Enzymes are biological
catalysts that speed up
chemical reactions by
reducing the activation
energy.
They are globular proteins that
have an active site which has a
specific shape that is determined
by the tertiary structure.
The active site is complementary to the substrate, if
it's not, it won't bind and the reaction won't be
catalysed.
How an enzyme
works
Enzymes reduce the activation energy of a
reaction, meaning the reaction will happen at a
lower temperature, speeding up the rate of
reaction.
Annotations:
Activation Energy = the minimum amount of energy needed to be supplied for a reaction to occur.
When a substrate binds to an enzyme's active
site, an enzyme-substrate complex is formed.
The formation of these is
what lowers the activation
energy.
If two substrate molecules need to be joined, they are held
by the enzyme which reduces repulsion so they can join
more easily.
If the enzyme is catalysing a breakdown
reaction, fitting into the active sites strains
the bonds in the substrate which means it
breaks up more easily.
Enzyme
Models
Lock and
Key
The substrate fits into the enzyme
in the same way a key fits into a
lock.
'Induced Fit'
This model helps to explain why
enzymes are so specific.
The substrate doesn't only have to be the right
shape, it has to make the active site change shape
slightly.
Factors Affecting Enzyme
Activity
Temperature
Increases the kinetic energy,
making the molecules in the
enzyme vibrate more.
The increased vibration breaks some of
the the bonds which changes the shape
of the active site, denaturing the
enzyme.
The changed active site means the
substrate will no longer fit.
pH
If pH is too high, the increased H+ ions will
bond with the negative R groups in the
enzymes active site, meaning the substrate
won't fit in.
It can also break the bonds that hold the
enzymes tertiary structure in place,
denaturing the enzyme.
Enzyme
Concentration
The more enzymes there
are, the more likely a
substrate molecule is to
successfully collide.
If the amount of substrate is
limited, there comes a point
when there's more than
enough enzyme but not
enough substrate so more
enzyme has not effect.
Substrate
Concentration
The more substrate there is, the more
likely there is to be a successful collision.
At a saturation point, there are so many
substrate molecules that the enzymes active
sites are all full so adding more substrate has no
effect.
Measuring Enzyme
Activity
Measuring how fast the product
appears.
e.g. collect the gas produced
and measure how fast it's given
off.
Measure the
disappearance of the
substrate.
e.g. regularly sample the
solution and measure the
time it takes for the
substrate to disappear.
Describing an
Experiment
Describe
method &
apparatus
Say what's
being
measured
Describe how the independent
variable is being varied
Describe what variables are
being kept constant.
State that the experiment
needs to be repeated at least
3 times.
State that a control
is needed
Cofactors/Coenzymes
Inorganic
They help the enzyme and substrate bind
together. They don't participate in the
reaction so aren't used up or changed in
any way.
Organic
These are coenzymes. They
participate in the reaction so
are changed by it and end up
as a 'second substrate'.
They act as carriers, moving chemical
groups between different enzymes. They
are continually recycled during the
process.
Inhibitors
Competitive
They have a similar shape to that of
the substrate molecule and they
compete with the substrate to bind to
the active site but no reaction takes
place.
They block the active site so no
substrate can get in. If there is a
high concentration of inhibitor then
it will take up nearly all the active
sites and hardly any of the substrate
will fit in.
Non-Competitve
They have a complementary shape to the secondary site. This
causes the active site to change shape so the substrate
molecules can no longer fit. They don't 'compete' with the
substrate molecules to bind to the active site because they are a
different shape.
Increasing the concentration
of substrate won't make any
difference, the enzyme
activity will still be inhibited.
Metabolic
Poisons
Cyanide =
irreversible
inhibitor of
cytochrome c
oxidase.
Malonate =
inhibits
succinate
dehydrogenase.
Arsenic = inhibits
the action of
pyruvate
dehydrogenase.
Drugs
Some antivirals
inhibit the enzyme
reverse
transcriptase