Acid-Base Catalysis

Now, remember that we said that enzymes are made of proteins? Proteins are important biological compounds. You probably know some foods that are high in protein, like meat and eggs. Have you ever fried an egg? The protein in the liquid egg white changes to a solid white mass when heated, because the heat changes how the protein is put together. This change is permanent. Since enzymes are proteins, they too can be changed by heating. The addition of acids and bases can also affect how a protein is put together. In this investigation, you will treat the catalase enzyme in yeast with acids and bases and then see how well the enzyme is able to break down hydrogen peroxide afterwards.

Problem: How do acids and bases affect enzymes?  

Materials

5 Clear glass containers of equal size (beakers or test tubes are ideal)

Permanent marker

Tape

5 Clean spoons

Distilled water

Small cup

Baking soda

Set of measuring teaspoons

Measuring cup

Hydrogen peroxide

Dry yeast

Ruler

Lemon juice

Procedure

Label the containers: 1- Control, 2- Low Acid, 3- High Acid, 4- Low Base, and 5-High Base. 

Put a spoon in each of the containers, and make sure to never move a spoon from one container to the other.

Pour a ½ cup of distilled water in the cup.

Add one teaspoons of baking soda, and stir it to dissolve.  

Add two teaspoons of distilled water to container 1- Control.

Stir in ¼ cup of hydrogen peroxide to container 1- Control.

Stir in 1/8 teaspoon of yeast.

Place the ruler alongside the container, and record the highest height the bubbles reach

Now that you have an idea of what happens when catalase is added to hydrogen peroxide, predict the height of the bubbles for the other containers which you will treat with acids and bases.  Record your predictions.

Create the acid-treated containers. Add one teaspoon of lemon juice to container 2- Low Acid and two teaspoons of lemon juice to container 3-vHigh Acid.

 Add one teaspoon of distilled water to container 2- Low Acid so it is the same volume as container 3.

Stir in ¼ cup of hydrogen peroxide to containers 2 and 3.  

Add 1/8 teaspoon yeast to both container 2 and 3. Stir and observe.  

Record the maximum height of the yeast bubbles.

Create the base-treated containers.  Add one teaspoon of the baking soda solution to container 4- Low Base and two teaspoons of the baking soda solution to container 5- High Base.

Add one teaspoon of distilled water to container 4- Low Base so it has the same volume as container 5.

Stir in ¼ cup of hydrogen peroxide to containers 4 and 5.

 Add 1/8 teaspoon yeast to both container 4 and 5. Stir and observe.  

Record the maximum height of the yeast bubbles.

Compare your predictions with your actual observations.

Results

The bubble height is likely to be highest is the 1- Control.  The bubble height is likely to be higher in both the Low Acid and Low Base than in the High Acid and High Base.

Why?

The catalase enzyme works best around pH 7, which is pH neutral. This indicates that it works best when there is neither excess acid nor base. The addition of a little acid (lemon juice) changes the catalase protein a little, causing it not to work quite as well, whereas the addition of more acid changes the catalase protein more, further reducing its ability to create oxygen bubbles.  The addition of a little base (baking soda) changes the catalase protein a bit, causing it not to work quite as well, whereas the addition of double the amount of baking soda further damages the catalase protein.

Going Further

If you want a more accurate picture of what the acid and base are doing in the reaction, use some pH paper to see how the pH is changed.  Also, not all enzymes work best at neutral pH. The enzyme lactase evolved to work in the digestive system, where pH’s are not neutral. Do an experiment to determine at what pH level lactase works best.