2.4 Enzymes Flashcards
What are enzymes
Biological catalysts
‘Biological’ because they function in living systems
‘Catalysts’ because they speed up the rate of chemical reactions without being used up or undergoing permanent change
What type of proteins are enzymes
globular proteins with complex tertiary structures
What do enzymes control
Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions
Virtually every metabolic reaction within living organisms is catalysed by an enzyme
Enzymes are therefore essential for life to exist
How are enzymes produced
Protein synthesis inside cells
Intracellular enzymes
Are produced and function inside the cell
Extracellular enzymes
Secreted by cells ad catalyse reactions outside cells
Catalase
Intracellular enzymes
Converts hydrogen peroxide into water and oxygen, preventing any damage to cells or tissues
Amylase
Extra cellular enzyme
Hydrolyses starch into simple sugars
What secrets amylase
Salivary glands and the pancreas
Where is starch digested
The mouth and small intestine
Trypsin
Extracellular digestive enzyme
Breaks down proteins into peptides and amino acids
What secretes trypsin
Pancreas and small intestine
Active site
where specific substrates bind forming an enzyme-substrate complex
What can change the shape of the active site
Heat or pH
Desaturation
Extremes of heat or pH can change the shape of the active site, preventing substrate binding
What must happen in order for reaction to occur
Substrates collide with enzymes active site and this must happen at correct orientation and speed
Enzymes specificity
The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
What is the shape of the active site determined by
complex tertiary structure of the protein that makes up the enzyme:
How does structure of proteins determine shape of active site
Proteins are formed from chains of amino acids held together by peptide bonds
The order of amino acids determines the shape of an enzyme
If the order is altered, the resulting three-dimensional shape changes
Enzyme-substrate complex
An enzyme-substrate complex forms when an enzyme and its substrate join together
How long is the enzyme-substrate complex formed for
only formed temporarily before the enzyme catalyses the reaction and the product(s) are released
Lock and key hypothesis
that both enzymes and substrates were rigid structures that locked into each other very precisely
Induced-fit hypothesis
The enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme
Conformational changes
The slight change in shape in order for enzyme-substrate complex to be made
What does conformational change ensure
an ideal binding arrangement between the enzyme and substrate is achieved
Why does conformational changes happen
To maximise the ability of the enzyme to catalyse the reaction
Activation energy
the amount of energy needed by the substrate to become just unstable enough for a reaction to occur and for products to be formed
How do enzymes affect activation energy
Enzymes speed up chemical reactions because they reduce the stability of bonds in the reactants
What does the destabilisation of bonds lead to
Makes it more reactive
What would happen if without enzymes
extremely high temperatures or pressures would be needed to reach the activation energy for many biological reactions
Optimum pH
PH where enzymes operate best
What happens to enzymes at extremes pH
Hydrogen and ionic bonds that hold the tertiary structure are broken which alters the shape of the active site causing denaturation
Pepsin optimum pH
2
Buffer solutions
Maintain specific pH
Effect of pH practical
Iodine added to spotting tile
Add amylase + buffer solution mix and add starch solution
Start stopwatch
After 10 seconds add a drop of solution to spotting tile
Repeat every 10 seconds
Repeat steps using buffer solutions of different pH
What should be kept the same in pH practical
Equal volume and concentration of enzyme
How could you improve ph practical
Control temperature by using water bath at 35 degrees
Enzymes optimum temperature
Temperature at which they catalyse a reaction at the maximum rate
How do lower temperatures affect reaction
prevent reactions from proceeding or slow them down
Why do lower temperatures affect enzymes activity
Molecules move relatively slowly as they have less kinetic energy
Less kinetic energy results in a lower frequency of successful collisions between substrate molecules and the active sites of the enzymes which leads to less frequent enzyme-substrate complex formation
Substrates and enzymes also collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)
How do higher temperatures affect enzyme activity
Speed up reactions
Why do higher temperatures affect enzyme activity
Molecules move more quickly as they have more kinetic energy
Increased kinetic energy results
in a higher frequency of successful collisions between substrate molecules and the active sites of the enzymes which leads to more frequent enzyme-substrate complex formation
Substrates and enzymes also collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)
How does temperature affect enzyme activity if its too high
the rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature
Why does extreme temperatures cause denaturation
Increased kinetic energy and vibration puts strain on enzyme molecules causing weaker hydrogen and ionic bonds to start to break
