Interactions between these positive and negative charges are very important. This is what holds the structure together in an enzyme. These links are known as salt links, salt bridges or electrostatic interactions, and involve a positive to negative attraction. If I was to change the pH it would alter the properties of these salt bridges. Even a slight alter away from the optimum pH might mean one of these salt bridges is affected and therefore the shape, activity and stability of the protein will also be affected.
Lock and key theory
Protein molecule
The correct part of the protein molecule fits into the enzyme.
Chemical activity The protein molecule and
Enzyme activity combine
And chemical activity takes place.
The protein
molecule
has been
split.
The enzyme
is unaltered
ready for the
next reaction
Prediction
A graph to show how active a catalase is at different pHs
Optimum pH I will be looking at the pH
range of 4-9, and I think that a
pH of 7.5 will be the optimum
pH i.e. the Ph at which the
catalase will be most active
and therefore most likely to
produce the fastest rate of reaction. I could also predict that a low pH i.e. a pH of 4 will not make much of an enzyme activity. Also, if it’s too alkaline i.e. 9 it will not make much of an enzyme activity neither.
In this investigation I am looking at the effect of pH on the rate of reaction, I will be looking at 5 different pH values from a range of: - 4.4, 5.2, 6.5, 7.5 and 8.4. All other factors will be kept constant and I will be measuring the final time taken to collect 25ml of oxygen.
This is the result I got from my pre-test.
As you can see from the pre-test above, the experiment was carried out at a temperature of 24°C (room temperature). The time taken to collect 25ml of oxygen was neither too short nor too long and therefore, this is a good temperature to carry out the experiment. In my actual investigation I will carry out the experiment at room temperature. This will remain constant throughout the experiment. What will change is the pH of the solution. We had decided to change the amount of gas to collect to 20ml. This is because it took to long to collect 25ml of oxygen.
To carry out this investigation, I will need to use various apparatus. These are:-
- Stop watch
- Measuring cylinder
- Conical flask
- Syringe
- Thermometer
- Water
- 10ml Buffer solution (pH)
-
5ml Hydrogen Peroxide (H2O2)
- 5g Potato
- Water bath
- Top pan balance
- Beaker
- Tile
- Potato peeler
The apparatus should be set up as follows.
In order to carry out this investigation I will need to follow a method. This is to:-
- Peel potato
- Grate potato on tile
- Measure 5g of potato using a top pan balance and put into conical flask.
- Measure buffer solution in a measuring cylinder and add to the conical flask
- Measure the temperature of buffer solution
- Put water into water bath
-
Measure 5ml of H2O2 in cylinder for accuracy then put into beaker for easier access to syringe the H2O2 into the syringe
- Place measuring cylinder into water bath once thoroughly rinsed and make sure it is aerated, then stand up the cylinder.
- Place the long tube coming conical flask into the cylinder.
-
Once syringed the H2O2 into the flask start stop watch.
- Once 25ml of oxygen has been collected stop the stop watch.
I will repeat each experiment three times; I will find the average results for accurate measurements.
For this investigation to be accurate and for me to reach a firm conclusion I will need to insure that my experiment is a fair one. For this to happen I will need to make sure of and carry out the following:
- The temperature will need to be constant – this was done at room temperature, by using a thermometer.
- Use the same potato for each experiment.
- Make sure the potato is grated to a similar size.
- Use separate pipettes for each solution. Make sure it’s not cross contaminated.
- The Peroxide solution should come from the same bottle for each experiment.
I will need to follow a set of rules for safety measures. These are:
- To wear goggles at all times. This is because I will be using acids and alkalis.
- To wear gloves because there is a risk of getting burnt/corroded from the solutions.
- Place apparatus on the centre of the table- Do not leave anything on the side because it might break. Also there is a possibility of harmful solutions dropping.
These are the results of my actual tests.
The average time it took to collect 20ml of oxygen for the pH of:-
4.4= 169 5.2= 158
406+ 267+
428+ 122+
1003/3=334.3 547/3=182.3
6.5= 108 7.5= 83
54+ 31+
81+ 47+
243/3=81 161/3=53.67
8.4= 85
111+
113+
309/3=103
This table shows the result of enzyme activity.
Analysis
As you can see from my graphs and tables, enzymes work best at a particular pH. From my graph you can tell that at region A the pH wasn’t active enough i.e. at a pH of 4.4 the average time taken to collect 20ml of oxygen is 333.4seconds. Similarly region B wasn’t active enough neither i.e. at a pH of 8.4 the average time taken to collect 20ml of oxygen is 103 seconds. At a pH of 6.5 the average time taken to collect 20ml of oxygen is 81 seconds. However, at region B the enzyme had become extremely active i.e. at a pH of 7.5 the average time taken to collect 20ml of oxygen is 53.67 seconds. This is the region where the enzyme works best at. 7.5 is the optimum pH. This fits the line of best fit very accurately. At a pH 7.5, the most common amino acid groups are -COO and –NH3+. This particular combination of amino acid groups gives us a particular shape which in turn affects the structure of the active site. As enzymes are specific and only react with certain substrates, it is important that the enzyme or active site has an optimum shape that fits in with the substrate molecules. With this optimum shape – the enzyme molecule fits in neatly with the substrate molecules and therefore the substrate molecule will be more readily broken down. If the pH is too low or too high the groups will change i.e. At low pH e.g. 4.4, the most common amino acid groups will be -COOH and –NH3+ and at high pH e.g. 8.4 the most common amino acid group will be -COO- and -NH2. This alteration in the pH will change the combination of amino acid groups and so the shape will change. The substrate will no longer fit as well into the active site.
Evaluation
My task was to find the effect of pH on the activity of the enzyme catalase. The experiment had worked out very well. My selected method involved altering the pH environment, by using different buffer solutions. All other factors were kept constant and my aim was to measure the effect of pH on enzyme activity and this was done by timing how long it took to form 20ml of oxygen.
I had followed a good accurate method, which enabled me to gain an accurate set of results, which fits the line of best fit. The results prove to be very reliable because there were no anonymous results. All points fit the line of best fit. I had done fifteen experiments (3 for each pH). This enabled me to find the averages for each pH. The pHs are 4.4, 5.2, 6.5, 7.5 and 8.4. I had carried out enough experiments to reach a firm conclusion. I found out that the catalase works best at a pH of 7.5. This is the optimum pH. If the pH is above or below the optimum pH, it will begin to denature.
As you can see from my results table and my graphs, catalysts work best at a particular pH. Graph A shows that at a pH of 4.4 it takes a long time to collect 20ml of oxygen. This is because there is very little enzyme activity, as you can see on Graph B. At a pH of 5.2 and also 6.5 the time taken to collect 20ml of oxygen is very short. This you can see on Graph A. Graph B shows that there is still, very little enzyme activity. However, Graph A shows that the time taken to collect 20 ml of oxygen is a lot less than any other pH. This is for pH 7.5. Graph B shows that pH 7.5 has the most enzyme activity. This is can prevent the Graph B shows that at a pH of 8.4, the enzyme activity has dropped, Graph A shows that it is taking a long time to collect 20ml of oxygen. As you can see from both Graph A and B, 7.5 is the optimum pH. My predictions are correct.
