Module 2 Section 4: Enzymes Flashcards
What do enzymes do
Enzymes are globular proteins that speed up chemical reactions by acting as biological catalysts to lower activation energy
They catalyse metabolic reactions - both at cellular level ( e.g. respiration ) and for the organism as a whole ( e.g. digestion in mammals )
How do enzymes effect structures and functions in organisms
Enzymes can affect structures in an organism
E.g. enzymes are involved in the production of collagen, an important protein in the connective tissues of animals )
Can also affect functions ( e.g. respiration )
Where can enzyme action take place in the body
Enzyme action can be intracellular -within cells, or extra cellular - outside cells
Give an example of intracellular enzyme action
Catalase
Hydrogen peroxide (H2O2) is the toxic product of several cellular reactions
If left to build up it can kill cells
Catalase is an enzyme that works inside cells to catalyse the breakdown of hydrogen peroxide to harmless oxygen (O2) and water (H2O)
Give an example of extra-cellular enzyme action
Amylase and trypsin
Both work outside cells in human digestive system
Properties of enzymes
Globular proteins
Have an active site, which has specific shape
The active site is the part of the enzyme that the substrate molecules (substance the enzyme interacts with) bind to
Specific shape is determined by the enzymes’ tertiary structure
For the enzyme to work, the substrate has to fit into the active site (it’s shape has to be complementary)
If the substrate shape doesn’t match the active site, the reaction won’t be catalyses
This means that enzymes work with very few substrates - usually only one
What is activation energy
In a chemical reaction, a certain amount of energy needs to be supplied to the chemical before the reaction
This is called the activation energy - its often provided as heat
How do enzymes lower activation energy
Enzymes reduce the amount of activation energy that’s needed, often making reactions happen at a lower temperature than they could without an enzyme
This speeds up the rate of reaction
When a substance binds to an enzymes active site, an enzyme-substrate complex is formed
It’s the formation of the enzyme substrate complex that lowers the activation energy
How do enzymes cause the formation or breakdown of substrates
If two substrate molecules need to be joined, attaching to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily
If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate
This strain means the substrate molecule breaks up more easily
Explain the lock and key model
Enzymes only work with substrates that fit their active site
The lock and key model is where the substrate fits into the enzyme in the same way a key fits into a lock
Enzyme + substrate -> enzyme substrate complex -> enzyme product complex -> enzyme is unchanged after the reaction and products are released
How does the induced fit model work
The substrate must be the right shape to fit the active site and the active site must change shape in the right way when it binds
Enzyme + substrate -> enzyme substrate complex ( as substrate binds, active site changes shape slightly to fit substrate more closely ) -> enzyme product complex -> products released
How does temperature affect enzyme activity
The rate of an enzyme controlled reaction increases with temperature
More heat means more kinetic energy, so molecules move faster
Makes enzymes more likely to collide with the substrate molecules
Energy of these collisions also increases, which means each collision is more likely to result in a reaction
However, if the temperature gets too high, the reaction stops
What happens to enzymes when the temperature gets too high
The rise in temperature makes the enzyme’s molecules vibrate more
If the temperature goes above a certain level, this vibration breaks some of the bonds that hold the enzyme in shape
The active site changes shape and the enzyme and substrate no longer fit together
At this point, the enzyme is denatured - it no longer functions as a catalyst
How does pH affect enzyme activity
All enzymes have an optimum pH value
Most human enzymes work best at pH 7 ( neutral )
However, for example, pepsin works best at pH 2 as it works in the stomach
How do enzymes become denatured with different pHs
Above and below the optimum pH, the H+ and OH- ions found in alkalis and acids can mess up the ionic bonds and hydrogen bonds that hold the enzyme’s tertiary structure in place
This makes the active site change shape, so the enzyme is denatured
How does enzyme concentration affect the rate of reaction
The more enzyme molecules in a solution, the more likely a substrate molecule is to collide with one and form an enzyme substrate complex
Increasing the concentration of the enzyme increases the rate of reaction
However, if the amount of substrate is limited, eventually there is more than enough enzyme molecules than available substrate, so adding more enzyme has no further effect
How does substrate concentration affect rate of reaction
This affects substrate concentration up to a point
Higher the substrate concentration, the faster the reaction
More substrate molecule means a collision between substrate and enzyme is more likely and so more active sites will be used
Only true up to saturation point where all active sites are already being used so rate of reaction will not increase if more substrate is added
Substrate concentration decreases with time during a reaction ( unless more is adding to the reaction mixture ), so if no other variables are changed, the rate of reaction will decrease over time too
This makes the initial rate of reaction right at the start the highest rate of reaction
Function of amylase
Amylase ( found in saliva ) is secreted into the mouth by cells in the salivary glands
It catalyses the hydrolysis of starch and maltose in the mouth
Function of trypsin
Trypsin catalyses the hydrolysis of peptide bonds - turning big polypeptides into smaller ones ( which then get broken down into amino acids by other enzymes )
Trypsin is produced by the pancreas and secreted into the small intestine
How can enzyme activity be prevented
Enzyme activity can be prevented by enzyme inhibitors - molecules that bind to the enzyme that they inhibit
Types of inhibition
Inhibition can be competitive or non-competitive
How do competitive inhibition molecules prevent enzyme action
Competitive inhibition molecules has a similar shape to that of the substrate molecules
They compete with the substrate molecules to bind to the active, but no reaction takes place
Instead they block the active site, so no substrate molecules can fit in it
How do concentrations of inhibitors and substrates affect the enzyme activity for competitive inhibition
How much the enzyme is inhibited depends on the relative concentration of the inhibitor and the substrate
If there’s a high concentration of the inhibitor, it’ll take up nearly all the active sites and hardly any of the substrate will get to the enzyme
If there’s a higher concentration of substrate, then the substrate’s chances of getting to an active site before the inhibitor increase.
