2.1.4 - Enzymes

Question 1

What are enzymes?

Answer 1

They are biological catalysts made of globular proteins

Question 2

What do enzymes do?

Answer 2

They speed up rates of reaction but remain unchanged and can be used repeatedly

Question 3

What is an example of an intracellular reaction catalysed by enzymes?

Answer 3

Catalase is an intracellular enzyme inside liver cells that breaks down hydrogen peroxide into oxygen and water

Question 4

What are 2 examples of extracellular reactions catalysed by enzymes?

Answer 4

Trypsin is an extracellular enzyme in the small intestine that hydrolyses proteins. Amylase is an extracellular enzyme that breaks down starch into maltose.

Question 5

How do enzymes catalyse a reaction?

Answer 5

By lowering the activation energy which is the amount of energy needed for a reaction to occur

Question 6

How are enzymes specific to a particular substrate?

Answer 6

Their active site is specific and unique in shape due to the specific folding and bonding in the tertiary structure of the protein

Question 7

How does the lock and key model explain how enzymes work?

Answer 7

• 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

Question 8

How does the induced fit model explain how enzymes work?

Answer 8

• 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

Question 9

Which conditions affect the rate of enzyme controlled reactions?

Answer 9

• Temperature
• pH
• Enzyme concentration
• Substrate concentration

Question 10

How does temperature affect the rate of enzyme controlled reactions?

Answer 10

• 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

Question 11

What is the Q₁₀ temperature coefficient?

Answer 11

A measure of the rate of change of an enzyme controlled reaction as a result of increasing the temperature by 10°C

Question 12

What is the formula for the temperature coefficient?

Answer 12

R1/R2
R1 = rate of reaction at a temperature of X°C
R2 = rate of reaction at a temperature of (X + 10)°C

Question 13

How does pH affect the rate of enzyme controlled reactions?

Answer 13

• 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

Question 14

How does enzyme concentration affect the rate of enzyme controlled reactions?

Answer 14

• 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

Question 15

How does substrate concentration affect the rate of enzyme controlled reactions?

Answer 15

• 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

Question 16

What do coenzymes and cofactors do?

Answer 16

• 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

Question 17

What is a coenzyme?

Answer 17

An organic molecule that when present increases the activity of an enzyme

Question 18

What is an example of a coenzyme?

Answer 18

Vitamins

Question 19

What is a cofactor?

Answer 19

An inorganic ion

Question 20

What is an example of a cofactor?

Answer 20

Chloride ions assist amylase reactions

Question 21

What is a prosthetic group?

Answer 21

A tightly bound non amino acid component necessary for enzyme activity

Question 22

What is an example of a prosthetic group?

Answer 22

Zinc ions on carbonic anhydrase

Question 23

What are the 2 types of enzyme inhibitors?

Answer 23

• Competitive inhibitors
• Non-competitive inhibitors

Question 24

How do competitive inhibitors work?

Answer 24

• 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

Question 25

Are competitive inhibitors reversible?

Answer 25

Most are reversible

Question 26

How do non-competitive inhibitors work?

Answer 26

• 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

Question 27

What is end product inhibition?

Answer 27

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.

Question 28

How can enzyme inhibitors lead to illness/fatality?

Answer 28

• 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

Question 29

How can enzyme inhibitors be used in medicines to treat diseases?

Answer 29

• Penicillin is an enzyme inhibitor that interferes with cell wall production causing bacterial cells to burst

Question 30

What are inactive precursors?

Answer 30

Non-working enzymes that are synthesised to prevent cell damage

Question 31

How are precursors activated?

Answer 31

The binding of a cofactor causes a change in the tertiary structure so that the active site becomes complementary in shape to its substrate

Question 32

What is the precursor protein known as?

Answer 32

Apoenzyme

Question 33

What is the activated precursor known as?

Answer 33

Holoenzyme