Enzymes Flashcards
Enzymes
- Globular proteins that act as catalysts
- Lower the activation energy level (allows reactions to occur at lower temperatures)
Catalysts
Alter the rate of a chemical reaction without undergoing permanent change themselves
Why are enzymes effective in small amounts
Can be reused repeatedly
Activation energy
An energy barrier that must be overcome for the reaction to take place
Enzyme structure
- Have a specific 3D shape that is due to its primary structure
- Active site
Active site
- A specific region of the enzyme that is functional
- Made up of a relatively small number of amino acids
- Forms a small depression within the much larger enzyme molecule
Substrate
Fits neatly into the active site to form an enzyme-substrate complex
Enzyme-substrate complex
The substrate molecule is held within the active site by bonds that temporarily form
Induced fit model of enzyme action
- The enzyme and substrate interact
- The proximity of the substrate (a change in the environment of the enzyme) leads to a change in the enzyme that forms the functional active site
- The enzyme is flexible + can mould itself to the shape of the substrate
How does the presence of a substrate change the shape of the active site in the induced fit model?
Puts a strain on the substrate molecule which distorts a particular bond in the substrate + consequently lowers the activation energy needed to break the bond
What happens to the active site after the products are released
The active site reverts to its original shape
Limitation of the lock + key model
It is considered as a rigid structure
What has to occur for an enzyme to work?
- Come into physical contact with its substrate
- Have an active site which fits the substrate
Factors that influence the rate of enzyme activity, affect these 2 things
How to measure an enzyme-catalysed reaction
- Measure its time-course (how long it takes for a particular event to run its course)
- Measure the formation of the products of the reaction (amount)
- Measure the disappearance of the substrate (concentration)
Graph: x-axis = time, y-axis = volume of gas produced
Upwards curve
- At first there is a lot of substrate and no product
- It is very easy for substrate molecules to come into contact with empty active sites on the enzyme molecule
- Amount of substrate starts to decrease as its broken down -> increases amount of product
- More difficult for substrate to come into contact with enzyme due to product getting in the way
- Takes longer for substrate to be broken down so graph becomes plateued
- All substrate has been used up and no new product can be produced
How to measure the rate of change on a graph
Gradient of tangent at a given point
What do you have to do when investigating the effect of a certain variable
Keep all other variables constant
Factors that effect enzyme action
- Temperature
- pH
- Enzyme concentration
- Substrate concentration
Effect of temperature on enzyme action
- increases K.E. of molecules -> move around more rapidly + more frequent collisions = more successful collisions
- increasing temp. causes bonds in enzyme structure to break -> enzyme starts to change shape -> substrate fits less easily into the changed active site
- After a certain temperature, enzyme becomes denatured
Denaturation
Permanent change of the enzyme that stops it from functioning altogether
Why is human body temperature slightly lower than optimum temperature of enzymes
- More energy would be needed to maintain a high temp.
- Any further rise in temp. (e.g. during illness) would denature the enzymes
What does body temperature of an organism depend on
Its activity
- More active organisms have a higher metabolic rate so higher temp.
Effect of pH on enzyme action
- Change in pH alters the charges on the amino acids that make up the active site
- Depending on the significance of the change in pH, the bonds of the tertiary structure may break
Effect of enzyme concentration on the rate of reaction
- Low enzyme concentration: Too few enzymes to allow all substrate molecules to have an active site
Intermediate enzyme concentration: All substrate molecules can occupy an active site
High enzyme concentration: The addition of further enzymes has no effect on rate of reaction (graph is plateued)
Graph of effect of enzyme concentration on the rate of reaction
x-axis: enzyme conc.
y-axis: rate of reaction
Effect of substrate concentration on the rate of enzyme concentration
Low substrate concentration: There are too few substrates to fill all active sites
Intermediate substrate concentration: all active sites are occupied at one time. Maximum
High substrate concentration: The addition of more substrate has no effect as all active sites are occupied at one time
Graph of effect of substrate concentration on the rate of enzyme concentration
x-axis: substrate concentration
y-axis: rate of reaction
Enzyme inhibitors
Directly or indirectly interfere with the functioning of the active site
Types of enzyme inhibitors
- Competitive inhibitors
- Non-competitive inhibitors
Competitive inhibitors
- Have a molecular shape similar to that of a substrate
- So compete with the substrate for the available active sites
- Not permanently bound
How can the effect of a competitive inhibitor be reduced?
Increase substrate concentration
Non-competitive inhibitors
- Attach to the enzyme on a binding site which is not the active site -> alters the shape of the enzyme and so its active site as well
- Permanently stops the enzyme from functioning
