Catalase is an enzyme. All enzymes are proteins, made up of a string of chemically bonded amino acids which each contain variable -R groups that give them different properties. The theory that explains catalysation is known as the ‘lock & key’ theory. This states that the enzyme has an area on it, which is a specific shape that compliments the structure of the substrate; this is known as the active site. The substrate bonds with the active site and forms an enzyme-substrate complex, the bonds are strained and the ES complex splits releasing the new products. The pH of solutions is a measure of the concentration of H+ ions in the solution.
For each integer the pH falls by the number of H+ ions grows ten fold. These hydrogen ions interact with the bonds of the protein. pH can have an effect of the state of ionisation of acidic or basic amino acids. Acidic amino acids have carboxyl functional groups in their side chains. Basic amino acids have amine functional groups in their side chains. If the state of ionisation of amino acids in a protein is altered then the ionic bonds that help to determine the 3-D shape of the protein can be altered. This can lead an enzyme becoming inactive. This is known as denaturing.
When an enzyme is denatured the tertiary structure is altered, this affects the structure of the active site. If the active site’s structure is changed then, following the ‘lock & key’ theory, it no longer compliments the structure of the substrate and it can no longer bind. No ES complexes are formed and the reaction is not catalysed. At each pH the amount of oxygen evolved increased at a fairly constant rate, producing a reasonably steady gradient, this suggests the rate of catalysation altered very little during the five minutes. In Conclusion: The amount of oxygen evolved is an indication of the rate of catalysation of the catalase.
This is because the more hydrogen peroxide molecules that are broken down into water, and more importantly oxygen, the higher the measure recorded. At the lowest pH value the reaction rate was low. At the low pH values there were many more H+ ions in the solution these interacted with the bonds in the enzyme. The enzyme was denatured. It altered the structure of the active site and changed the charge, this would mean the enzyme could no longer bind to the substrate, nor form an ES complex, and therefore could not catalyse the breakdown of hydrogen peroxide. Less H2O2 molecules were broken down so less oxygen was evolved.
At high pH the high numbers of OH- ions causes a very similar effect. However at the pH values of 5. 2 and 6. 5 the gradient is high, suggesting many hydrogen peroxide molecules were broken down per unit time. This is because the concentration of H+ ions provided optimum conditions for the enzyme. The active site had the best structure for catalysing reactions, as it was not denature, and it had the optimum charge for attracting the substrate. By looking at the graph of average gradient against pH, it is possible to predict the optimum pH for the function of catalase.
It looks to be just under pH 7 although my research tells me the optimum pH should be roughly pH 7. 4. Evaluation Main sources of error There were sources of error within the experiment. Firstly the temperature at which each experiment was conducted had no control on it, and could have fluctuated with room temperature. As the experiment was performed in the morning, over the course of the procedure the temperature would have inevitably risen. This would have meant the rate at which oxygen was evolved would have increased as the experiments continued.
This could lead to unreliability in the results. However the temperature rise in the classroom was small and the effect should have been minor enough to still allow a firm conclusion to be drawn. If the experiment were repeated, conducting it within a water bath would solve this problem. A large source of error in the results was the fact the data was pooled. Each person could not conduct the experiment in exactly the same way. For example the swirling of the flask, whether a bubble produced on the 30 second mark was counted or not, or the time taken to fit the bung.
The procedure was standardised as much as possible but this will still have had a large effect upon the reliability of the results. There will have been error due to human limitations. The conical flask could not be swirled the same amount each time. This means the solution would have been agitated a different amount. The mixture that had been mixed more thoroughly would react faster as the molecules would have more kinetic energy. This would have lead to the catalase appearing to function better than it was in reality.
However the conical flask was swirled to a similar degree and averages were taken of results, which minimized the error. If repeated a mechanical swirling device could be used. Also the time taken to fit the bung will have caused unreliability between experiments, as oxygen evolved in this time would not have been recorded. This will have made little difference because it took nearly 30 seconds for the first bubble to be produced in most cases. The hydrogen peroxide could have been added using a syringe directly into the conical flask without removing the lid and thus avoiding this problem.
The part, shape and condition of the potato could have had an effect on the amount of catalase is provided to the solution. If the grated potato came from the edge of the potato it could have different concentrations of catalase than the centre. Also the concentration of catalase could vary from potato to potato. In addition the length of time the potato gratings had been left out, affected their moisture contents and they turned brown. This could have affected the osmotic balance and the speed at which enzymes diffused into the solution.
These affects were minimized by using the same grater, freshly grating the potato needed; using the same potato, skinning the potato and trying to use the central part as much as possible. The amount of potato each time may have been a source of error as it was hard to pack the same amount of potato into the syringe each time, therefore next time a cork borer could be used and the tube of potato cut down to the same repeated size as to ensure that the test is reliable. The equipment used had inaccuracy in them, which contributed to error in the experiment.
Anomalies None of the points fell on the lines of best fit, this is because of the errors outlined above, however most followed the trend although I believe that pH 5. 2 was for some reason too fast, as it does not seem to fit correctly, also this theory would prove the fact that the optimum pH for catalase is pH 7. 4. None of the results are far enough away from the trend of the other points at that pH value to be mark as anomalous and discarded although I believe that pH 5. 2 is not quite correct and therefore me considered an anomaly. Limitations
The error mentioned above place a limitation upon how definitely the conclusion can be stated. The optimum pH was concluded to be – 6. 8; however this could be out by at least i?? 1 and the optimum could be at least pH7. 4. Another limitation on drawing a firm conclusion was number of different pH values tested, in this case 4. If more values were tested, for example every integer value between pH 2 and pH 12, then a much more detailed picture of the average rate of catalase at different pH values good be established and the optimum could be estimated more accurately.
In my opinion the results I obtained were reliable enough to show that extreme pH’s denature the active sites of enzymes.
References Biology for life – second edition – M. B. V. Roberts- Thomas Nelson and Sons – (1986) Advanced Biology – Mary Jones and Geoff Jones-Press syndicate of the University of Cambridge (1999) Biology – second edition – Mike Boyle and Kathryn Senior – HarperCollinsPuclishers Limited (2002)