1.7 Enzyme Action

Question 1

What is an enzyme?

Answer 1

A protein that acts as a biological catalyst

Question 2

What is a catalyst?

Answer 2

Something that speed up reactions without being used up in the reaction

Question 3

Name 6 features of an enzyme

Answer 3

1. Catalyse metabolic reactions at a cellular level and the whole organism
2. Affect structure of an organism ~ involved in production of proteins
3. Affect functions of an organism eg respiration
4. Enzyme action can be intracellular (within cells) and extracellular (outside cells)
5. They have an active site which has a specific shape = where substrates bind to to form esc
6. Highly specific due to their tertiary structure

Question 4

What is activation energy?

Answer 4

• activation energy = certain amount of energy needed to be supplied to the chemicals before the reaction will start ~ often provided as heat

Question 5

How do enzymes lower activation energy?

Answer 5

• making reactions happen at lower temperature = increase rate of reaction
• substrate fits into active site to form esc
• esc lowers activation energy by reducing repulsion of molecules so they can bond more easily and making molecules break up more easily if enzyme is catalysing breakdown reaction

Question 6

Describe the lock and key model

Answer 6

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

Question 7

Describe the induced fit model

Answer 7

• explains why enzymes are so specific and only bond to one particular substrate
• substrate has to be in right place and make active site change shape to fit in right way

Question 8

Why is the induced fit better than the lock and key model?

Answer 8

• shows that esc changed shape slightly to completely fit into the active site
• more accurate than lock and key model

Question 9

How do enzyme properties relate to their tertiary structure? (5 points)

Answer 9

1. Enzymes are specific ~ usually only catalyse one reaction as only one complementary substrate will fit into the active site
2. Active site is determined by enzymes tertiary structure which is determined by primary structure
3. Different enzyme = different tertiary structure = different shaped active site ~ if substrate doesn’t match active site an esc won’t be formed and reaction isn’t catalysed
4. Tertiary structure is altered = shape of active site will change and substrate wont fit, esc won’t form and enzyme cannot carry out its function
5. Tertiary structure of an enzyme can be altered by pH or temperature

Question 10

Describe how temperature affects enzyme activity

Answer 10

• temperature increases = rate of enzyme-controlled reaction increases
• more heat = more kinetic energy = molecules move faster
• more collisions = increase in energy of collisions = more likely that collisions create a reaction

Question 11

What happens if the temperature is too high?

Answer 11

• All enzymes have an optimum temperature of 37
• increase temperature = enzymes vibrate more
• temperature increases above the optimum = active site changes as shape as bonds are broken
• enzyme and substrate can no longer fit together so the enzyme is denature = it no longer functions as a catalyst

Question 12

Describe how pH affects enzyme activity

Answer 12

• all enzymes have an optimum pH
• human enzymes work best at pH 7 (neutral) ~ some like pepsin work best at acidic pH 2 as its found in the stomach
• above and below optimum pH the H+ and OH- ions found in acids and alkalis can mess up the ionic hydrogen bond
• these bonds are what hold the enzymes tertiary structure in place = can change the enzymes active site so enzyme is denatured

Question 13

Describe how enzyme concentration affects the rate of reaction

Answer 13

• more enzyme molecules in a solution = more likely to collide with a substrate molecule and form an esc = increase rate of reaction
• limited amount of substrate = more than enough enzyme molecules so adding more enzymes has no further affect

Question 14

Describe how substrate concentration can affect the rate of reaction up to a point

Answer 14

• higher substrate concentration = faster concentration
• more likely for collisions to occur between substrate and enzymes = more active site used up
• only true up until a ‘saturation’ point ~ after this all the active sites are full so adding more enzymes doesn’t make a difference

Question 15

How does a competitive inhibitor affect an enzyme-controlled reaction?

Answer 15

• have a similar shape to substrate molecules = compete with substrate molecules
• they block the active site so no molecules can fit in it
• high concentration of inhibitor = take up all active sites
• higher concentration of substrate = substrate will have more of a chance of binding to the active site
• increasing concentration of substrate = increases rate of reaction

Question 16

How does a non-competitive inhibitor affect an enzyme-controlled reaction?

Answer 16

• bind to the enzyme away from its active site so the active site changes shape so substrate can no longer bind
• don’t ‘compete’ with the active site because they are a different shape
• increasing concentration of substrate won’t make a difference as enzyme will still be inhibited