaction. A greater substrate concentration increases the chances of an enzyme binding to its substrate instead of the inhibitor.

19. R P Q Questions The diagram below represents a metabolic pathway.
In which direction will the pathway proceed if more of metabolite P is added? P R Q
Enzyme 2
Enzyme 1

20. Answers
Left to right. (P would be converted to Q and Q would be converted to R).