2. Enzymes

Question 1

Enzymes

Definition

Answer 1

Enzymes are biological catalysts. They speed up the rate of chemical reactions without themselves being chemically changed at the end of the chemical reactions. Hence, they can be reused and are effective in small concentrations.

Question 2

Catabolic reactions

Definition + Examples

Answer 2

Catabolic reactions are chemical reactions which break up complex molecules into simpler molecules.

E.g. Hydrolysis, respiration, deamination, glycolysis

Question 3

Anabolic reactions

Definition + Examples

Answer 3

Anabolic reactions are chemical reactions which build up simpler molecules into complex molecules.

E.g. Condensation reaction, photosynthesis

Question 4

Structure of enzymes

What are they made of?

Answer 4

Enzymes are proteins with a unique 3-dimensional structure.
Each enzyme has an active site, whose shape is complementary to the shape of the substrate(s) it binds to.

Substrate is the molecule which an enzyme acts on

Question 5

Characteristics of enzymes

Answer 5

1. Enzymes can be reused - due to themselves being chemically unchanged at the end of chemical reactions
2. Enzymes are effective in small concentrations - same reason
3. Enzymes are specific in action - attributed to the complementary shape between the substrate and the active site, enzyme has a unique 3D shape
4. Enzymes speed up the rate of chemical reactions by lowering the activation energy required to start the chemical reaction

Question 6

Activation energy

Definition

Answer 6

Activation energy is the energy that must be supplied to the reactant molecules for them to react.

Activation energy is often supplied in the form of heat

Question 7

How does activation energy help the reactant molecules to react?

Answer 7

Thermal energy absorbed increases the speed of reactant molecules, causing them to collide more frequently and more forcefully in the correct orientation for the chemical reaction to occur.
Thermal agitation of the atoms within the molecules also makes the bonds more likely to break.

Question 8

Why is heating inappropriate for a biological system to speed up the rate of chemical reactions?

What do high temperatures do to an organism on the cellular level?

Answer 8

High temperatures denature proteins and kill cells. Heating will also speed up all the chemical reactions, not just the intended ones.

Organisms therefore use an alternative: enzymes

Question 9

How do enzymes speed up chemical reactions?

activation energy

Answer 9

Enzymes lower the activation energy required to start a chemical reaction.

Question 10

Summarise how enzymes catalyse chemical reactions

Steps

Answer 10

1. Effective collision between specific substrate(s) and enzyme at correct orientation causes substrate molecule(s) to enter the active site of the enzyme.
2. Substrate molecule(s) bind to the active site of the enzyme and forms the enzyme-substrate complex.
3. The formation of an enzyme-substrate complex lowers the activation energy.
4. Chemical reaction occurs and products are formed.
5. The enzyme-substrate complex dissociates to release the products and the chemically unchanged enzyme is ready for another cycle of chemical reaction.

Question 10

Name the 2 models of enzyme action and specifity

Answer 10

1. Lock and key hypothesis
2. Induced fit model

Question 11

What is the lock and key hypothesis?

Steps

Answer 11

The lock and key hypothesis suggests that:

• The substrate is a “key” and the enzyme is a “lock”
• The shape of the substrate is complementary to the shape of the active site of the enzyme
• The active site of the enzyme has a specific shape into which the subtrate(s) fit exactly
• The substrate binds to the active site of the enzyme, forming an enzyme-subtrate complex, lowering the activation energy
• Chemical reaction occurs
• Once the products are formed, they no longer fit into the active site of the enzyme and are released into the surrounding medium

Question 12

What is the induced fit model?

Steps

Answer 12

The induced fit model suggests that:

• The active site of the enzyme is complementary in shape but not a perfect fit to the substrates it catalyses
• However, when the substrate binds to the active site of the enzyme, it induces a change in the shape of the active site
• This allows the subtrate to fit more tightly into the active site
• The substrate binds to the active site of the enzyme, forming an enzyme-substrate complex, lowering the activation energy
• Chemical reaction occurs
• Once the products are formed, they no longer fit into the active site of the enzyme and are released into the surrounding medium

Question 13

What happens when an enzyme becomes denatured?

Answer 13

The enzyme will lose its unique 3D shape.
The substrate will no longer be able to bind to the active site of the enzyme.
The enzyme cannot catalyse reactions anymore.
Denaturation is irreversible.

Bonds that break include: hydrogen bonds, hydrophobic interactions, ionic bonds, disufide bridges

Question 14

4 Factors affecting the rate of enzyme-catalysed reaction

Answer 14

1. Temperature
2. pH
3. Enzyme concentration
4. Substrate concentration

Question 15

How does temperature affect the rate of enzyme-catalysed chemical reaction?

From low temperature to extreme temperature

Answer 15

At low temperatures:

• molecules have little kinetic energy
• enzymes are inactive
• hence, low number of effective collisions and formation of enzyme-substrate complexes - low rate of reaction

As temperature increases:

• molecules increase in kinetic energy
• enzymes become more active
• hence, more effective collisions and formation of enzyme-substrate complexes - rate of reaction increases
• The reaction rate doubles for every 10°C rise in temperature until optimum temperature

At optimum temperature (temperature at which enzyme activity is at its maximum):

• rate of reaction is at its maximum

Beyond optimum temperature:

• enzyme is denatured, loses its unique 3D shape and is unable to bind to the subtrate (irreversible)
• rate of reaction decreases rapidly

Question 16

How does pH affect the rate of enzyme-catalysed chemical reaction?

At optimum pH? At extreme pH?

Answer 16

At the optimum pH (pH at which enzyme activity is at its maximum):

• rate of reaction is at its maxmum

At extreme pH:

• enzyme is denatured, loses its unique 3D shape and is unable to bind to the substrate (irreversible)
• rate of reaction decreases

Question 17

How does enzyme concentration affect the rate of enzyme-catalysed chemical reaction?

From low enzyme concentration to high enzyme concentration

Answer 17

At low enzyme concentrations:

• low rate of reaction
• all the active sites of the enzyme molecules are saturated with substrate molecules (not enough enzymes, enzyme concentration is the limiting factor)

As enzyme concentration increases:

• more effective collisions and formation of enzyme-substrate complexes - rate of reaction increases

At the plateau:

• rate of reaction becomes constant
• not enough substrates to occupy the active sites of the enzymes (substrate concentration is the limiting factor)

Question 18

How does substrate concentration affect the rate of enzyme-catalysed chemical reaction?

From low substrate concentraiton to high substrate concentration

Answer 18

At low substrate concentrations:

• low rate of reaction
• not enough substrates to occupy the active sites of the enzymes (substrate concentration is the limiting factor)

As substrate concentration increases:

• more effective collisions and formation of enzyme-substrate complexes - rate of reaction increaes

At the plateau:

• rate of reaction becomes constant
• all the active sites of the enzyme molecules are saturated with substrate molecules (not enough enzymes, enzyme concentration is the limiting factor)

Question 19

2 Ways to measure the rate of chemical reaction

Answer 19

1. Measuring the rate of concentration/volume/mass of products formed
2. Measuring the rate of concentration/volume/mass of reactants used up

Question 20

What control set-up is used in an experiment to show the effect of temperature on the speed of enzyme-catalysed chemical reaction?

Temperature affects enzymes

Answer 20

Boiled reactants => denature enzymes