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Enzymes are globular proteins that act as catalysts.

Catalysts alter the rate of a chemical reaction without undergoing permanent changes

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…

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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…

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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…

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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…

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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…

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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

For most enzymes this is about pH 7-8.

A change in pH reduces the effectiveness of…

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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

Normally enzymes are…

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Effect of Substrate Concentration on Enzyme Action

The higher the substrate concentration, the faster the

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

At higher concentrations the enzyme…

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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.


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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…


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