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Enzymes are biological catalysts; they are all globular proteins with a tertiary structure. The tertiary structure is the twisting and bending of the polypeptide helix, which may contain both the alpha helix and the beta pleated sheet, and results from interactions of all the different 'R' groups on the constituent amino acids. Hydrogen, ionic and disulphide bonds contribute to the maintenance of the tertiary structure. (Toole and Toole 1999).
A catalyst is a substance which speeds up a chemical reaction within a cell, without being used up. They do not alter with the reaction being catalysed and so can be used again. Enzymes do not cause reactions; they just increase the rate of reaction and are effective in small amounts. The enzyme molecule is larger than the substrate molecule it acts upon. The enzyme has an area of a few amino acids that makes up the active site. (See fig. 2). It is the active site that comes into contact with the substrate. The active site works like a lock and key mechanism; meaning that the substrate fits the active site of the enzyme. When the two molecules lock together, they form a temporary structure called the enzyme-substrate complex. The enzyme products form a different shape from the substrate and then leave the active site, leaving it free to be used again. It is thought that the active site is flexible and so changes shape to fit the substrate. This is called induced fit. This process speeds up a reaction by lowering its activation energy. (Toole and Toole 1999).
Fig. 2: Diagram to show the active site of an enzyme.
(Taken from eapbiofield.wikispaces, 2009)
Two tests were carried out using cylinders of potato 2 cm in length, mixed with 5 ml of hydrogen peroxide and 3 drops of detergent. From the results collected from the experiment, it is possible to see a pattern when these results are plotted in a graph. From the graphs, it can be clearly seen that the amount of oxygen foam produced increases, showing enzyme activity, or the rate of reaction increases as the temperature rises. Generally, an increase of 10°C causes double rate of reaction. To define the temperature coefficient (Q10) of the reaction:
Q10 = rate of reaction at (X + 10) °C ÷ rate of reaction at X°C
Q10 ≈ 2 for most reactions. (Clegg and Mackean 1994).
However, this only happens up to 40°C. After this, the amount of O2 foam produced starts to decrease. At 60°C there is only very little O2 foam produced and at 100°C the foam produced is negligible.
With reference to the results obtained, it can be seen that the results do more or less support the hypothesis. Although it had predicted that there would be no foam produced at or after 60°C, there was still a very small amount. 5 mm of O2 foam was produced at 60°C, so it would be fair to say that enzyme activity would probably be almost zero at 70°C. However, a test was not carried out at this temperature. The rest of the prediction does support the hypothesis. Thus enzyme activity does increase with temperature up to 40°C. This is because, as temperature rises, the enzyme and substrate molecules gain kinetic energy. As they gain more and more kinetic energy, they move around faster. The faster they move, the more collisions they have with one another, thus resulting in an increased rate of reaction. After 40°C the rate of reaction starts to decrease. Although the kinetic energy carries on increasing, and so the rate of reaction should keep increasing, this is not the case. This is because, as temperature rises above 40°C, (the optimum temperature for catalase enzyme activity), the more the atoms which make up the enzyme molecules, vibrate and move around with increased kinetic energy. This results in the weakening of the bonds that hold together the tertiary structure of the enzyme. Eventually, the hydrogen bonds and other forces holding the molecules in their precise shape, break. This results in the decrease of the rate of reaction and the enzyme becomes denatured; i.e. the three dimensional shape of the enzyme molecule is changed to such a magnitude, that the substrate molecule can no longer fit into its active site, and the enzyme looses its catalytic properties. (Clegg and Mackean 1994).
The results from the two experiments do support the hypothesis. However, there were a few errors when carrying out the experiment, and so the results collected cannot be entirely accurate. Firstly, it was very difficult to measure out equal amounts of potato. Keeping the surface area exactly the same and measuring them all precisely to 2 cm long, was impossible. Also different parts of the potato used would not all have exactly the same catalase content. Using a measuring cylinder was not ideal for measuring out 5 ml of hydrogen peroxide, as it was very difficult to keep the cylinder straight, while keeping the bottom of the meniscus at eye level. It was also extremely difficult to measure out only 3 drops of detergent. Another problem encountered, especially in test 1, was timing. As other students were carrying out the experiment at the same time, all using the same water baths, it was difficult to get to the test tubes at exactly 5 minutes to measure the O2 foam. Even though there was some inaccuracy, there were not any real anomalies in the results. It can be seen however, that the results from test 1, were indeed higher than those from test 2. This can be explained by the difficulty in the timing of test 1. In test 2, students were not at the baths, so this made it much easier to time the reaction to exactly 5 minutes each. In test 1, they tended to be left slightly longer, this could possibly have increased the amount of O2 foam produced.
To improve this investigation, the following could be done:
Crush the potato to a paste and drain the filtrate for use as the catalase sample, then measure out a volume of the liquid so the exact amount of enzyme can be calculated.
Use a graduated pipette and filler to measure out the amounts of hydrogen peroxide and detergent. This would be far more accurate, as there would be a clear line and no need to worry about the meniscus.
Carry out the experiment when there are no other students doing the same experiment, or time it in intervals, so other students are not at the water baths at the same time.
With the above taken into account, it can be said that the experiment was a success; showing that enzyme activity does increase with temperature up to 40°C and then decreases thereafter. It is denatured at 60°C - 70°C.