Enzymes Flashcards
What are enzymes?
They are biological catalysts
They are proteins
What do enzymes do within a reaction?
- They speed up the rate of reaction
- They lower the activation energy by bending the bonds within the substrate
Are enzymes specific to one reaction?
Yes
Are enzymes used up or changed by the recation?
No
Do enzymes alter the product of the reaction?
No
Can they create reactions?
No - they only catalyse reactions which already occur
What kind of proteins are enzymes?
Globular
What do enzymes have that makes them strong?
They have a high molecular weight, they are substantial, solid and have a certain amount of stability
(so we try not to change the environment)
What are enzymes sensitive to?
- Temperature and pH
- They are denatured at high temperatures and at pH’s that they are not optimal in
Ho do high temperatures affect enzymes?
They are denatured as the bonds are broken - the active site changes shape
How do different pH’s affect enzymes?
They are denatured. Only hydrogen and ionic bonds in enzymes are affected by pH
What is the active site?
It is where the chemical reactions take place. It has a complimentary 3D tertiary structure to one specific substrate
What are the two types of reactions that enzymes catalyse?
Anabolic - building up
Catabolic - breaking down
What kind of reactions are digestion reactions?
Catabolic
Why are the majority of enzymes soluble in water?
Due to hydrophilic R variable groups on their amino acids
What is the reaction known as when an enzyme is inside the cell?
Intracellularly e.g. DNA replication happens inside cells
What is the reaction known as when an enzyme is outside the cell?
Extracellularly - it occurs outside of the cells which have constructed it e.g. in fluids
e.g. extracellular action of pepsin and amylase
What are the main groups of enzymes?
Oxidoreductase
Transferases
Lysases
Hydrolases
Ligases
Isomerases
What is oxidoreductase?
The transfer of electrons in oxidation and reduction reactions
What are transferases?
Transfer of a functional group from one molecule to another
What are lysases?
Splitting of bonds (not hydrolysis or oxidation)
What are hydrolases?
Hydrolysis of bonds (all digestive enzymes)
What are ligases?
Joining of 2 molecules by formation of covalent bonds
What are isomerases?
Isomerisation of molecules e.g. conversion of glucose and fructose to sucrose
What do some enzymes need before they catalyse a reaction?
Non-protein substances
What are activators?
Inorganic groups permanently bound to enzymes and are a type of prosthetic group e.g. iron, copper
What are co-enzymes?
Organic molecules that bind only temporarily to the enzyme, transferring a chemical group necessary required for the reaction
What is the lock and key model?
- Substrate binds to active site
- This creates an enzyme/substrate complex
- Bends the bonds of the substrate
- The reaction occurs and we have an enzyme/product complex
- The active site releases products
- The product molecules diffuse away from the active site
What does the lock and key model propose?
The substrate is an exact complimentary shape to the active site
What does the induced fit model take into account?
The fact that proteins (enzymes) have some 3 dimensional flexibility
How does the induced fit model work?
- Substrate binds to the enzyme at the active site which is a similar complimentary shape to the substrate but not exactly
- Binding of the substrate induces the enzyme to change shape such that there is an exact fit in the active site once the substrate has bound
According to the induced fit model, when can the reaction only take place?
After induced fit has occured
What happens to the active site, according to the induced fit model, after the products are released?
It goes back to its original shape
What is the activation energy?
The minimum amount of energy required for a reaction to start
What affects the rate of enzyme catalysed reactions?
- Temperature
- pH
- Substrate concentration
- Enzyme concentration
- Inhibitors
- Activators
At a low temperature, what is the rate of reaction like in enzymes?
Slow
Enzymes and substrates have little kinetic energy and move around less
So fewer successful collisions
Fewer enzyme/substrate complexes
We don’t achieve the activation energy
Less product per second being generated
What happens as we increase the temperature up to the optimum to the rate of reaction in enzymes?
We increase kinetic energy
Increased number of successful collisions
Increased number of enzyme/substrate complexes
Increased rate of reaction
More product formed per second
What happens as we increase the temperature by 10 degrees in enzyme controlled reactions?
The rate of reaction doubles
Q10 = 2
What happens at the optimum temperature in enzyme controlled reactions?
Optimum is around 37 degrees
Fastest rate of reaction
Most successful collisions per second
Highest rate of enzyme/substrate complexes
Highest rate of product formation per second
What happens at temperature above the optimum in enzyme controlled reactions?
The active site denatures
Its 3D tertiary structure is no longer complimentary to the substrate
There are fewer successful collisions per second
Lower rate of enzyme/substrate complexes forming
Lower rate of product formation per second
Decrease in rate of reaction
What happens when the active site denatures?
