76 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.4 Enzymes And Biological Reactions) Flashcards
What did observations suggest about the flexibility of an enzyme’s shape?
It was obseerved that an enzyme’s shape was altered by binding its substrate.
This suggested that it was flexible and not rigid, as originally thought.
What is the alternative model of enzyme action called?
What does it say?
Induced fit model
It suggests that the enzyme shape alters slightly to accommodate the substrate.
Draw a diagram of the induced-fit model of enzyme action
Give an example of an enzyme for which the lock and key model fits well.
Tyrosine kinase
It seems more rigid than others so the lock-and-key model fits their behaviour well.
Give an example of an enzyme where the induced fit model works well
Lysozyme.
It seems to be much more flexible and the induced fit model describes their behaviour better.
What is lysozyme?
Where is it found?
It is an anti-bacterial enzyme
It is found in human saliva, mucus and tears.
Describe the mechanism of action of lysozyme in terms of enzyme model
- The active site is a groove.
- The sugars on the bacterial cell wall fit into it.
- The groove closes over the sugars.
- The lysozyme molecule changes shape around the sugars 5. This hydrolyses the bonds holding them together.
- The cell wall is weakened.
- The bacteria absorb water by osmosis and burst.
What do molecules need to have to react?
They must have enough kinetic energy to approach each other closely enough to react.
What is the activation energy?
It is the minimum energy required for molecules to react by
a. breaking existing bonds in the reactants
b. making new ones in the products
What does heat do to reactions in non-living systems?
- Heat increases kinetic energy.
- When kinetic energy is increased, successful collisons between chemicals is more likely.
- Hence reactions between chemicals is more likely.
- This speeds up reactions in non-living systems.
What does heat do in living organisms?
In most living organisms, temperatures above about 40°C cause irreversible damage to proteins, and they denature.
How do enzymes work?
Enzymes work by modifying the substrate so that the reaction requires a lower activation energy.
When a substrate enters the active site of an enzyme, the shape of the molecule alters.
This allows reactions to occur at lower temperatures, i.e. with lower kinetic energy, than in the absence of enzymes.
Draw a diagram to show how enzymes alter the activation energy
What needs to happen before a reaction can start?
Explain with a model
Reactions need to overcome an energy barrier before they can get started.
Consider a person pushing a boulder over the top of a hill.
They have to exert energy to get the boulder to the top of the hill and over.
Only after this happens, it can roll down the other side.
Induced-fit (Key-Term)
The change in shape of the active site of an enzyme, induced by the entry of the substrate, so that the enzyme and substrate bind closely.
Explain what happens when the sugards in the bacterial wall enter the active site of lysozyme
When the sugars in the bacterial cell wall enter the active site of lysozyme:
- Some of the amino acids at the active site move 7.5 nm
- This causes the groove closes over the chain of sugars.
- This strains the bonds holding the sugars together.
- This lowers the activation energy needed to break them.
Activation energy (Key-Term)
The minimum energy that must be put into a chemical system for a reaction to occur.
How do enzymes allow reactions to take place in cells?
By lowering the activation energy, enzymes allow reactions to take place at the lower temperatures found in cells.
Describe the activity of enzymes in relation to temperature?
Most enzymes are inactive at 0°C.
If the temperature is raised they become active again.
Enzymes in most organisms denature above 40°C.
Lowering the temperature after it has been increased to above 40°C does not restore their activity.
