Biological molecules AQA AS Biology PART 4 of 8 TOPICS: Enzymes

Quick summary on everything that needs to be known about enzymes for the AQA exam

It includes the lock and key model as well as the unfamiliar method - the induced fit model.

It has the factors affecting the rate of enzyme activity as well as another key concept that you may not ouch upon in class but is required by the exam board - hydrogen ion concentration. Check it out!

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Preview of Biological molecules AQA AS Biology PART 4 of 8 TOPICS: Enzymes

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Biological molecules (AQA AS Biology) PART 4 of 8 TOPICS: Many
proteins are enzymes
Many proteins are enzymes:
At GCSE, the lock and key model was the method that had to be known where the substrate could fit
into the active site of the enzyme to make the products. At A Level, this is not the only method that is
performed. The induced fit model is another method that should be known alongside the lock and key
model. This follows the same procedure as the lock and key model however, the active site of the
enzyme is not complementary to the substrate (complementary just means that the active site and
the substrate are similar shape as they are not identical. NB: Remember that they are not the same
because you will lose a mark). Therefore when the substrate needs to bind to the active site, the
active site changes shape so the substrate can fit inside. This fitting when the substrate is in the active
site is called enzyme-substrate complexes. Then the products are made as usual like the lock and key
As an enzyme is a biological catalyst, there are factors that will affect the rate of enzyme activity.
Simple factors such as just increasing the number of enzymes will increase the rate of reaction and
just increasing the number of substrates will decrease the rate of reaction. Typical factors such as
temperature and pH can change how fast the enzyme works. If temperature is low, there will be a
slow rate of reaction. As the temperature will start increasing, so would the rate of reaction until
reaching the optimum temperature where the enzyme is working at its fastest rate. Once the
temperature goes over the optimum temperature, the enzyme will start to denature, meaning it will
no longer be able to work as an enzyme as the bonds that held the quaternary structure will break
causing fewer enzyme-substrate complexes forming therefore a low yield in products. Different
enzymes require different temperatures and also pH levels. pH levels will also have the same effect on
the enzyme if the pH is not right or at the optimum.
There are two complicated factors that will be explained that affects the enzymes' activity and they
are competitive inhibitors and non-competitive inhibitors. Inhibitors as a whole means that they
invade the site at which reactions occur - being the active site. This is known as a competitive inhibitor
because they compete with the substrate for the active site therefore reducing the number of
enzyme-substrate complexes. However not all inhibitors invade the enzyme in this way such as
non-competitive inhibitors. These invade at a place other than the active site known as allosteric site
where they are not competing with the substrate for a site on the enzyme. This behaviour reduces the
number of enzyme-substrate complexes because once the non-competitive inhibitor is in, the active
site changes shape and stays changed whilst the non-competitive inhibitor is in. The effects are seen
on the graphs illustrated.

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