1.1.1 Enzymes are biological
catalysts that speed up
chemical reactions by reducing
the activation energy.
1.1.2 They are globular proteins that have an active site
which has a specific shape that is determined by the
1.1.3 The active site is complementary to the substrate, if it's
not, it won't bind and the reaction won't be catalysed.
1.2 How an enzyme works
1.2.1 Enzymes reduce the activation energy of a reaction,
meaning the reaction will happen at a lower
temperature, speeding up the rate of reaction.
Activation Energy = the minimum amount of energy needed to be supplied for a reaction to occur.
1.2.2 When a substrate binds to an enzyme's active site,
an enzyme-substrate complex is formed.
22.214.171.124 The formation of these is what
lowers the activation energy.
1.2.3 If two substrate molecules need to be joined, they are held by the
enzyme which reduces repulsion so they can join more easily.
1.2.4 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.
2 Enzyme Models
2.1 Lock and Key
2.1.1 The substrate fits into the enzyme in
the same way a key fits into a lock.
2.2 'Induced Fit'
2.2.1 This model helps to explain why
enzymes are so specific.
2.2.2 The substrate doesn't only have to be the right shape,
it has to make the active site change shape slightly.
3 Factors Affecting Enzyme Activity
3.1.1 Increases the kinetic energy, making
the molecules in the enzyme vibrate
3.1.2 The increased vibration breaks some of the the bonds which
changes the shape of the active site, denaturing the enzyme.
3.1.3 The changed active site means the
substrate will no longer fit.
3.2.1 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.
126.96.36.199 It can also break the bonds that hold the enzymes
tertiary structure in place, denaturing the enzyme.
3.2.2 Enzyme Concentration
188.8.131.52 The more enzymes there are, the more likely a
substrate molecule is to successfully collide.
184.108.40.206 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.
3.3 Substrate Concentration
3.3.1 The more substrate there is, the more likely
there is to be a successful collision.
3.3.2 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.
4 Measuring Enzyme Activity
4.1 Measuring how fast the product appears.
4.1.1 e.g. collect the gas produced and
measure how fast it's given off.
4.2 Measure the disappearance of
4.2.1 e.g. regularly sample the
solution and measure the
time it takes for the
substrate to disappear.
4.3 Describing an Experiment
4.3.1 Describe method
4.3.2 Say what's
4.3.3 Describe how the independent variable is being varied
4.3.4 Describe what variables are being kept constant.
4.3.5 State that the experiment needs
to be repeated at least 3 times.
4.3.6 State that a control is needed
5.1.1 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.
5.2.1 These are coenzymes. They participate in the reaction so
are changed by it and end up as a 'second substrate'.
5.2.2 They act as carriers, moving chemical groups between different
enzymes. They are continually recycled during the process.
6.1.1 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.
6.1.2 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.
6.2.1 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.
6.2.2 Increasing the concentration of
substrate won't make any
difference, the enzyme activity
will still be inhibited.
6.3 Metabolic Poisons
6.3.1 Cyanide =
of cytochrome c