3.1.3 Biochemical Reactions in Cells are Controlled by Enzymes

Question 1

Enzymes can affect ______ and _____ in organisms

Answer 1

Enzymes can affect structures and functions in organisms

Question 2

Enzymes are highly specific due to their ____ _______

Answer 2

tertiary structure

Question 3

Explain how forming an enzyme-substrate complex lowers activation energy

Answer 3

1. If 2 substrate molecules need to be joined:
   
   1. Being attached to the enzyme holds them close together = reducing any repulsion between molecules
   2. ∴ they bond more easily
2. If enzyme is catalysing a breakdown reaction:
   
   1. Fitting into the active site puts a strain on bonds in substrate
   2. ∴ substrate molecule breaks up more easily

Question 4

How is a substrate molecule held within an active site?

Answer 4

By bonds that form temporarily between amino acids of active site and groups on substrate molecules

Question 5

Describe the ‘lock and key’ model

Answer 5

Where substrate fits into enzyme in the same way a key fits into a lock

Question 6

What is the problem with the ‘lock and key’ model?

Answer 6

Too simplistic = enzyme-substrate complex actually CHANGES shape slightly to complete the fit

Question 7

What model is better than the ‘lock and key’ model?

Answer 7

‘Induced Fit’ Model

Question 8

Why do enzymes only bond to one particular substrate?

Answer 8

∵ substrate has to be right shape (complementary) to fit active site & has to make active site CHANGE SHAPE

Question 9

Why are enzymes very specific?

Answer 9

∵ only one complementary substrate will fit into their active site

Question 10

Each enzyme has a different tertiary structure ∴ …

Answer 10

It has a different shaped active site

Question 11

How might a mutation in a gene affect the stucture of an enzyme?

Answer 11

• Primary structure is determined by gene
• If mutation occurs in gene = could change tertiary structure of enzyme produced

Question 12

How can a change in the primary structure affect an enzyme?

Answer 12

• Sequence of amino acids changes
• Bonds form in different places as it alters hydrogen, disulfide, ionic bonds
• Tertiary structure changes

Question 13

Describe how tertiary structure of a protein determines the shape of an active site (3)

Answer 13

• Primary structure: unique sequence of amino acids
• Different arrangement of R groups determine how it’ll fold into the secondary structure & then tertiary structure
• Tertiary structure determines 3D shape & shape of active site

Question 14

Explain in terms of primary structure why an enzyme is usually specific to one substrate (4)

Answer 14

• Primary structure: unique sequence of amino acids
• Different arrangement of R groups determine how it’ll fold into the secondary structure & then tertiary structure
• Tertiary structure = specific 3D shape & active site is apart of this shape (made up of around 10 amino acids)
• Certain R groups will be exposed that allow substrate to bind

Question 15

Explain why increasing the temperature increases rate of reaction (enzyme activity)?

Answer 15

• More heat = more kinetic energy so molecules move faster
  
  • ∴ enzymes more likely to collide with substrate molecules
  • Energy of collisions increase = collisions more likely to result in reaction (be successful)
• More enzyme-substrate complexes form

Question 16

What happens to the rate of reaction when the temperature is too low (enzyme activity)?

Answer 16

• Little kinetic energy = little movement
• Substate won’t collide with active site = rate of reaction decreases
• Less enzyme-substrate complexes form

Question 17

Explain what happens to enzymes when they get too hot

Answer 17

• Lots of kinetic energy = enzymes vibrate too much
• This breaks some of bonds (e.g. weaker hydrogen/ionic) that hold enzyme in shape
• Changes in tertiary structure
• Active site changes shape
• Enzyme substrate complex can’t form
• Enzyme = denatured

Question 18

What happens when an enzyme is placed in a solution that is above or below its optimum pH?

Answer 18

• H⁺ and OH^- ions found in acids/alkalis form extra/break ionic & hydrogen bonds that hold enzyme’s tertiary structure
  
  • Active site change shape ∴ enzyme = denatured
• (Happens ∵ ions can bind to any negatively charged R groups active site)

Question 19

Describe and explain how increasing the substrate concentration affects the rate of a reaction

Answer 19

1. Increases rate of reaction
   
   • ∵ More substrate molecules = more collisions between substrate & enzymes = more enzyme substrate complexes
2. BUT up to a ‘saturation’ point where it then levels off
   
   • ∵ active sites are full

Question 20

If the substrate concentration decreases with time then the rate of reaction will…

Answer 20

decrease over time

Question 21

Why does the rate of reaction decrease over time?

Answer 21

∵ fewer substrate molecules to collide with enzymes & products may ‘get in the way’ of substates reaching active site

Question 22

The initial rate of reaction is the…

Answer 22

highest rate of reaction

Question 23

Why does increasing the concentration of enzymes increase the rate of the reaction?

Answer 23

More enzymes molecules in solution = more likely for substrate molecules to collide with one & form enzyme-substrate complex

Question 24

What happens to rate of reaction if the amount of a substrate is limited, when increasing the enzyme concentration, & why?

