Enzymes are protein catalysts that speed up chemical reactions without being used up in the process. They work by lowering the activation energy of reactions. Each enzyme is highly specific and will only work on one or a few substrate molecules due to their complementary shapes. Temperature and pH can affect the rate of enzyme reactions by changing the shape of the active site. Most enzymes work best around body temperature and neutral pH.

1. Enzymes
1
Chapter 5

2. 2
Enzymes
Enzymes are proteins that act as catalysts.
They are made in all living cells.
Enzymes, can be used over and over again
 they are not used up during the reaction
only a small amount is needed to speed the reaction up
A catalyst is a substance that increases the rate of a chemical
reaction and is not changed by the reaction.
An enzyme is a protein that functions as a biological catalyst.

3. 3
enzymes can create long chains
by joining molecules together
example:
Long chains of glucose 
Starch, cellulose, Glycogen
Long chains of amino acids 
Protein molecules

4. 4
How an enzyme join two molecules together to form a more complicated
substance (the product)
Enzyme action
Example: joining up of two molecules of glucose to produce maltose.
Enzyme and substrate molecules have complementary shapes
 they fit together.

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Anabolic reaction = Building reaction = synthesis
Reactions in which large molecules are built up from smaller molecules
The product is released
The enzyme is free to repeat the
reaction with another substrate.
Action of enzyme
(Anabolic reaction)
enzyme-substrate
complex is formed
temporarily.

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Action of enzyme
(Catabolic reaction)
Catabolic reaction = breakdown reaction
Reactions that split large molecules into smaller ones

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The shape of an enzyme can control what substances it combines with.
the active site, exactly fits the substances on which it acts
So, the shape of the active site of the enzyme molecule and the
substrate molecule are
complementary.
This means that an enzyme which normally acts on one substance will
not act on a different one.
Enzymes are specific
Specificity
if a reaction takes place in stages, e.g.
Starch  maltose (stage 1)
Maltose  glucose (stage 2)
a different enzyme is needed for each stage.

8. Lock and key model
Substrate
Enzyme
product
product

9. Enzyme names
• Almost all enzymes end in the letters –ase
• Named after their substrate: the substance that they act upon
• Example: the enzyme that works on lipids is called lipase
• Now you try:
1) Maltose: _________________
2) Sucrose: _________________
3) Lactose: _________________
4) Proteins: _________________

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Molecules of the two substances might have combined without the enzyme being
present, but they would have done so very slowly (it could take hours or days
to happen without the enzyme).
In the presence of enzymes the process can be extremely fast
Example : catalase, can break down 40 000 molecules of hydrogen peroxide
every second!
Enzymes speed up the reaction
If starch is mixed with water it will break
down very slowly to sugar, taking several
years.
In your saliva the amylase that can break
down starch to sugar in minutes or seconds.

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Factors affecting enzyme action
Temperature

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A rise in temperature increases the rate of most chemical reactions;
a fall in temperature slows them down.
above 50 °C the enzymes, being proteins, are denatured and stop working.
 the shapes of enzymes are permanently changed and the enzymes can no longer
combine with the substances
One way to test whether a substance is an enzyme is to heat it to boiling point. If it
can still carry out its reactions after this, it cannot be an enzyme.
This technique is used as a ‘control’ in enzyme experiments.
Functional enzyme Nonfunctional enzyme
Factors affecting enzyme action
Temperature

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Generally, a rise of 10 °C will double the rate of an enzyme-controlled reaction in a
cell, up to an optimum temperature of around 37 °C (body temperature).
the enzyme and substrate molecules are constantly moving, using kinetic energy.
Temperature（OC）0 10 20 30 40 50
As the temperature is increased,
the molecules gain more kinetic
energy, so they move faster and
there is a greater chance of
collisions happening
 rate of reaction increases.
Increasing
number of
collisions
The reaction only occurs when the enzyme and substrate molecules come into contact
with each other.