Breaking of bonds cause tertiary structure to change
Active site permanently damaged and its shape is no longer complementary to the substrate preventing the substrate from binding
Temperature coefficient
the ratio between the rates of that reaction at two different temperatures
Temperature coefficient equation
Temperature coefficient = (rate of reaction at (x + 10) °C) ÷ (rate of reaction at x °C)
What does a higher enzyme concentration mean
The higher the enzyme concentration in a reaction mixture, the greater the number of active sites available and the greater the likelihood of enzyme-substrate complex formation
How does enzyme concentration affect rate of reaction
As long as there is sufficient substrate available, the initial rate of reaction increases linearly with enzyme concentration
If the amount of substrate is limited, at a certain point any further increase in enzyme concentration will not increase the reaction rate as the amount of substrate becomes a limiting facto
what does a higher substrate concentration mean
The greater the substrate concentration, the higher the rate of reaction
Why does a higher substrate concentration increase rate of reaction
As the number of substrate molecules increases, the likelihood of enzyme-substrate complex formation increases
How does increased substrate concentration affect rate of reaction
Increases however If the enzyme concentration remains fixed but the amount of substrate is increased past a certain point, however, all available active sites eventually become saturated and any further increase in substrate concentration will not increase the reaction rate
When the active sites of the enzymes are all full, any substrate molecules that are added have nowhere to bind in order to form an enzyme-substrate complex
Reversible inhibitor
can reduced or stop enzyme activity temporarily
Two types of reverse inhibitors
Competitive and non-competitive
Competitive inhibitor
have a similar shape to that of the substrate molecules and therefore compete with the substrate for the active site
Non-competitive inhibitor
bind to the enzyme at an alternative site, which alters the shape of the active site and therefore prevents the substrate from binding to it
What happens if you increase concentration of an inhibitor
reduces the rate of reaction and eventually, if inhibitor concentration continues to be increased, the reaction will stop completely
What happens if you increase the substance concentration to counter the competitive inhibitor
increasing the substrate concentration can increase the rate of reaction once more (more substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes)
What happens if you increase the substance concentration to counter non-competitive inhibitors
increasing the substrate concentration cannot increase the rate of reaction once more, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form
What can reversible inhibitors act as
regulators in metabolic pathways
End-product inhibition
End-product of reaction binds to alternative site on enzymes preventing esc’s being formed
The end product can then detach allowing active site to reform
Non-reversible inhibitors
form covalent bonds with enzymes, inhibiting them permanently
Why are non-reversible inhibitors dangerous
can cause the biological reaction the enzyme is catalysing to be completely stopped
What is the only way to avoid biological reaction the enzyme is catalysing to be stopped by non-reversible inhibitors
produce more of the enzyme being inhibited, which can only be achieved by transcribing and translating the gene(s) for that enzyme, which is a relatively slow process
Cofactors
Non-protein substance that changes enzymes tertiary structure which allows enzyme to function properly
Cofactor for amylase
Chloride ions
inorganic cofactor
Particular inorganic ions may help to stabilise the structure of the enzyme or may actually take part in the reaction at the active site
Coenzymes
Organic non-protein cofactors
What do coenzymes do
link different enzyme-catalysed reactions into a sequence during metabolic processes, such as photosynthesis and respiration
An example of a source of coenzymes
Vitamins
Prosthetic groups
Cofactors that are a permanent part of the structure of the enzymes they assist
Why are profactors essential to enzymes functioning properly
they help to form the final 3D shape of the enzyme
How can enzyme catalysed reactions be investigated
Measuring the rate of formation of a product
Measuring the rate of disappearance of a substrate
Rate of product formation investigation
Catalase is an enzyme found in the cells of most organisms that breaks down hydrogen peroxide into water and oxygen
Hydrogen peroxide and catalase are combined and the volume of oxygen generated is measured in a set time
The rate of reaction can then be calculated
Investigating amylase activity using iodine
Rate of substrate disappearance
Amylase and starch are combined and this reaction mixture is then tested for starch at regular time intervals
This can be done by taking samples from the reaction mixture at each time interval and adding each sample to some iodine in potassium iodide solution
Starch forms a blue-black colour with this solution
If no starch is present, the iodine solution remains yellow-brown
In this way, the time t
Amylase
digestive enzyme that hydrolyses starch into maltose and glucose
