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Summary Notes – BY1 1
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• Inactivation and denaturation
• The principles of competitive and non-competitive inhibition (references to
reversible and irreversible action not required)
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4 Enzymes)
1
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2
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3
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4
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5
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Bonds are broken and hence the configuration of the active site is
altered
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High temperatures and extreme pH changes cause permanent change in protein
structure, causing denaturation
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Small changes in pH cause small reversible changes in enzyme structure, causing
inactivation
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Inhibition is when enzyme action is slowed down or stopped by another substance
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Enzyme inhibition may be competitive whereby an inhibitor, which is structurally
similar to the substrate, associates with the enzyme active site
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10
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The rate of
reaction is unaffected by substrate concentration
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g
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At extremes of pH:
Ø H-bonding is affected
Ø The 3-D shape of the enzyme changes
Ø So the shape of the active site changes

Buffer: maintains a constant pH when a buffer is used in an experiment the pH changes little
when a volume of acid/alkalis is added
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Extremes of pH can denature an enzyme:
Ø The change in the side chains of the enzymes active site
Ø Are affected by free H+/OH- ions

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In the formation of an enzyme-substrate-complex:
Ø The charge of the active site

It may be said that a buffer “soaks up H+ ions”

Controls: are duplicate experiments identical in every respect to the actual experiment except
for keeping the variable being investigated constant
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• If the concentration of enzyme is constant:
Ø The rate of reaction increases as the concentration of the substrate increases
At low concentration of substrate

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•

As more substrate is added

The effect of pH on the rate of enzyme action:
Most enzymes have an optimum pH:
Ø Where the rate of reaction is at its maximum
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e
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e
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Provided the temperature & pH are favourable & there is an excess of
substrate
Ø The rate of reaction will be directly proportional to the enzyme
concentration
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• So only a small concentration of enzyme is needed to catalyse a large number of
substrate molecules
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E
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catalase turnover rate = several million

The rate of an enzyme-catalysed reaction will vary with a change in the
concentration of the enzyme
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(IV)
Enzyme inhibition:
Inhibition occurs when enzyme action is slowed down or stopped by another substance
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Competitive inhibition:
Competitive inhibitors: have a molecular shape similar to that of the substrate
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The substrate & the competitive inhibitor compete for the active site
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Ø

•
•

•

E
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malonic acid competes with succinate for the active site of succinic
dehydrogenase
(an enzyme in the Krebs cycle of aerobic respiration)

If the substrate concentration is increased, the substrate will decrease the effect of
the inhibitor
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The competitive inhibitor is not permanently bound to the active site
Ø When the inhibitor leaves the active site
Ø Another molecule can take its place
Ø This could be another competitive inhibitor or a substrate molecule
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Non-competitive inhibition:
Non competitive inhibitor: is a chemical that binds to the enzyme at a location that is not the
active site,
Ø In such a way that the active site can no longer be complementary to the substrate
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E
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Cyanide (= a respiratory poison)
ü Attaches itself to part of the enzyme cytochrome oxidase
ü & inhibits aerobic respiration

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