AQA AS/A Level Biology - Enzymes and Nucleic Acids (2020-21)
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Course
Enzymes and Nucleic Acids
Institution
AQA
An in-depth informative booklet on Enzymes and Nucleic acids such as enzyme action, DNA replication and ATP. Also includes required practicals on this topic, useful summary questions and the specification.
• They are all tertiary proteins with globular shape which has a pocket or cleft area which has a
specific 3D shape called an active site where the reaction takes place.
• Act as biological catalysts – proteins that speed up metabolic reactions without being used up.
• Enzymes provide an alternative pathway during a reaction.
• They are specific - they only catalyse one shape of substrate.
• The specific substrate has a complementary shape to the active site
• Their activity is affected by temperature and pH.
• They are very important in digestion as they hydrolyse (break bonds using water) polymers into
monomers. E.g. Polypeptides into amino acids.
An individual cell contains over a 1000 different enzymes which are involved in metabolic reactions
such as hydrolysis reactions in digestion and photosynthesis and respiration.
Substrate and Product
enzyme
Substrate Product
maltase
e.g. maltose glucose + glucose
(The name of an enzyme is usually derived from its substrate and it has the suffix ‘ase’)
, Activation energy
The extra energy that is required to
enable a reaction to occur is known as the
activation energy.
This energy is often supplied through heat
in a laboratory reaction. Enzymes are
biological catalysts i.e. proteins that are
able to lower the activation energy of a
reaction to allow it to proceed more
quickly at lower temperatures in the body
so that molecules can be broken down
and new ones formed. Enzymes provide
an alternative route during a chemical
reaction, lowering the activation energy.
Enzyme Action – Models
Models are a simplified visual representation used to explain how a process may work. There is now
more scientific evidence that supports the induced fit model of enzyme activity, so this model is now
accepted over the lock and key model.
Lock and Key Model:
Enzymes have a specific
shaped active site which is
complementary to the shape of
the substrate molecules being
used in the reaction. This is
known as the ‘Lock and Key’
model of enzyme action.
Induced Fit model:
• Substrate, not complementary, collides and binds with the active site.
• The active site shape changes to fit more closely, becomes complementary, around the
substrate molecule and is held in
position by oppositely charged R
groups. The tertiary structure of the
enzyme changes.
• An enzyme –substrate complex is
formed (ESC).
• A change in the enzyme shape
places a strain on the bonds in the
substrate molecule (so they are
more likely to break) allowing the
reaction to occur more easily. This
strain lowers the activation energy needed for the reaction to occur.
• An enzyme – product complex now forms
• The product no longer fits into the active site.
• Product is released. The enzyme can catalyse another reaction.
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