2.1.4 - Enzymes Flashcards
What are enzymes?
They are biological catalysts made of globular proteins
What do enzymes do?
They speed up rates of reaction but remain unchanged and can be used repeatedly
What is an example of an intracellular reaction catalysed by enzymes?
Catalase is an intracellular enzyme inside liver cells that breaks down hydrogen peroxide into oxygen and water
What are 2 examples of extracellular reactions catalysed by enzymes?
Trypsin is an extracellular enzyme in the small intestine that hydrolyses proteins. Amylase is an extracellular enzyme that breaks down starch into maltose.
How do enzymes catalyse a reaction?
By lowering the activation energy which is the amount of energy needed for a reaction to occur
How are enzymes specific to a particular substrate?
Their active site is specific and unique in shape due to the specific folding and bonding in the tertiary structure of the protein
How does the lock and key model explain how enzymes work?
- The shape of the active site on an enzyme is complementary to the substrate molecule
- The substrate fits into the fixed shape active site exactly on collision
- An enzyme substrate complex is formed
- The charged groups within the active site are thought to distort the substrate which lowers the activation energy
- The reaction occurs
How does the induced fit model explain how enzymes work?
- The shape of the active site of an enzyme is not fully complementary to the substrate molecule
- When the substrate molecule collides with the active site the enzyme molecule changes shape slightly to fit the active site around the substrate
- An enzyme substrate complex is formed which puts a strain on the bonds lowering the activation energy
- The reaction occurs
Which conditions affect the rate of enzyme controlled reactions?
- Temperature
- pH
- Enzyme concentration
- Substrate concentration
How does temperature affect the rate of enzyme controlled reactions?
- Enzymes have different optimum temperatures which are the temperatures at which the enzymes work at their maximum rate
- If the temperature is too low there is insufficient kinetic energy for successful collisions
- If the temperature is too high this causes bonds to break and the tertiary structure alters meaning the enzyme denatures, the active site changes shape and enzyme substrate complexes cannot form
What is the Q₁₀ temperature coefficient?
A measure of the rate of change of an enzyme controlled reaction as a result of increasing the temperature by 10°C
What is the formula for the temperature coefficient?
R1/R2
R1 = rate of reaction at a temperature of X°C
R2 = rate of reaction at a temperature of (X + 10)°C
How does pH affect the rate of enzyme controlled reactions?
- Enzymes have different optimal pHs which are the pHs at which the enzymes can work at their maximum rate
- Too high or too low a pH will interfere with the charges in the amino acids in the active site. This causes ionic and hydrogen bonds to break which alters the tertiary structure and changes the shape of the active site and the enzyme denatures
How does enzyme concentration affect the rate of enzyme controlled reactions?
- At low enzyme concentrations there will be a lower rate of reaction
- Increasing the enzyme concentration will increase the rate of reaction as enzyme substrate complexes will be more likely to form
- At high enzyme concentrations unless unlimited substrate is added the rate of reaction plateaus as there will be insufficient substrate to bind with the large number of enzymes so substrate concentration will become the limiting factor
How does substrate concentration affect the rate of enzyme controlled reactions?
- If there is a low concentration of substrate the reaction will be lower as there will be fewer collisions between the enzyme and substrate
- Increasing the substrate concentration will increase the rate of reaction as there will be a greater number of enzyme substrate complexes formed
- At high substrate concentrations the rate of reaction will plateau because the enzymes are saturated meaning all the active sites are in use so enzyme concentration becomes the limiting factor
What do coenzymes and cofactors do?
- Carry atoms from one reaction to the next in multi step pathway reactions
- Bind to the active site to make it complementary to the substrate
What is a coenzyme?
An organic molecule that when present increases the activity of an enzyme
What is an example of a coenzyme?
Vitamins
What is a cofactor?
An inorganic ion
What is an example of a cofactor?
Chloride ions assist amylase reactions
What is a prosthetic group?
A tightly bound non amino acid component necessary for enzyme activity
What is an example of a prosthetic group?
Zinc ions on carbonic anhydrase
What are the 2 types of enzyme inhibitors?
- Competitive inhibitors
- Non-competitive inhibitors
How do competitive inhibitors work?
- They are the same shape as the substrate and complementary in shape to the active site so they compete for and bind to the active site
- An active site blocked by the competitive inhibitor prevents the substrate from binding and forming an enzyme substrate complex so the rate of reaction is lower
Are competitive inhibitors reversible?
Most are reversible
How do non-competitive inhibitors work?
- They bind to a non-functional part of the enzyme away from the active site called the allosteric site
- This causes the active site to change shape so the substrate can no longer bind and enzyme substrate complexes cannot form so the rate of reaction is lower
What is end product inhibition?
Metabolic pathways are made up of a chain of reactions each controlled by enzymes. Enzyme inhibitors play a role in controlling these reactions. If there is a lot of product present the final product will inhibit the enzymes and cause the reaction to slow or stop which prevents resources from being wasted.
How can enzyme inhibitors lead to illness/fatality?
- Potassium cyanide is an irreversible competitive inhibitor of the enzyme cytochrome C oxidase which is part of the electron transport chain of respiration
- Carbon monoxide is a competitive inhibitor with oxygen for haemoglobin which makes it potentially fatal
How can enzyme inhibitors be used in medicines to treat diseases?
- Penicillin is an enzyme inhibitor that interferes with cell wall production causing bacterial cells to burst
What are inactive precursors?
Non-working enzymes that are synthesised to prevent cell damage
How are precursors activated?
The binding of a cofactor causes a change in the tertiary structure so that the active site becomes complementary in shape to its substrate
What is the precursor protein known as?
Apoenzyme
What is the activated precursor known as?
Holoenzyme
