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Summary Enzymes Biology OCR A detailed everything you need to know
Detailed notes on Enzymes everything you need to know (OCR A). Can be used for other exam boards
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The role of enzymesAs intracellular and extracellular catalysts
Enzymes are globular proteins. They act as catalysts to metabolic reactions in living organisms,
which means they usually speed up metabolic reactions so that they occur at a reasonably fast
pace even at body temperature.
Enzymes are required to build all the structures of the body (e.g. the cytoskeleton of a cell can
be built up and reduced by enzyme activity), as well as to control the activity of the body.
Enzymes may be intracellular (working inside cells), such as catalase which converts hydrogen
peroxide to oxygen and water. Alternatively, enzymes may be extracellular (working outside
cells), such as the digestive enzymes amylase and trypsin, which are released into the digestive
system.
The mechanism of enzyme action
Enzyme properties
Enzymes have particular properties. These include:
• the molecule has a three-dimensional shape — its tertiary structure
• part of the molecule is an active site that is complementary to the shape of the substrate
molecule
• each enzyme is specific to the substrate
• there is a high turnover number
• they have the ability to reduce the energy required for a reaction to occur
• their activity is affected by temperature, pH, enzyme concentration and substrate concentration
• the enzyme is left unchanged at the end of the reaction
Specificity and the lock and key hypothesis
The specificity of an enzyme refers to its ability to catalyse just
one reaction or type of reaction. Only one particular substrate
molecule will fit into the active site of the enzyme molecule. This is
because of the shape of the active site.
The shape of the active site is caused by the specific sequence of
amino acids. This produces a specific tertiary structure — the
three-dimensional shape of the molecule. This is referred to as the
lock and key hypothesis.
Catalysing the reaction
Enzymes can speed up the rate of a reaction at body temperature. They lower the activation
energy required for the reaction to occur. The activation energy is the amount of energy required
to set off the reaction and break the bonds in the substrate molecule.
The induced-fit hypothesis
, The induced-fit hypothesis helps to explain how the activation energy may be reduced.
The active site of an enzyme molecule does not have a perfectly complementary fit to the shape
of the substrate. When the substrate moves into the active site, it interacts with the active site
and interferes with the bonds that hold the shape of the active site. As a result, the shape of the
active site is altered to give a perfect fit to the shape of the substrate. This changes the shape of
the active site, which also affects the bonds in the substrate, making them easier to make or
break (and therefore reducing the activation energy).
The course of an enzyme-controlled reaction
In an enzyme-controlled reaction, substrate is complementary in shape to the enzyme’s active
site, therefore the substrate enters the enzyme’s active site, combining to form the
enzyme–substrate complex (ESC). This
destabilises and strains the bonds in
the substrate, forming the enzyme
product complex. The product is finally
leaves the active site and the enzyme
is then free to take up another
substrate molecule.
Different enzymes are used in different parts of complex processes such as digestion as
enzymes are specific to particular substrates as the shapes of the active site and substrate are
complimentary, so only certain substrates will be able to form the enzyme substrate complex
and have their bonds destabilised via induced fit to form a product with particular enzymes.
Effects of conditions on enzymes
pH
All enzymes have an optimum pH — the pH at which
they work best. Therefore, they will not work as
quickly at a pH outside their optimum range. This is
because the hydrogen ions that cause acidity affect
the interactions between R groups in the tertiary
structure of the enzyme, so hydrogen and/or ionic
bonds may break.
This affects the tertiary structure of the molecule and
so alters the shape of the active site. The shape will no longer be complementary to the shape
of the substrate molecule, so the enzyme substrate complex will no longer form, meaning that
the enzyme is denatured. The higher the percentage of
enzymes that are denatured, the slower the rate of reaction
will be.
Temperature
The effects of temperature change on enzyme action vary
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