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Chapter 5 - Enzymes
5.1 What Are Enzymes?
5.2 Classification of Enzymes
4.3 Characteristics of Enzymes
Sec 3 Bio
1
Learning Objectives
Candidates should be able to:
• Define enzymes as proteins which
function as biological catalysts.
2
5.1 What Are Enzymes?
Enzymes are:
• Biological catalysts,
• Protein in nature,
• Catalyze chemical reactions without
being changed at the end of the reaction.
3
Enzymes as catalysts
• Enzymes lower the activation energy of a
reaction so that it occurs more readily.
4
Activation Energy
1
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).
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Activation Energy
1
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).
2
Activation energy is the
energy needed to roll the stone
up the hill.
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Activation Energy
1
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).
3
Once over the hill, the rest of
the reaction occurs.
2
Activation energy is the
energy needed to roll the stone
up the hill.
7
Activation Energy
1
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).
3
Once over the hill, the rest of
the reaction occurs.
2
Activation energy is the
energy needed to roll the stone
up the hill.
4
The stone rolls down and breaks into
tiny pieces (products are formed).
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Activation Energy
1
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).
3
Once over the hill, the rest of
the reaction occurs.
2
Activation energy is the
energy needed to roll the stone
up the hill.
4
The stone rolls down and breaks into
tiny pieces (products are formed).
5
The energy needed to start a chemical
reaction is called activation energy.
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Digestion: An Enzyme-Catalysed 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.
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5.2 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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5.3 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.
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A synthesis reaction
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active sites
enzyme molecule
(the ‘lock’)
Lock and Key
Hypothesis
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.
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Effect of temperature
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Effect of Temperature on the Rate of Reaction
Rate of reaction
(enzyme activity)
3
The optimum temperature is reached.
Enzyme is most active.
4
Beyond the optimum
temperature, enzyme
activity decreases.
2
1
An enzyme
is less active
at very low
temperatures.
0
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 10°C rise in
temperature until the
optimum temperature is
reached.
5
At point D, the enzyme
has lost its ability to
catalyse the reaction.
K (optimum temperature)
D
Temperature
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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
A
+
reactants
B
+
C
D
reactants
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Enzymes
Functions
Characteristics
Mode of Action
affected by
Limiting factors
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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.
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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
Functions
Characteristics
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Enzymes
Functions
Characteristics
• 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
Functions
Characteristics
Mode of Action
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Enzymes
Functions
Characteristics
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
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Enzymes
Functions
Characteristics
Mode of Action
affected by
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Enzymes
Functions
Characteristics
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.
44
Enzymes
Functions
Characteristics
Mode of Action
affected by
Limiting factors
e.g.
Temperature / pH
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Enzymes
Functions
Characteristics
Mode of Action
affected by
• 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
Limiting factors
e.g.
Temperature / pH
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Enzymes
Functions
Characteristics
Mode of Action
Classes
affected by
Limiting factors
e.g.
Temperature / pH
47
Enzymes
Functions
Characteristics
Mode of Action
Classes
based on the
type of reaction
catalysed e.g.
affected by
Hydrolases
Limiting factors
e.g.
Temperature / pH
48
Enzymes
Functions
Characteristics
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
49