The tertiary structure bonds are broken
This changes the 3D tertiary structure of the active site so no longer complimentary shape to the substrate
Do specific enzymes all work over the same range of pH?
No- each one can only work over a particular range of pH
Does each enzyme have its own optimum pH where the rate of reaction is maximum?
Yes
What do changes in pH result in?
- It affects the hydrogen and ionic bonds responsible for the tertiary structure shape of active sites
- Extremes of pH break these bonds and denature the active sites
- So it is no longer a 3D tertiary complimentary shape to the substrate
- So fewer successful collisions per second
- So fewer enzyme/ substrate complexes
- So less product per second and rate of reaction drops
If we have 10 active sites, until when is the substrate the limiting factor on rate of reaction?
Until there are 10 substrates
When does the number of active sites become the limiting factor in the rate of reaction?
- When the number of substrates exceeds the number of active sites
- At this moment, all the active sites are occupied
- The rate of reaction is at its fastest and an increase in substrate concentration does not increase the rate of reaction
- There is a maximum number of enzyme substrate complexes
Is rate of reaction directly proportional to enzyme concentration?
Yes - as enzyme concentration increases, the rate of reaction increases
What happens when all the substrates are used up in a reaction?
The reaction is finished
In living cells, what is the enzyme concentration like compared to the substrate concentration?
Enzyme concentrations are usually much lower than substrate concentration
Is substrate concentration ever a limiting factor?
Rarely
What is an inhibitor?
It is something that slows down the rate of reaction - inhibits the enzyme
What is a competitive inhibitor?
- It competes with the substrate to bind with the active site
- It has a similar 3D shape to the substrate or a similar 3D complimentary shape to the active site
What happens when a competitive inhibitor binds to the active site?
-It prevents enzyme/substrate complexes forming
- Active site is occupied so fewer products formed per second
- So lower rate of reaction
What do inhibitors help us control?
The speed/rate of reaction of the enzyme
Apart from the active site, are there any there binding sites on an enzyme?
Yes
There is a binding site/allosteric site away from the active site
What kind of inhibitors bind to the binding site?
Non-competitive inhibitors as it does not bind to the active site
What happens when a non-competitive inhibitor binds to the binding site?
- It causes a 3D shape change to the active site so it is no longer a 3D complimentary shape to the substrate
- It also prevents active site bending bonds
How does a non-competitive inhibitor lower the rate of reaction?
It prevents successful enzyme/substrate complexes forming
What happens if the non-competitive inhibitor is removed from the binding site?
The active site will return to its original 3D complimentary shape
Are these types (competitive and non-competitive) of inhibitors reversible?
Yes and they can be removed
When competitive inhibitors are present, what happens to the rate of reaction as substrate concentration is increased?
- There is an increase in probability that the competitive inhibitor will not bind to the active site
- So increasing number of enzyme/substrate complexes
- Higher rate of reaction
What can happen to the rate of reaction, with competitive inhibitors present, if we have a large enough substrate concentration?
They can out-compete the inhibitors and eventually reach the same rate of reaction as if there were no inhibitors
What happens when there is a high substrate concentration but non-competitive inhibitors are present?
- All enzyme active sites are occupied
- But the substrate molecules bound to enzymes with attached inhibitors are not converted into product
- So the maximum rates are never achieved
If we have a large enough substrate concentration with non-competitive inhibitors present, can a maximum rate of reaction ever be achieved?
- No
- The effect of the inhibitor is not overcome by increasing the substrate concentration
- Active sites do not convert substrate to product
- The effect is equal to lowering enzyme concentration
How do poisons act as inhibitors?
They are non-competitive inhibitors that bond to the binding site permanently
What kind of reactions do poisons inhibit?
They stop metabolic reactions from occurring and essentially cause someone to die
(no energy from respiration so no energy for all reactions in the body etc)
Apart from poisons, what can also act as inhibitors?
Drugs
What is allosteric regulation?
The term used to describe any case in which a protein’s function at one site is affected by binding of a regulatory molecule at another site
(e.g non-competitive inhibitors or activators)
What is end product inhibition important for?
Regulating metabolic pathways
In end product inhibition, what does the end product often act as?
A regulator
In end product inhibition, what happens when the end product concentration is high?
It binds non-competitively to an enzyme in the pathway, blocking further production of itself
(e.g. if there is an enzyme A in the beginning, it inhibits enzyme A)
In end product inhibition, what happens when the product concentration begins to fall after the product has inhibited the reaction?
The product leaves the binding site and the active site is then available
How is ATP synthesis regulated?
Through end product inhibition
