1.4 Enzymes

Question 1

What are enzymes?

Answer 1

• Examples of proteins
• Biological catalysts for intra and extracellular reactions
• specific tertiary structure determines shape of active site, complementary to a specific substrate
• formation of enzyme-substrate complexes lowers activation energy of metabolic reactions

Question 2

What is Ea?

Answer 2

This is the activation energy (energy necessary to start any chemical reaction)

Question 3

How do enzymes split substrates?

Answer 3

Using hydrolysis reactions

Question 4

What factors affect the rate of enzyme controlled reactions?

Answer 4

• Temperature
• pH
• Substrate Concentration
• Inhibition
• Enzyme concentration

Question 5

What does a low temperature mean and why?

Answer 5

• Low temp = Low rate
• Enzymes have too little kinetic energy and move slowly. Few successful collisions between enzyme and substrate leads to low rate of reaction

Question 6

What does an optimum temperature mean and why?

Answer 6

• Optimum temp = high rate
• Enzymes have a lot of kinetic energy and move quickly. Lots of successful collisions between enzyme and substrate leads to high rate of reaction

Question 7

What does a high temp mean and why?

Answer 7

• High temp = Low rate
• Heat breaks the bonds maintaining the tertiary structure of the enzyme. The active site becomes denatured and substrate no longer fits. No successful collisions leads to low rate of reaction

Question 8

What does a low pH mean and why?

Answer 8

• Low pH = low rate
• Acidity breaks the bonds maintaining the tertiary structure of the enzyme. The active site becomes denatured and substrate no longer fits. No successful collisions leads to low rate of reaction

Question 9

What does an optimum pH mean and why?

Answer 9

• Optimum pH = High rate
• Many collisions between working enzyme and substrate means high rate of reaction

Question 10

What does a high pH mean and why?

Answer 10

• High pH = Low rate
• Alkalinity breaks the bonds maintaining the tertiary structure of the enzyme. The active site becomes denatured and substrate no longer fits. No successful collisions leads to low rate of reaction

Question 11

What does a low substrate concentration mean and why?

Answer 11

Low sub. conc. = Low rate
- Few substrate molecules limits chance of successful collisions between enzyme and substrate. Rate of reaction is low

Question 12

What does a high substrate concentration mean and why?

Answer 12

• High sub. conc. = High rate
• More substrate molecules increase the chance of successful collisions between enzyme and substrate. Rate of reaction is high

Question 13

What are competitive inhibitors?

Answer 13

• Similar shape to substrate = Bind to active site of enzyme
• Can be reversed if the substrate concentration is increased
• Do not stop reaction; ES complex forms when inhibitor is released
• Increasing substrate concentration decreases their effect

Question 14

What are non-competitive inhibitors?

Answer 14

• Bind to the enzyme (not in the active site) and change its shape
• Substrate no longer fits
• Increasing substrate concentration has no impact on their effect
• May permanently stop reaction; triggers active site to change shape

Question 15

Explain the induced fit model of enzyme action

Answer 15

• 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 16

How have models of enzyme action changed?

Answer 16

• 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 17

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

Answer 17

difference between free energy of substrate and peak of curve

Question 18

How does substrate concentration affect rate of reaction?

Answer 18

Given that enzyme concentration is fixed, rate increases proportionally to substrate concentration.
Rate levels off when maximum number of ES complexes form at any given time

Question 19

How does enzyme concentration affect the rate of reaction?

Answer 19

Given that substrate is in excess, rate increases proportionally to enzyme concentration.
Rate levels off when maximum number of ES complexes form at any given time

Question 20

How does temperature affect the rate of reaction?

Answer 20

Rate increases as kinetic energy increases and peaks at optimum temperature.
Above optimum, ionic and H-bonds in 3° structure break = active site no longer complementary to substrate

Question 21

How does pH affect rate of reaction?

Answer 21

Enzymes have a narrow optimum pH range.
Outside range, H+/OH- ions interact with H-bonds and ionic bonds in 3° structure = denaturation

Question 22

How do you calculate the rate of reaction from a graph?

Answer 22

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

Question 23

How do you calculate the rate of reaction from raw data?

Answer 23

Change in concentration of product or reactant/time

Question 24

Why is it advantageous to calculate initial rate?

Answer 24

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