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ENZYMES
A huge number of chemical reactions take place inside every living cell.
Some release energy, some synthesise new substances, some break down waste
products.
Enzymes play a vital role in these reactions
Enzymes are biological catalysts

They are made by…

They can be…

They regulate the chemical reactions which take place in many biological
processes.

They are required in very…

They are used over and over again.
All enzymes are proteins

Their activity depends on the tertiary structure of the protein and this influences
their requirements for optimal activity. The enzyme will be sensitive to
environmental conditions for example changes in.

Each enzyme is coded for by one gene.
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Why Are enzymes Important?
The substances that react together in an enzyme controlled reaction are called
______________.
E.g. What are the substrates for the chemical reaction of respiration?
To actually start this reaction would need a lot of energy to break the bonds already
in existence between the molecules of the substrates. The energy needed is usually in
the form of heat, e.g. sugar will only react with oxygen when it is heated.
Animals need to release the energy in sugar and break it down inside their bodies.
Such high temperatures are likely to cause damage to or kill the animal! The chemical
reactions necessary to support life need to occur quickly and within a suitable
temperature range. Body temperature is _______. Temperatures greatly above this
would cause damage to chemicals such as proteins, so the high activation energy
needs to be lowered. This is where enzymes are important:
*___________________________
The minimum amount of energy needed to bring molecules together so that they will
react with each other is called the ACTIVATION ENERGY
Enzymes, in the same way as all catalysts, work by lowering the amount of energy
needed to start a reaction.
Mechanisms could be:
 binding the reactants next to each other increases the chance of a reaction
or
 reactants are held in such a way that bonds are exposed to attack
Enzymes cannot alter a reaction – they cannot
cause it to proceed in a different way, or influence
the final concentrations of reactants and products.
All the enzyme does is _____________________
_______________.
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Activation Energy Graphs
In any reaction, the substrates will only bond together and release products if they
have enough energy. The following graph shows some of the energy changes involved
in a chemical reaction which does not involve enzymes.
The amount of energy needed is very high and is not suitable in living organisms.
Enzymes LOWER the activation energy required by………….
The following graph compares the activation energy required for a reaction with and
without an enzyme
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How do enzymes function?
1, Lock and key hypothesis
2, Induced Fit hypothesis
Show using diagrams how this enzyme would function to catalyse a breakdown
reaction.
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Factors affecting enzyme activity
Enzyme and Substrate Concentration
Enzymes work by forming enzyme-substrate complexes. The rate of reaction depends
on the number of substrate molecules that bind with enzyme molecules in any given
time. The more enzyme molecules in the solution, the greater the chance of a
substrate molecule finding an active site, and the faster the rate of reaction.
Similarly, the more substrate molecules in a solution, the greater the chance that an
enzyme-substrate complex will form and the greater the rate of reaction.
1)
2)
1. a) Referring to the diagram and the graph, describe how changes in substrate
concentration affect rate of reaction.
b) Explain using your knowledge of enzyme activity why this happens.
c) Mark on the graph with a letter X the point of saturation (when all active
sites are occupied)
2. a) Referring to the diagram and the graph, describe how changes in enzyme
concentration affect rate of reaction
b) Draw out the shape of the graph for the change in rate of reaction as
enzyme concentration increases if substrate concentration was not a limiting
factor.
3. Describe the differences between the lock and key hypothesis and the induced
fit hypothesis.
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Temperature and enzyme activity
As the temperature rises, the rate of a chemical reaction normally increases. This is
because the molecules gain_________ ___________ so they move faster, collide
more often and the __________ are more likely to lead to a reaction. This also holds
true for reactions controlled by enzymes.
However, as temperature rises the atoms within the enzyme molecules also gain
_________ _____________ and ___________ so rapidly that the weak bonds
that maintain the __________ structure of the protein molecule break and the
protein can _________. This means a change in shape of the ______ _______which
will inactivate the __________. Once broken, the __________ bonds do not reform
their original positions and even if ________, the damage is ______________.
The enzymes is said to be ___________________.
Most enzymes in the body are denatured by temperatures above approximately 45ºC
(however denaturation is not instantaneous). They work fastest just below this, at
about 40ºC. This is their __________ ____________. However other organisms
living in extreme temperatures have enzymes which function best at other
temperatures.
1. Why is the rate of reaction so low below 10ºC?
2. Divide the y axis into an even scale from 0-10 (use a ruler!) to represent the
rate of reaction in arbitrary units.
3. What is the rate of reaction at 10ºC, 20ºC and 30ºC.
4. What is the effect of raising the temperature by 10ºC?
5. what would the rate of reaction be if some of the enzyme and its substrates
were frozen and then warmed to 40ºC?
6. A protease enzyme in washing powder works best at 55ºC.
a) What is the advantage of using a protease with this optimum temperature in a
washing powder?
b) Explain how proteases in washing powder help to remove protein stains such as
blood or egg from clothes.
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pH and enzyme activity
As well as having an optimum temperature, enzymes also have an optimum pH. Most
enzymes are denatured in solutions that are strongly ____________ or strongly
____________. This is because the Hydrogen ions in an acid or the hydroxyl ions in
an alkali are attracted to the charges on the amino acids in the polypeptide chains
that make up the enzyme. They interact with the amino acids and disrupt the bonds
that are maintaining the molecules 3-D shape, destroying the active site of the
enzyme. Once the original structure is lost, it cannot reform and the enzyme no
longer ______ to its substrate.
7. The optimum pH for this enzyme is pH 8. If this were a digestive enzyme
suggest where it may be produced within the human digestive system.
8. Describe how changes in pH affect this enzyme and explain why.
9. Draw onto the graph the curve that would show how the enzyme activity of a
protease enzyme found in the stomach would change with pH.
10. The enzyme catalase speeds up the breakdown of hydrogen peroxide into water
and oxygen. Describe how you would carry out an experiment to investigate the
effect of pH on the activity of the enzyme catalase in yeast cells.
HINT 1: you would need hydrogen peroxide, a suspension of yeast cells, a range of
buffer solutions at different pH’s, test tubes, a water bath at 30ºC, measuring
cylinders
HINT 2: enzyme activity can be determined by measuring the rate of production
of product
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Factors Affecting Enzyme Activity
Competitive and Non-competitive Inhibition
1) Competitive Inhibition
2) Non-Competitive Inhibition
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