14. Factors affecting enzyme action
Temperature
Temperature（OC）0 10 20 30 40 50
Above the optimum temperature the reaction
will slow down.
Enzyme molecules are proteins.
Protein molecules start to lose their shape
at higher temperatures
the active site becomes deformed.
Substrate molecules cannot fit together
with the enzyme
Reaction stop
Denaturation of the enzyme is permanent.
Not all the enzyme molecules are affected straight away, so the reaction does not
suddenly stop – it is a gradual process as the temperature increases above 37 °C.
Increasing
number of
collisions
Denaturation
Optimum

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Temperature（OC）0 10 20 30 40 50

16. • Each enzyme works best at a particular temperature
called_____________________.
• When temperature is lower than optimum temperature, activity of enzyme
become ________. At low temperature, enzymes become _____________.
They become _________________ when the temperature is raised.
• Above optimal temperature enzymes become _____________ because they
______ shape.
• The active site and substrate don’t _______________ anymore.

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Factors affecting enzyme action
pH
Most enzymes work best at a particular level
of acidity or alkalinity (pH),
Digestive enzyme in the stomach, works well
at an acidity of pH 2.
At this pH, the enzyme amylase, from your
saliva, cannot work at all.
Inside the cells, most enzymes will work best
in neutral conditions (pH 7).
Conditions in the duodenum are slightly
alkaline: the optimum pH for pancreatic
lipase is pH 8.

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Extremes of pH may denature some enzymes irreversibly
 the active site of the enzyme molecule can become deformed
 the enzyme and substrate will not fit together.
Although changes in pH affect the
activity of enzymes, these effects are
usually reversible,
i.e. an enzyme that is inactivated by a
low pH will resume its normal activity
when its optimum pH is restored.

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the rate of an enzyme-controlled reaction depends on
 the temperature
 pH.
 the concentrations of the enzyme
 The concentration of the substrate.
If the cell produce more enzyme  faster the reaction will proceed,
(provided there are enough substrate molecules available).
If the substrate concentration increases  faster the reaction will proceed
( provided there are enough enzyme molecules to cope with the additional
Substrate).
Rates of enzyme reactions

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All enzymes are made inside cells. Most of them remain inside the cell to
speed up reactions in the cytoplasm and nucleus
 These are called intracellular enzymes
Intra- and extracellular enzymes
Example: Enzymes
found in the nucleus
involved in making a
new copy of the DNA

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In a few cases, the enzymes made in the cells are let out of the cell to do
their work outside
 These are extracellular enzymes
In the digestive systems of animals,
extracellular enzymes are released into
the stomach and intestines in order to
digest the food.

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Fungi and bacteria release extracellular enzymes in order to digest their food.
A mould growing on a piece of bread releases starch-
digesting enzymes into the bread and absorbs the
soluble sugars that the enzyme produces from the bread.

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Catalase experiments
Certain reactions in the cell produce hydrogen peroxide, which is
poisonous.
Catalase makes the hydrogen peroxide harmless by breaking it down to
water and oxygen.

24. 1. Measure 20cm3 hydrogen peroxide into a
measuring cylinder and add to a 250cm3
beaker.
2. Place the beaker in a tray.
3. Add a small piece of liver and observe.
4. Feel the side of the beaker during the
reaction (be careful not to get solution on
your hands – wash it off if you do)

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26. DISCUSSION/think
1. Describe what happened.
2. Explain what happened.

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Catalase experiments
Effect of temperature
Explain what happened.

28. 28
Catalase experiments
Effect of pH
Explain what happened.

29. 29
Amylase experiments
We will need :
+ Amylase

30. 30

31. 31
Amylase experiments
Effect of temperature
After 5 min
10 °C 20 °C 35 °C

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What will happen when you add the Amylase to the starch solution?
As the amylase breaks down the starch, it will cause the blue colour to
disappear.

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Questions
1 At what temperature did the amylase break down starch most rapidly?
2 What do you think would have been the result if a fourth water bath at
90 °C had been used?

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Amylase experiments
Effect of pH
5 ml of a
1% starch
solution
in each
tube

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Add acid
or alkali
to each
tube

36. 36

37. 37
Questions
1 What is the optimum pH for amylase?
2 Your stomach pH is about 2. Would you expect starch digestion to take place in
the stomach?