Chapter 5: Enzymes
Enzymes are Biological
Catalysts
What are enzymes?
Biological catalysts made up of protein which alter
rate of chemical reaction without themselves being
chemically changed at the end of reactions
Catalysts : Substance that increase rate of reaction without being used up.
Enzymes are Biological
Catalysts
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Enzymes are made up of protein
Denatures (enzyme loses it shape and
function) if exposed to high temperature or
extreme changes in pH
Speeds up reaction without the need to
increase temperature.
Mode of actions of enzymes
Breaking
down of fats
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Heat at high
temperature
Using
enzymes
Body cannot withstand high temperatures
Energy is required to start breaking down
fats – activation energy
Enzymes lower activation energy >
Required to start chemical reaction.
Enzymes-catalysed Reactions
1.

Digestion
Process whereby large insoluble food
substances are broken down into small
soluble molecules
Carbohydrates  Monosaccharides
Protein  Amino Acids
Fats  Fatty Acids + Glycerol
Enzymes-catalysed Reactions
2. Synthesis of complex substances from
simpler ones.
 Synthesis of proteins
 Synthesis of glycogen for storage in the
body
 Synthesis of fats
Enzymes-catalysed Reactions
3. Oxidation of Glucose
 Release of Energy to do work
 Involves a series of enzyme catalysed
reactions
4. Breakdown of toxic materials
 Hydrogen peroxide to water and oxygen
catalysed by catalase.
Enzymes catalyse almost all reactions in body.
There are many different types of enzymes and each
is specific and will be produced only when required.
Classifying Enzyme
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According to chemical reaction they catalyse
Usually ends up with –ase
Example:
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Breaking down of starch = amylase
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Breaking down of Protein = Protease
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Breaking down of Lipids/Fats = Lipase
Enzyme combine with molecules it acts upon (substrate) to
form an enzyme-substrate complex.
Enzyme controlled reaction can be either
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Anabolic : building up larger molecules
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Catabolic: breaking down large molecules into smaller molecule
Characteristics of Enzymes
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Speeds up rate of chemical reactions
Required in minute amount (small) – (itself not
breakdown)
Specific (Lock & Key hypothesis)
Lock & Key Hypothesis
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Substrate which the enzyme acts on is called substrate
Example: Breaking down of starch, amylase is the
enzyme and starch is substrate. End-product = Maltose
Enzyme(lock)
Substrate (Key)
Product
Amylase
Starch
Maltose
Lipase
Lipid
Fatty acids + glycerol
Protease
Protein
Amino acids
Enzyme molecules are usually larger than substrate
Small part of enzyme, called active site comes into
contact with substrate
Enzyme (lock) and substrate(key)
Active
site
Enzyme
substrate
complex
amylase
starch
amylase
2
maltose
units
Enzyme are affected by temperature
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Enzyme and substrate in constant motion
Increase temperature results in increased kinetic
energy
Molecules move faster
Rate of enzyme colliding with substrate is higher
Rate of enzyme-substrate complex formed higher
Only up to an optimum temperature
Low temperature = enzymes are inactive
Enzyme are affected by temperature
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Around 37 ℃ in mammals
Too high a temperature will result in protein being
denatured
Active sites change shape and activity gradually decline
Different enzymes can withstand different temperature
range
Most enzymes will be completely denatured above 60 ℃
Molecules move slower and rate of collision lower
Q: How does temperature affect Enzyme activity
What happens at low temperature?
Enzyme are inactive at low temperature
What happens as temperature increase?
As temperature increase, enzymes gain more kinetic energy
causing them to move faster. When they move faster, chances of
colliding into a substrate is higher. Hence, rate of forming
enzyme-substrate complex is higher. Rate of reaction increases
What happens at optimum temperature?
At optimum temperature, enzyme are most active
What happens above optimum temperature?
Enzymes loses active sites, denatured.
Activeness of
enzymes
0℃
Enzymes are affected by
temperature:
50 ℃
100 ℃
Enzyme are affected by pH
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Enzyme usually work efficiently within a narrow pH
range.
Some enzymes works best in slightly acidic conditions
(pepsin and rennin) while others in slightly alkaline
conditions.
Extreme pH will result in denaturation of enzymes
Low
High
Temperature
Inactive
Denatured
pH value
Denatured
denatured
Activeness of
enzymes
pH 1
Enzymes are affected by pH:
pH 7
pH 14
Enzyme are affected by Enzyme
concentration
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Increase enzyme concentration, increased rate of
reaction
Temperature and pH are at optimum values
Substrate level must be high.
Enzyme are affected by substrate
concentration
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Concentration of enzyme remains the same
Increased substrate concentration doesn’t necessarily
increase rate of reaction
Active sites become saturated, therefore could not work
any faster
For example, you are making burgers with buns
and the number of patties you have is limited.
Buns = Substrate, Patties = Rate of reaction
5
No. of
patties
(Rate of
Reaction)
4
3
2
1
2
4
6
8
10
Number of buns
(substrate)
Enzyme are affected by substrate
concentration
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How do you know that substance is not enough?
When I increase number of substance, I can produce
more burgers (Also: When I increase number of that
substance, there is an increase of rate of reaction)
Limiting Factors
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Any factor that directly affects the rate of reaction if its
quantity is changed
Value of this factor has to be increased in order to
increase rate of reaction
Reversible Reactions
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Most reactions in living cells are reversible.
  
REACTANTS
A
PRODUCTS
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PRODUCTS
C
B

D
REACTANTS
Enzyme can catalyse reversible reactions
Usually not allowed as products formed are used up or
transported away.