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Alevel Biology - Enzymes
What are enzymes?
Enzymes are globular proteins, with an active site
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It has a specific shape, and the shape of the
active site will be complementary to the substrate (the same, will fit); this shape of the active site is
determined by the tertiary structure of the polypeptides that make up the enzymes
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When a substrate does bind, it forms an
enzyme-substrate complex
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How do they work?
In a chemical reaction, a specific amount of energy is needed to be supplied before the reaction will
start, often in the form of heat
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Enzymes lower the activation
energy, making it easier to achieve, and therefore speeding up the rate of the reaction
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If two substrate molecules need to be joined, they can both attach to an enzyme to hold them close
together
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2
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Models of Enzyme Action
'The Lock and Key Model' - This idea uses how a lock is complementary to a specific key, and
applies this to enzymes with their active sites, and the specific complementary substrate
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While the previous method shows the complementary fitting, it does not explain
how the enzyme lowers the activation energy
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However, the active site changes shape slightly when it forms the enzymesubstrate complex, which puts pressure on the substrate and the bonds inside of it
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Factors Affecting Enzyme Activity
1
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This is because molecules are given more kinetic energy and move faster, and
therefore the are more likely to collide successfully with the active site as the activation energy is
most likely going to be met
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If the
temperature gets too high, the substrate can vibrate too fast, breaking the bonds within the tertiary
structure of the enzyme
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This is called denaturing
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pH - all enzymes also have an optimum pH value, which is the pH in which the enzyme works the
fastest at
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Above or below to optimum pH, the H+ and
OH- ions in acids and alkalis can interfere with the bonds within the tertiary structure, breaking
original ones and forming them with the acid or alkali molecules
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3
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However, if there are a limited amount of substrate molecules, there will be a point when
there is more than enough enzyme molecules to bind with the available substrates
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4
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This is
because the more substrate molecules means there is more likely to be a collision with an enzyme, so
the rate of reaction will increase
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Adding more substrate at this point will have no effect, as there are no available active
sites to bind to
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1
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They work by helping the substrate and
enzyme bind together
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An example of these are chloride ions (Cl-) which are inorganic cofactors of amylase (an
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enzyme that breaks down starch)
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Organic cofactors - these are organic molecules
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They often act as carriers, moving chemicals between the
enzymes
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3
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An
example of these are zinc ions (Zn2+) for carbonic anhydrase, which is an enzyme in red blood cells
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Competitive Inhibitors - these have a similar shape the the substrate molecule
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If the inhibitor binds with the active site, the substrate cannot
enter, preventing a reaction
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If the substrate has a higher concentration, it is more likely to reach the active
site first and reacting
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Non-Competitive Inhibitors - these binds to the enzyme at the allosteric site, away from the
active site
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Increasing the
concentration of substrate will not affect the rate of reaction with this inhibitor, as the enzyme is still
inhibited
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If it is bonded by weak hydrogen or ionic bonds, it can be removed and inhibition is
reversible
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Antiviral drugs - 'reverse transcriptase inhibitors' treat HIV
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This prevents the virus from
replicating
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Antibiotics - Penicillin inhibits 'transpeptidase', which catalyses the formation of proteins in
bacterial cell walls
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The cell therefore bursts
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Cyanide - a non-competitive, irreversible inhibitor of 'cytochrome c oxidase', which catalyses
respiration reactions
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Arsenic - a non-competitive inhibitor or 'pyruvate dehydrogenase', which also catalyses respiration
reactions
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Many enzymes are inhibited by the product of the reaction
they catalysed , which is called product inhibition
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This is a convenient way of regulating amounts
of product
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Part of the precursor inhibits its action as an enzyme; when this is removed, the enzyme becomes
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active again
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Their cell walls are made from protein, but they break down protein - being an inactive
precursor means they do not damage themselves

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