1.4.2 Enzymes

Question 1

What are enzymes?

Answer 1

Enzymes are biological catalysts that increase the rate of reaction by lowering the activation energy of the reaction they catalyse

Question 2

How do enzymes lower activation energy?

Answer 2

Specific tertiary structure determines shape of active site, complementary to a specific substrate
Formation of enzyme-substrate (ES) complexes lowers activation energy of metabolic reactions

Question 3

What are enzymes made up of?

Answer 3

Enzymes are 3D tertiary structured globular proteins whose shape is determined by the primary sequence of amino acids

Question 4

Describe the active site of an enzyme

Answer 4

The active site is an area of the enzyme that is made up of only a few amino acids and forms a small depression in the overall enzyme

Question 5

Describe the substrate

Answer 5

The substrate is the part of the molecule that the enzyme acts upon

Question 6

Explain the induced it model of enzyme action

Answer 6

• Shape of active site is not directly complementary to substrate and is flexible
• Conformational change enables ES complexes to form
• This puts strain on substrate bonds, lowering activation energy

Question 7

How have models of enzyme action changed?

Answer 7

• Initially lock and key model: rigid shape of active site complementary to only 1 substrate
• Currently induced fit model: also explains why binding at allosteric sites can change shape of active site

Question 8

How could a student identify the activation energy of a metabolic reaction from an energy level diagram?

Answer 8

Difference between free energy of substrate and peak of curve

Question 9

Name the 5 factors that affect the rate of enzyme-controlled reactions

Answer 9

1. Enzyme concentration
2. Substrate concentration
3. Concentration of inhibitors
4. pH
5. Temperature

Question 10

How does substrate concentration affect rate of reaction?

Answer 10

As concentration of substrate increases, rate of reaction increases as more ES complexes are formed
Beyond a certain point, the rate of reaction no longer increases as enzyme concentration becomes the limiting factor

Question 11

How does enzyme concentration affect rate of reaction?

Answer 11

The rate of reaction increases as enzyme concentration increases as there are more active sites for substrates to bind to
Increasing the enzyme concentration beyond a certain point has no effect on the rate of reaction as there are more active sites than substrates so substrate concentration becomes the limiting factor

Question 12

How does temperature affect the rate of reaction?

Answer 12

Rate of reaction increases up to the optimum temperature as the kinetic energy of the enzyme increases
Above the optimum temperature rate of reaction decreases beyond the optimum temperature as the enzyme becomes denatured

Question 13

How does pH affect rate of reaction?

Answer 13

pH affects the enzymes shape as it can disrupt the bonds in the tertiary structure of the enzyme. Like temperature all enzymes work at different optimum pH’s

Question 14

Describe competitive inhibitors

Answer 14

• Similar shape to substrate = bind to active site
• Don’t stop reaction ; ES complex forms when inhibitor is released
• Increasing substrate concentration decreases their effect

Question 15

Describe non-competitive inhibitors

Answer 15

• Bind at allosteric binding site
• May permanently stop reaction ; triggers active site to change shape
• Increasing substrate concentration has no impact on their effect

Question 16

Outline how to calculate rate of reaction from a graph

Answer 16

• Calculate gradient of line or gradient of tangent to a point
• Initial rate: draw tangent at t=0

Question 17

Outline how to calculate rate of reaction from raw data

Answer 17

Change in concentration of product or reactant / time taken

Question 18

Why is it advantageous to calculate initial rate?

Answer 18

Represents maximum rate of reaction before concentration of reactants decreases and ‘end product inhibition’

Question 19

State the formula for pH

Answer 19

pH = -log10(H+)

Question 20

How do competitive inhibitors affect the rate of reaction?

Answer 20

As concentration of competitive inhibitors increase, rate of reaction decreases as active sites are temporarily blocked by inhibitors so substrates cannot bind to them

Question 21

How do non-competitive inhibitors affect the rate of reaction?

Answer 21

As concentration on non-competitive inhibitors increases, rate of reaction decreases as shape of the enzyme is altered by the inhibitors