Answer 24

Comes to a point where there’s more than enough enzymes to deal with available substrates = so rate of reaction levels off

Question 25

How can enzyme activity can be prevented?

Answer 25

By enzymes inhibitors

Question 26

Describe how a competitive inhibitor prevents enzyme activity

Answer 26

• Competitive inhibitor molecules have similar shape of substrate molecules
• Compete with substate molecules to bind to active site
• They block the active site = no substrate molecules can fit
• Less enzyme substrate complexes can form

Question 27

How much enzyme is inhibited depends on…

Answer 27

relative concentration of inhibitor and substrate

Question 28

Competitive Inhibition

What happens if you increase the concentration of the inhibitor?

Answer 28

• They take up nearly all active sites (little substate will get to enzyme)
• Decreases rate of reaction

Question 29

Competitive Inhibition

What happens if you increase the concentration of the substrate?

Answer 29

• Substrate’s chance of getting to active site before inhibitor increase
• Increases rate of reaction

Question 30

Sketch a graph to show how increasing the substrate concentration affects the rate of the reaction

Answer 30

Question 31

Sketch a graph to show how increasing the enzyme concentration affects the rate of the reaction

Answer 31

Question 32

Describe how a non-competitive inhibitor affects an enzyme

Answer 32

1. Non-competitive inhibitor binds to enzyme away from its active site
2. This causes active site to change shape so substate molecules can’t bind to it
3. i.e. Tertiary structure changes
4. No enzyme substrate complexes can form

Question 33

Why are non-competitive inhibitors different shapes?

Answer 33

∵ they don’t compete

Question 34

Non-Competitive Inhibition

Does increasing the concentration of substrate affect the rate of the reaction?

Answer 34

NO

Question 35

Sketch graph showing an enzyme-controlled reaction without an inhibitor and the same reaction with a competitive inhibitor

Answer 35

Question 36

Sketch graph showing an enzyme-controlled reaction without an inhibitor and the same reaction with a non-competitive inhibitor

Answer 36

Question 37

Name 2 ways you can measure the rate of an enzyme-controlled reaction

Answer 37

1. You can measure how fast the product of reaction is made
2. You can measure how fast the substrate is broken down

Question 38

How can you investigate the effect of pH on an enzyme-controlled reaction? (brief answer needed)

Answer 38

By adding a buffer solution with a different pH to each test tube

Question 39

How can you investigate the effect of substrate concentration on an enzyme-controlled reaction? (brief answer needed)

Answer 39

Use serial dilutions to make substrate solutions with different concentrations

Question 40

Casein is protein found in milk & _____ is the enzyme that digests it

Answer 40

Trypsin

Question 41

When trypsin is added to a _____ _____ of ___ ____, casein is digested and solution goes ____

Answer 41

When trypsin is added to a dilute solution of milk powder, casein is digested and solution goes clear

Question 42

Describe how you can investigate the effect of temperature on the rate of an enzyme-controlled reaction

Answer 42

1. Draw ‘X’ halfway down each of the 3 test tubes
2. Add 10 cm³ of solution of milk powder to each test tube
3. Add 2cm³ of trypsin & 2cm³ of pH 7 buffer to another 3 test tubes
4. Put all 6 test tubes into water bath at 20°C for 10 minutes
5. Add trypsin and buffer solution to solution of milk powder
   
   1. Place bung in them and invert it 5 times
6. Place it back into water bath & time how long it takes for milk to clear (i.e. until you see the ‘X’)
   
   1. Record this time
7. Use same method to find out how long it takes trypsin to digest the protein at 30°C, 40°C, 50°C, 60°C

Question 43

When interpreting graphs about enzyme-controlled reactions, what should you look at first?

Answer 43

The initial rate of reactions

Question 44

At 37°C, why does the graph plateau?

Answer 44

∵ all the substrate has been used up

Question 45

At 65°C, why does the graph plateau earlier than 37°C?

Answer 45

• ∵ the high temperature = enzyme denatured so reaction stopped sooner
• Not as much product was made ∵ not all substrate was converted to product (before the enzyme was denatured) so there’s still substrate left

Question 46

What is happening at 25°C and why?

Answer 46

Rate of reaction is remaining constant and the volume of product is continuing to increase ∵ not all of the substrate has been used up

Question 47

Describe how you would calculate the initial rate of a reaction

Answer 47

Question 48

Explain why maltase only catalyses the hydrolysis of maltose (3)

Answer 48

1. Active site of maltase has specific shape/tertiary structure which is complementary to maltose
2. Only maltose can bind to it
3. To form enzyme substrate complex

Question 49

Suggest 3 reasons why it is more efficient to attach lactase to beads than adding it directly to cow’s milk

Answer 49

• (Lactase/beads) can be reused
• No need to remove from milk
• Allows continuous process
• The enzyme is more stable
• Avoid end product inhibition

Question 50

Describe the induced fit model of enzyme action (3)

Answer 50

• Active site / enzyme not complementary
• Active site changes (shape) / is flexible
• (Change in enzyme allows) substrate to fit / E-S complex to form