Many proteins are enzymes

Biological molecules (AQA AS Biology) PART 4 of 8 TOPICS: Many proteins are enzymes



Many proteins are enzymes:

TOPICS: Carbohydrates  Lipids  Proteins  Many proteins are enzymes  Nucleic acids  ATP  Water  Inorganic Ions

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 model.

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.


You may well be asked to calculate a value for the hydrogen concentration of a solution with enzymes The equation is:

[H+] = 10-pH where

[H+] = hydrogen concentration (needs to be square brackets as it a concentration value)

If a pH is given and you need to find the concentration, you would do 10 to the power negative of the pH value to give the hydrogen ion concentration. E.g. if the pH of a solution was 7 then the hydrogen ion concentration would be 10-7 which equals 1 x 10 -7.

If a hydrogen ion concentration is given and you need to find the pH, you would do the negative of log([H+]) to give the pH. E.g. if the hydrogen ion concentration is 1 x 10-7 then the pH would be log((1×10-7)) which equals 7. Incase if log is not clear what it is, it is just a function to know what the power would be to give a known answer ([H+]) to a known base (10). It is on your standard Casio calculator shown as ‘log’ as well as on other makes such as Sharp.