So increasing the concentration of substrate will increase the rate of reaction (up to a point)
How do non-competitive inhibitors affect enzyme activity
Non competitive inhibitor molecules bind to the enzyme away from its active site
The site they bind to is known as the enzymes’ allosteric site
This causes the active site to change shape so the substrate molecules can no longer bind to it
They don’t compete with the substrate molecules to bind to the active site because they are a different shape
How do concentrations of substrate affect the enzyme activity for non-competitive inhibition
Increasing the concentration of substrate won’t make any difference to the reaction rate - enzyme activity will still be inhibited
Different types of inhibitions and what does this depend on
Inhibition can be reversible or non-reversible
Which one they are depends of the strength of the bonds between the enzyme and the inhibitor
What makes a inhibitor irreversible
If they’re strong, covalent bonds, the inhibitor can’t be removed easily and the inhibition is irreversible
What makes a inhibitor reversible
If they’re weaker hydrogen bonds or weak ionic bonds, the inhibitor can be removed and the inhibition is reversible
How can inhibitors be used in antiviral drugs
Some antiviral drugs ( drugs that stop viruses )
E.g. reverse transcriptase inhibitors inhibit the enzyme reverse transcriptase, which catalyses the replication or viral DNA
This prevents the virus from replicating
How can inhibitors be used in antibiotics
Antibiotics e.g. penicillin inhibits the enzyme transpeptidase, which catalyses the formation of proteins in bacterial cell walls
This weakens the cell wall and prevents the bacterium from regulating its osmotic pressure
As a result the cell bursts and the bacterium is killed
What are metabolic poisons
Metabolic poisons interfere with metabolic reactions (the reactions that occur in cells), causing damage, illness or death - they’re often enzyme inhibitors
Types of metabolic poison that use enzyme inhibitors
Cyanide is an irreversible inhibitor of cytochrome c oxidase, an enzyme that catalyses respiration reactions
Cells that can’t respire will die
Malonate inhibits succinate dehydrogenase (which also catalyses respiration reactions)
Arsenic inhibits the action of pyruvate dehydrogenase, another enzyme that catalyses respiration reaction
What are enzyme cofactors
Some enzymes will only work if there is another non-protein substance bound to them
These non-proteins substances are called cofactors
How do inorganic molecules or ions act as cofactors
Some cofactors are inorganic molecules or ions
They work by helping the enzyme and substrate to bind together
They don’t directly participate in the reaction so aren’t used up or changed in any way
E.g. chloride ions are cofactors for the enzyme amylase
How do organic molecules or ions act as coenzymes
Some cofactors are organic molecules - these are called coenzymes
They participate in the reaction and are changed by it (like a second substrate)
They often act as carriers, moving chemical groups between different enzymes
They’re continually recycled during this process
Vitamins are often sources of coenzymes
What are prosthetic groups on enzymes
If a cofactor is tightly bound to the enzyme, it’s known as a prosthetic group and forms a permanent feature of the enzyme
E.g. zinc ions are a prosthetic group for carbonic anhydrase (an enzyme in red blood cells), which catalyses the production of carbonic acid (from water and carbon dioxide)
The zinc ions are a permanent part of the active site
What is a metabolic pathway
A metabolic pathway is a series of connect metabolic reactions
The product of the first reaction takes part in the second reaction - and so on
Each reaction is catalysed by a different enzyme
What is product inhibition
Many enzymes are inhibited by the product of the reaction they catalyse
This is known as product inhibition
What is end product inhibition
End product inhibition is when the final product in a metabolic pathway inhibits an enzyme that acts earlier in the pathway
This is a good way of regulating the pathway and controlling the amount of end product that gets made
Example of end product inhibition
E.g. phosphofructokinase is an enzyme involved in the metabolic pathway that breaks down glucose to make ATP
ATP inhibits the action of phosphofructokinase - so a high level of ATP prevent more ATP from being made
How does end product inhibition work
Both product and end-product inhibition are reversible
So when the level of product starts to drop, the level of inhibition will start to fall and the enzyme can start to function again - this means that more product can be made
How can enzyme inhibition be used to protect cells
Enzymes can be synthesised as inactive precursors in metabolic pathways to prevent them causing damage to cells
E.g. some proteases are synthesised as inactive precursors to stop them damaging proteins in the cell in which they’re made
Part of the precursor molecule inhibits its action as an enzyme
Once this part is removed (e.g. via a chemical reaction) the enzyme becomes active
What is precursor activation
Many enzymes are produced in an inactive form
These are known as inactive precursors enzymes
These can be enzymes which can cause damage within the cells that are producing them
Or enzymes whose actions must be controlled and only activated under certain conditions
How does precursor activation occur with cofactors
Precursor enzymes often need to undergo a change in shape of their active site to be activated
Can be achieved through addition of a cofactor
The precursor protein is called an apoenzyme before the cofactor is added
It’s then called a holoenzyme when the cofactor is added and the enzyme is activated
How does precursor activation occur with actions of another enzyme or a change in conditions
The change in tertiary structure can be caused by the action of another enzyme, such as protease
This can cleave certain bonds in the molecules
A change in pH or temperature can change the tertiary structure and activates a precursor enzyme
These precursor enzymes are called zymogens or proenzymes
Graph for how enzyme conc affects rate of reaction
Graph for how substrate conc affects rate of reaction
What to talk about when asked for process of enzyme action
Describe induced fit
ESC lowers activation energy
What is the temperature coefficient
A measure of how much the reaction rate increases with a 10°C increase in temperature
Rₜ: rate at higher temperature
Rₜ₋₁₀: rate a temperature 10° lower
What is the Q₁₀ value in a biological system
Q₁₀ is equal to around 2
