Enzymes

Question 1

Define “enzyme”

Answer 1

Biological catalysts that increase the rate of reaction by lowering activation enthalpy

Question 2

Define “activation enthalpy”

Answer 2

Amount of energy needed for a reaction to occur

Question 3

What is activation enthalpy for? 2

Answer 3

• To make substrates into products, energy is needed to break the chemical bonds between substrates and start reaction
• Heat energy agitates atoms within molecules, they become unstable, reaction starts

Question 4

Why is it that enzymes reducing activation enthalpy is useful in body cells? 2

Answer 4

• More collisions of substrate particles have sufficient energy for a successful reaction
• Allows reactions to occur at cellular temperature much faster

Question 5

How do enzymes interact with substrates? 2

Answer 5

• Substrate fits into enzyme active site

- Forms enzyme-substrate complex

Question 6

What 2 types of reaction do enzymes catalyse?

Answer 6

• Breakdown of molecules

- Joining together of molecules

Question 7

How do enzymes catalyse the breakdown of molecules? 2

Answer 7

• Fitting into the active site puts strain on substrate bonds
• Substrate molecule breaks up more easily

Question 8

How do enzymes catalyse the joining together of molecules? 4

Answer 8

• 2 substrate molecules attached to the enzyme
• Holds them closer together
• Minimises repulsion between molecules
• Bonds form more easily

Question 9

Define “active site”

Answer 9

The part of the enzyme that binds to the substrate to form an enzyme-substrate complex

Question 10

Where levels do enzymes catalyse a reaction at?

Answer 10

• Cellular level e.g respiration

- Organism as a whole e.g digestion

Question 11

What 2 things do enzymes affect?

Answer 11

• Structures

- Functions

Question 12

What are the 2 models of enzyme theory?

Answer 12

• Lock and Key

- Induced Fit

Question 13

Explain the lock and key model 4

Answer 13

• Substrate has complementary shape and charge to active site
• Substrate binds to active site and forms an
  enzyme-substrate complex
• Substrate converted to product
• Product no longer complementary in shape and charge and so is released

Question 14

Explain the induced fit model 4

Answer 14

• Substrate binds to active site
• Active site shape changes - closer fit between
  active site and substrate
• More bonds form between substrate and
  active site
• Forms enzyme-substrate complex

Question 15

Why is each enzyme specific to its function? 3

Answer 15

• Tertiary structure of the active site
• Complimentary to the shape of the substrate
• Forms specific enzyme-substrate complex

Question 16

How does a mutation make an enzyme not functional? 5

Answer 16

• Changes primary structure
• Changes tertiary structure
• Changes shape of active site
• Substrate no longer complimentary
• No enzyme - substrate complex can be formed

Question 17

How does the enzyme denaturing make it not functional? 5

Answer 17

• Changes ionic bonds
• Changes tertiary structure
• Changes shape of active site
• Substrate no longer complimentary
• No enzyme-substrate complex formed

Question 18

What 4 things affect the rate of reaction for enzyme controlled reactions?

Answer 18

• Temperature
• pH
• Substrate concentration
• Enzyme concentration

Question 19

Define “denatured”

Answer 19

Permanent change to the active site so no more E-S complexes are formed

Question 20

Explain how increasing temperature up to optimum increases enzyme rate of reaction - 4

Answer 20

• Increase in temperature: increase in molecules vibrating
• More collisions
• Higher number of collisions with activation energy
• More E-S complexes are formed

Question 21

Explain how increasing temperature past optimum decreases rate of reaction - 4

Answer 21

• Molecules vibrate and break internal bonds e.g ionic and disulphide bridges
• Changes shape of active site
• No E-S complexes can be formed
• Enzyme is denatured

Question 22

Explain how changes in pH affects rate of enzyme reaction 7

Answer 22

• Enzymes have an optimum pH
• If pH is increased/decreased away from optimum enzyme activity decreases
• The OH- group of alkali or H+ of an acid will interact with hydrogen/ionic bonds
• Changes shape of tertiary structure
• Changes shape of active site
• Fewer/no E-S complexes formed
• Enzyme is denatured

Question 23

How does increase in enzyme concentration affect rate of reaction? 8

Answer 23

• At first, increase in enzyme concentration : increase in rate of reaction: positive correlation
• More enzyme molecules : more active sites
• More likely for substrate molecules to collide with an active site to form enzyme-substrate complex
• But after optimum increase in enzyme concentration has no effect on rate of reaction
• Amount of substrate is limited so there are more than enough enzyme molecules
• Adding more enzyme molecules has no effect and no more enzyme-substrate complexes formed
• As substrate molecules go down, products go up
• Products formed hit the enzymes and stop substrate reacting with them

Question 24

How does increase in substrate concentration affect rate of reaction?

Answer 24

• At first: higher substrate concentration: increased rate of reaction: positive correlation
• More substrate molecules: more collisions between substrate and enzyme
• More active sites used: more enzyme-substrate complexes are formed
• After the optimum the rate of reaction decreases
• All active sites are occupied
• No E-S complexes can be formed
  Adding more substrate will make no difference as enzyme concentration is the limiting factor

Question 25

What is an intracellular enzyme?

Answer 25

Enzyme catalysing reactions within the cell

Question 26

What is an extracellular enzyme? 3

Answer 26

• Enzymes all made within cells in transcription and translation
• Exported by exocytosis
• To catalyse reactions outside the cells