AQA AS Biology Unit 1: Enzymes

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  • Created on: 22-08-13 16:18
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Enzymes
Enzymes are globular proteins that act as catalysts.
Catalysts alter the rate of a chemical reaction without undergoing permanent changes
themselves.
They can be used repeatedly and are therefore effective in small amounts.
Enzymes alter the speed of reactions.
Enzymes Lower the Activation Energy
In a chemical reaction, a certain amount of energy needs to be supplied to the chemicals
before the reaction will start.
This is called activation energy ­ it's often provided as heat.
Enzymes lower the amount of activation energy that's needed, often making reactions
happen at lower temperatures than normal.
This speeds up the rate of reaction.
This enables some metabolic processes to occur rapidly at the human body temperature of
37°, which is relatively low in terms of chemical reactions.
Without enzymes these reactions would proceed too slowly to sustain life.

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Enzyme Structure
Enzymes, being globular proteins, have a specific 3D shape.
Their shape is the result of their sequence of amino acids.
An enzyme molecule is large, yet only a small region of it is functional.
This is known as the active site.
The active site forms a small, hollow depression within the much larger enzyme molecule.
The molecule on which the enzyme acts is called the substrate.
This fits neatly into this depression to form an enzyme-substrate complex.…read more

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Enzyme Lock and Key Model
Enzymes only work with substrates that fit their active site.
The lock and key model shows how the substrate fits into the enzyme in the same way that
a key fits into a lock.
One limitation of this model is that the enzyme, like a lock, is considered to be a rigid
structure.
However, observations showed that other molecules could bind to enzymes at sites other
than the active site.
In doing so, they altered the activity of the enzyme.…read more

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Measuring the Rate of Enzyme Reactions
For an enzyme to work, it must:
Come into physical contact with its substrate
Have an active site which fits the substrate
To measure the progress of an enzyme-catalysed reaction you measure its time course ­
how long it takes for a particular event to run its course.…read more

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Effect of Temperature on Enzyme Action
A rise in temperature increases the kinetic energy of molecules.
As a result, the molecules move around more rapidly and collide
with each other more often.
In enzyme-catalysed reactions, this means that the enzyme and
substrate molecules come together more often in a given time, so
that the rate of reaction increases.
This results in more enzyme-substrate complexes.
If the temperature gets too high, the reaction stops.…read more

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Effect of pH on Enzyme Action
The pH of a solution is a measure of its hydrogen ion concentration.
Each enzyme has an optimum pH, that is a pH at which is works
fastest.
For most enzymes this is about pH 7-8.
A change in pH reduces the effectiveness of an enzyme and may eventually cause it to stop
working altogether ­ becomes denatured.…read more

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Effect of Enzyme Concentration on Enzyme Action
As the enzyme concentration increases, the rate of reaction
increases linearly.
This is because there are more enzyme molecules available to
catalyse the reaction.
At very high enzyme concentration the substrate
concentration may become rate-limiting, so the rate stops
increasing.
Normally enzymes are present in cells in rather low concentrations.…read more

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Effect of Substrate Concentration on Enzyme Action
The higher the substrate concentration, the faster the
reaction.
More substrate molecules means a collision between
substrate and enzyme is more likely and so more active sites
will be used.
This means more enzyme-substrate complexes will be
formed.
At higher concentrations the enzyme active sites become saturated with substrate, so there
are few free enzyme molecules.
Eventually, increasing the substrate concentration yet further will have no effect.…read more

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Enzyme Inhibition
Enzyme inhibitors are substances that directly or indirectly interface with the functioning of
the active site of an enzyme and so inhibit its activity.
Sometimes the inhibitor binds itself so strongly to the active site that it cannot be removed
and so permanently prevents the enzyme functioning.
Most inhibitors only make temporary attachments to the active site.…read more

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Competitive Inhibitors
Competitive inhibitors have a molecule shape
similar to that of the substrate.
This allows them to occupy the active site of
an enzyme.
They compete with the substrate for the
available active sites.
It is the difference between the concentration of the inhibitor and the concentration of the
substance that determines the effect that this has on enzyme activity.
If the substrate concentration is increased, the effect of the inhibitor is reduced.…read more

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