Chapter 5 - Enzymes

1
Chapter 5 - Enzymes

• What Are Enzymes?
• Classification of Enzymes
• Characteristics of Enzymes

2
Learning Objectives

• Candidates should be able to
• Define enzymes as proteins which function as
  biological catalysts.

3
What Are Enzymes?

• Enzymes are
• Biological catalysts,
• Protein in nature,
• Catalyze chemical reactions without being changed
  at the end of the reaction.

4
Enzymes as catalysts

• Enzymes lower the activation energy of a reaction
  so that it occurs more readily.

5
Activation Energy
Imagine a chemical reaction as the process of
rolling a huge stone (reactant) up a hill so that
it rolls down and breaks into tiny pieces
(products).
1
6
Activation Energy
Imagine a chemical reaction as the process of
rolling a huge stone (reactant) up a hill so that
it rolls down and breaks into tiny pieces
(products).
1
Activation energy is the energy needed to roll
the stone up the hill.
2
7
Activation Energy
Imagine a chemical reaction as the process of
rolling a huge stone (reactant) up a hill so that
it rolls down and breaks into tiny pieces
(products).
1
Once over the hill, the rest of the reaction
occurs.
3
Activation energy is the energy needed to roll
the stone up the hill.
2
8
Activation Energy
Imagine a chemical reaction as the process of
rolling a huge stone (reactant) up a hill so that
it rolls down and breaks into tiny pieces
(products).
1
Once over the hill, the rest of the reaction
occurs.
3
Activation energy is the energy needed to roll
the stone up the hill.
2
The stone rolls down and breaks into tiny
pieces (products are formed).
4
9
Activation Energy
Imagine a chemical reaction as the process of
rolling a huge stone (reactant) up a hill so that
it rolls down and breaks into tiny pieces
(products).
1
Once over the hill, the rest of the reaction
occurs.
3
Activation energy is the energy needed to roll
the stone up the hill.
2
The stone rolls down and breaks into tiny
pieces (products are formed).
4
The energy needed to start a chemical reaction
is called activation energy.
5
10
Digestion An Enzyme Process

• Why do we need to digest our food?
• Starch, proteins and fats are very large.
• They cannot diffuse across cell membranes for
  absorption.
• Therefore, they must be digested into
• Simpler, smaller and soluble substances.
• Diffusible across cell membranes.

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Other applications of Enzymes

• Anabolic processes
• Eg. Synthesis of proteins from amino acids.
• Catabolic processes
• Eg. Oxidation of glucose (tissue respiration)
• Catalase production
• Catalase catalyses the breakdown of toxic
  hydrogen peroxide into harmless water and oxygen.
• Catalase is abundant in liver and blood.

12
Classification of Enzymes

• Enzymes are classified
• according to the chemical reaction involved in
• Enzymes that catalyse hydrolysis reactions are
  called hydrolases.
• Example of hydrolases
• Carbohydrases, proteases, lipases.
• Enzymes involved in oxidation of food as called
  oxidation-reduction enzymes.

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Learning Objectives

• Candidates should be able to
• Explain enzyme action in terms of the lock and
  key hypothesis.
• Investigate and explain the effects of
  temperature and of pH on the rate of enzyme
  catalyzed reactions .

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Characteristics of Enzymes

• Enzymes alter or speed up the rates of chemical
  reaction that occur in a cell.
• Enzymes are required in minute amounts.
• Since enzymes are not altered in a chemical
  reaction, a small amount can catalyse a huge
  reaction.

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Enzymes are specific

• Specificity of enzyme is due to its shape (or
  surface configuration).
• The substrate will fit into an enzyme, forming an
  enzyme-substrate complex.
• The product will then be released.

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Lock and key hypothesis

• What is the lock and key hypothesis?
• It is the old view of enzyme specificity, that
  there was an exact match between the active site
  and the substrate.

17
A synthesis/ dehydration Rx.
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Lock and Key Hypothesis
active sites
enzyme molecule (the lock)
B
A
substrate molecules ( A and B) can fit into the
active sites
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Lock and Key Hypothesis
active sites
enzyme molecule (the lock)
B
A
substrate molecules ( A and B) can fit into the
active sites
enzyme-substrate complex
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Lock and Key Hypothesis
active sites
enzyme molecule (the lock)
B
A
enzyme molecule is free to take part in another
reaction
substrate molecules ( A and B) can fit into the
active sites
enzyme-substrate complex
AB
a new substance (product) AB leaves the active
sites
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Induced fit hypothesis

• What is induced fit hypothesis?
• shape of the active site adjusts to fit the
  substrate.

