Factors affecting enzyme rates
Joanna George-Johnson, Contributor

Last week, we learnt about some aspects of enzymes. This week, we will take a deeper look at their actions and factors that affect their rate.

Factors influencing the rate of an enzyme-controlled reaction:

1) Temperature: As temperature increases, so does the rate of reaction, up to a point where the enzyme begins to get denatured. So if the temperature goes beyond the optimum temperature the rate of the reaction will decrease.

2) pH: The optimum pH of an enzyme is the pH at which it works best. So as the pH goes towards this optimum pH, the rate of the reaction will continue to increase. Any pH below or higher than the optimum pH of the enzyme, the rate of the reaction will decrease.

3) Substrate concentration: As the amount of substrate increases, the rate of an enzyme-controlled reaction also increases. This is due to an increase in the chances of a collision between enzymes and substrate to form an enzyme-substrate complex. There comes a point, however, where all the active sites of enzymes become saturated with substrates, and so, an increase in substrate will no longer increase the rate of an enzyme-controlled reaction. The graph above illustrates this.

4) Enzyme concentration: You would expect that if you have more backhoes carrying dirt, the dirt would be transported faster. It is the same with enzymes. The more enzymes present, the faster the reaction will be completed, once the substrate is in an excess.

5) Presence of heavy metals: Some metal ions (like copper, zinc and iron) act as cofactors for enzymes and assist in their proper function. What do you expect to happen in an increase of these metals?

Others, like lead and mercury, can interfere with the bonds in the enzyme and cause distortion of the active sites. These heavy metals are called non-competitive inhibitors. What do you expect to happen when there is a high concentration of these metals in the reaction?

Questions

1. The enzyme catalase, liberates oxygen from hydrogen peroxide. In an experiment carried out by Stacy, it was observed that the rate of the reaction was half that of her friend, Patrick. This confused Stacy because both of them used similar amounts of hydrogen peroxide and catalase. When she checked the pH of the solution, it was 7, which is the pH required for the reaction to occur. The pH of Patrick's reaction was also 7. Stacy's experiment was carried out at room temperature. Neither Patrick nor Stacy used any heavy metals in their experiment. Using the information given, deduce what Patrick may have done differently for his reaction rate to have doubled. State how this affected the rate of his reaction.

2. Equal samples of a food sample are placed in two separate test tubes. The test tubes were labeled 'A' and 'B'. After carrying out food tests on test tube 'A', it tests positive for starch, negative for reducing sugar, and positive for protein. A solution is then added to test tube 'B'. The food is then tested again after 30 minutes. The results showed a negative test for starch, a positive test for reducing sugar and positive again for protein.

Using your knowledge of enzymes, explain the results of the experiment.

These girls forcefully get a ball over the net as they play a practice volleyball game during school activities at Jonathan Grant High School in Spanish Town, on Tuesday, February 26. -Anthony Minott/Freelance Photographer

Joanna George-Johnson teaches at Ardenne High School.