2. • Biological catalysts – a molecule which
speeds up a chemical reaction but remains
unchanged at the end of the reaction
• Names end in –ase
• Globular proteins (soluble)
• Active site – region (cleft or depression) to
which another molecule or molecules
(substrate) can bind
• Substrate fit perfectly and is held in place by
temporary bonds which form between the
substrate and some of the R groups of the
enzyme’s amino acids (enzyme-substrate
complex)
• Specific - shape of active site will only allow
one shape of molecule to fit
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5. Activation energy
• Enzymes increase the
rate at which chemical
reactions occur
• Activation energy –
extra energy
temporarily given to
substrate to convert it
to product
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7. The course of a reaction
• Oxygen that is released can be collected and
measured
• Initial rate of reaction is fastest (large
volume of oxygen is collected in the first minute
of the reaction)
• Initial rate is measured by calculating the slope
of a tangent to the curve, as close to the time 0
as possible
• As reaction continues, the rate at which oxygen
is released gradually slows down until it
eventually stops completely
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8. Effect of enzyme concentration
• Reaction rate is directly proportional
to the enzyme concentration
• The more enzyme present, the more
active sites will be available for the
substrate to slot into
• As long as plenty of substrate available,
initial rate of reaction increases linearly
with enzyme concentration
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10. Effect of substrate concentration
• As substrate concentration increases, the initial rate of
reaction also increases
• In the same way that increasing the temperature
increases the chances of a favourable collision, so
increasing the substrate concentration increases the
chances - because there are simply more substrate
molecules kicking about!
• Unfortunately this doesn't carry on forever. Eventually
you reach an optimum value. If you increase the
concentration of the substrate passing this point, it
makes no difference.
• Increasing the substrate concentration increases the
activity of the enzyme, up to a point, after which
increasing the substrate concentration has no effect
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12. Temperature and enzyme activity
• Temperature increases reaction rate (temperature also
increases the activity of an enzyme)
• The more times they hit each other, the more likely it is
that the substrate will slot into the active site, leading to
the product being formed
• The higher the temperature, the more energy the
molecules have, so they'll be moving more quickly, and
are more likely to collide successfully. So, the higher the
temperature, the greater the activity of the enzyme
• So as you increase temperature, the activity of an
enzyme will increase, until you reach the optimum value.
After that, any further increase in temperature will result
in denaturation of the enzyme, and a steep drop in
activity
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15. pH and enzyme activity
• pH is a measure of the concentration of hydrogen ions in
a solution
• Hydrogen ions can interact with the R groups of amino
acids, affecting the way in which they bond with each
other and therefore affect 3D arrangement
• If the pH is changed sufficiently, the enzyme will be
completely altered due to this effect, and it is said to be
denatured. However, unlike the effect of extreme heat,
which causes the enzyme to be irreparably damaged,
denaturation due to pH change is reversible. Restore the
pH to its original level, and the enzyme will return to its
original capability.
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17. Enzyme inhibitors
• There are two types of inhibition
– competitive inhibition – a substance that reduces
the rate of activity of an enzyme by competing with
the substrate molecules for the enzyme’s active site.
Increasing the concentration of substrate reduces the
degree of inhibition
– non-competitive inhibition - a substance that
reduces the rate of activity of an enzyme, but where
increasing the concentration of the substrate does not
reduce the degree of inhibition. Many non-
competitive inhibitors bind to areas of the enzyme
molecule other than the active site itself
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18. • Competitive Inhibition
This is where the inhibitor is a molecule which has
a similar shape to the molecule which is supposed
to be binding to the active site. In the case of
enzymes, a competitive inhibitor may have the
same shape as that of the substrate, but it doesn't
react in the same way. Rather than turning into
the product, it simply uses up time and prevents
the substrate from getting to the active site. It
blocks the way.
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21. • Non-competitive Inhibition
This is the kind that exists when a molecule binds
to a different site on the protein, rendering it
inactive. Sometimes it does this before the
substrate reaches the active site, sometimes
afterwards, but in either case it stops the protein
doing its job, and prevents a product being
formed.
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23. • In both competitive and non-competitive
inhibition, it is possible to have both
reversible and irreversible inhibitors.
As the name suggests, a reversible
inhibitor does not have a permanent affect
- it will stop the protein doing what it is
supposed to do, but it will move off again
and allow the protein to function later on;
an irreversible inhibitor, on the other
hand, permanently renders the protein
inactive, so it will have to be replaced by a
brand new one - the inhibitor will not
budge
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24. • Penicillin binds to the
enzyme
(transpeptidase) that
links the
peptidoglycan
molecules in bacteria,
and this weakens the
cell wall of the
bacterium when it
multiplies
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