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