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Induced fit hypothesis

• How did induced fit hypothesis come about?
• - recent imaging technology demonstrated changes
  in the 3-D conformation of enzymes when
  interacting with their substrates.

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Effect of temperature

• At low temp
• Rate of reaction is slow.
• Enzymes are inactive at low temp.
• Every 10oc rise in temp, rate of reaction
  increases by double
• (till it reaches optimum temp).

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Effect of temperature

• At optimum temp
• Rate of reaction is the highest.
• Enzymes are most active.
• Beyond optimum temp
• Rate of enzyme activity decreases sharply.
• Enzymes are being denatured.
• Hydrogen bonds are easily disrupted by increasing
  temperature.

25
Effect of temperature
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Effect of Temperature on the Rate of Reaction
The optimum temperature is reached. Enzyme is
most active.
3
Rate of reaction (enzyme activity)
Beyond the optimum temperature, enzyme
activity decreases.
4
As the temperature rises, enzyme activity
increases as indicated by the increase in the
rate of reaction it catalyses. Usually the enzyme
is twice as active for every 10C rise in
temperature until the optimum temperature is
reached.
2
At point D, the enzyme has lost its ability to
catalyse the reaction.
5
An enzyme is less active at very low
temperatures.
1
0
Temperature
K (optimum temperature)
D
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Effect of pH

• Enzymes have an optimum pH.
• Deviation from the optimum pH will decrease
  enzyme activity.

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Effect of pH on Enzyme Activity
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Effects of substrate and enzyme concentration on
rate of reaction

• Increasing substrate concentration will increase
  rate of reaction until a certain limit.
• Cause
• Enzyme molecules are saturated.
• Enzyme concentration is now the limiting factor.

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What is a limiting factor?

• Any factor that directly affects the rate of a
  process if its quantity is changed
• The value of the limiting factor has to be
  increased in order to increase the rate of the
  process.

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Coenzymes

• What are coenzymes?
• Some enzymes require a coenzyme to be bound to
  them before they can catalyse reactions.
• Usually, coenzymes are non-protein organic
  compounds.
• Eg. Vitamins, especially the B complex vitamins.

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Coenzymes

• Coenzymes are altered in some way by
  participating in enzyme reaction.

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Enzymes

• catalyse reversible reactions

products
reactants
C
D
B
A


reactants
reactants
34
Enzymes
Characteristics
Functions
Mode of Action
affected by
Limiting factors
35
Enzymes
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Enzymes
Biological catalysts, which are mainly made of
proteins. They speed up the rate of chemical
reactions without themselves being chemically
changed at the end of the reactions.
37
Enzymes
Functions
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Enzymes
Functions

• Building up or synthesising complex substances
• Breaking down food substances in cells to
  release energy (cellular respiration)
• Breaking down poisonous substances in cells

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Enzymes
Characteristics
Functions
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Enzymes
Characteristics
Functions

• Speed up chemical reactions
• Required in small amounts
• Highly specific
• Work best at an optimum temperature and pH
• May need coenzymes for activity
• Some catalayse reversible reactions

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Enzymes
Characteristics
Functions
Mode of Action
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Enzymes
Characteristics
Functions
Mode of Action

• Lower the activation energy of a reaction
• Interact with the substrate according to lock
  and key hypothesis to form an enzyme-substrate
  complex

43
Enzymes
Characteristics
Functions
Mode of Action
affected by
44
Enzymes
Characteristics
Functions
Mode of Action
affected by
Limiting factors
Factors that directly affect the rate at which a
chemical reaction occurs if their quantity is
changed. The value of a limiting factor must be
increased in order to increase the rate of
reaction.
45
Enzymes
Characteristics
Functions
Mode of Action
affected by
Limiting factors
e.g.
Temperature / pH
46
Enzymes
Characteristics
Functions
Mode of Action
affected by
Limiting factors

• Increase in temperature increases the rate of
  enzyme reaction until optimum temperature is
  reached
• Increase in pH increases the rate of enzyme
  reaction until optimum pH is reached

e.g.
Temperature / pH
47
Enzymes
Characteristics
Functions
Mode of Action
Classes
affected by
Limiting factors
e.g.
Temperature / pH
48
Enzymes
Characteristics
Functions
Mode of Action
Classes
based on the type of reaction catalysed e.g.
affected by
Hydrolases
Limiting factors
e.g.
Temperature / pH
49
Enzymes
Characteristics
Functions
Mode of Action
Classes
based on the type of reaction catalysed e.g.
affected by
Hydrolases
Limiting factors
Oxidation-reduction enzymes
e.g.
Temperature / pH