Factors Affecting Enzyme Activity

AP Biology

Introduction

Hydrogen peroxide (H2O2) is a toxic substance to most living organisms. However, many organisms are capable of breaking down H2O2 though enzymatic activity before it can do much damage. Enzymes effectively convert H2O2 to oxygen and water, as shown below:


2 H2O2 ----> 2 H2O + O2

Research Question

How does pH level affect enzymatic activity?

Hypotheses

Explanatory Hypothesis: Changes in pH level will have an effect on enzymatic activity because deviations from the optimal pH level will decrease enzymatic activity due to changes in the structure of the enzyme and the protein, rendering them unable to bind and preventing the substrate from effectively being catalyzed.

Prediction: I predict changes in pH level will affect enzymatic activity by decreasing activity as pH level deviates from the optimal level and increasing activity as it nears the optimal level.

Null Hypothesis: Changes in pH levels will have no effect on enzymatic activity.

Alternative Hypothesis: Changes in pH level will have an effect on enzymatic activity.

Method

In order to measure the effect of pH level on enzymatic activity, three test tubes were set up labeled pH 4, pH 7, and pH 10, respectively. All three test tubes were prepped with 3 mL 3% H2O2 solution and 3 mL pH buffer of a pH level corresponding to each label (independent variable). Each test tube then had 2 drops of enzyme solution (constant) added and was capped with a gas pressure sensor to measure oxygen produced by the reaction over a course of 180 seconds (dependent variable). Results were recorded on LabQuest.

Data

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Results

The results show that enzymatic activity decreased as the pH level became more basic. The pH level with the greatest rate of reaction was pH 4. Average rate of reaction for pH 7 was lower than that of pH 4 but greater than the average rate of reaction for pH 10. The graph depicting the average rate of reaction of varying pH levels shows the second half of a bell curve, with pH 4 being the optimal point.

Claim

I support the hypothesis stating changes in pH will affect enzymatic activity by decreasing activity as pH level deviates from the optimal level and increasing activity as it nears the optimal level.

Discussion

As supported by the results, enzymatic activity decreased as pH levels deviated from the optimal pH level of 4. This was calculated through the amount of gas produced during the reaction, which was measured by increases in gas pressure. The decrease in enzymatic activity is due to the denaturing of proteins and enzymes as pH level changes. The denaturing of these molecules prevents substrates and enzymes from binding, effectively decreasing the rate of catalysis. Only half of the bell curve relationship is shown, but if a more acidic level of pH were to be tested, a decrease in enzymatic activity would be depicted as it deviates from the optimal level in pH 4. Sources of error in our experiment include the pressure reading due to the pressure sensor. Because someone had to hold the cap on the pest tube, they could have unintentionally added to the pressure reading by holding the cap down too tightly and/or inadvertently changing the pressure with which they held the cap down. Another source of error in our results would be the lack of trials performed. There was only enough time to perform one trial, but the best and most accurate results are derived from a plethora of trials so that one can take a more representative average rate of reaction. A further question for investigation along the lines of this experiment include differences in optimal pH levels for different enzymes.