Enzymes: A Study of Biochemical Reactions
A chemical reaction occurs when chemicals come together (the reactants) and their molecules interact by the
breaking or formation of bonds. The results are new chemicals (the products). Sometimes chemical reactions happen by
themselves. These reactions are usually very fast and spontaneous and give off energy. Other chemical reactions need
energy to happen, and without energy proceed v e r y s l o w l y or not at all.
These types of chemical reactions can be helped to occur more quickly by using enzymes. Enzymes are made out
of protein (amino acid chains!) and they speed up the rate of a chemical reaction by acting as a catalyst. A catalyst provides
the necessary environment for the chemical reaction to occur, which speeds up the reaction. Certain catalysts work for
certain kinds of reactions. In other words, each enzyme has a particular type of reaction that it can activate.
Enzymes, as we just said, are proteins, which are molecules that are very large and dynamic. They can be very
fussy, and sometimes need to be in certain environments or conditions to work, and the ideal conditions are usually
reflective of where the enzyme must normally function in the body. In other words, different tissues are home to different
enzymes, and an enzyme should be able to function in the conditions that surround it in the tissue it's supposed to function
in. Some enzymes can be damaged under certain conditions, such as heat. A damaged enzyme will no longer work to
catalyze a chemical reaction. (NOTE! This is a huge hint for later in the lab!)
One source of enzymes is the liver, which needs to break down many substances in the body. Catalase is one
enzyme from liver that breaks down harmful hydrogen peroxide into oxygen gas and water. When this chemical reaction
occurs, you can see the oxygen gas bubbles escaping and causing the reaction to foam. In our cells there are many reactions
occurring. One by-product from these reactions is hydrogen peroxide. Hydrogen peroxide can be toxic to our bodies if a
sufficient amount is built up, so our cells contain an enzyme, catalase, which helps break down hydrogen peroxide into nonharmful substances. The reaction that takes place is shown below:
2H2O2 (Hydrogen Peroxide) ---> 2H2O (water) + O2 (Oxygen)
2 hydrogen peroxide molecules react to form 2 water molecules and 1 oxygen molecule.
This reaction is V E R Y S L O W without an enzyme. Since enzymes remain unchanged in a reaction, catalase is able
to catalyze a new reaction over and over until all of the hydrogen peroxide is broken down. The cells we are extracting the
enzymes from are chicken liver cells because they contain large amounts of catalase, since the function of the liver is to detoxify
and purify substances in our bodies.
Purpose: To compare the action of catalase to a non-protein catalyst under different conditions.
Materials: manganese dioxide stirring rod
potato
test tubes
250 mL beaker
scalpel
fresh or frozen liver
hydrogen peroxide
sand
hot water bath
ice
scale
graduated cylinder
Procedure with Analysis Questions (record answers to questions in your logbook)
A. Catalytic reactions
Add 2 mL of hydrogen peroxide to two test tubes. Place 0.1 g of sand in one test tube and add 0.1 g of manganese dioxide to the
second tube. Observe and record the rates of reaction. The manganese dioxide and hydrogen peroxide reaction is a
decomposition reaction - the bonds within the H2O2 are broken. As you already know, the MnO2 is a catalyst, so it is not
included in the balanced equation: 2H2O2 → 2H2O+O2 A1) What gas do you think evolves? A2) What role do you think the
sand and H2O2 test tube played in this procedure?
B. The effect of an enzyme
Add 2 mL of hydrogen peroxide to each of two clean test tubes. In one place a small piece of liver and in the other a small piece
of potato. Record the rates of reaction and compare these results to those with manganese dioxide. Do not discard these
materials.
C. Re-using an enzyme
Divide the liquid portion of the previous tube in B containing the liver into two test tubes. Cut the liver from procedure B into
two equal portions and add these to the two test tubes. To the first add a fresh piece of liver and to the second add 1 mL of
hydrogen peroxide. Record your observations. C1) What could explain the reaction in the test tube containing the fresh liver?
C2) What do you think would happen if additional hydrogen peroxide were added to the second tube?
D. Effect of particle size
Place a small piece of liver in one test tube and a small piece of potato in a second. Add a pinch of sand to each tube and crush
these materials with separate stirring rods. Add 2 mL of hydrogen peroxide to each tube, then observe and record the rates of
these reactions. D1) How do these results compare with those of the uncrushed liver and potato in part B?
E. Effect of temperature
Place a small piece of liver into each of two test tubes. Place one test tube in a 37'C water bath for 5 minutes and the second test
tube in an ice-water bath for the same length of time. Remove both test tubes from the water baths and add 2 mL of hydrogen
peroxide to each tube. Record the rates of the reactions.
Observations: Record your results in a chart similar to the one shown below. Rates of reaction can be designated as follows: 0 = no
reaction, + = slow, ++ = moderate, +++ = fast.
0 = no reaction, + = slow, ++ = moderate, +++ = fast.
Mixture
Part A
H2O2 + Sand
H2O2 +MnO2
Part B
H2O2 + Liver
H2O2 + Potato
Part C
Re-use + Fresh Liver
Re-use + H2O2
Part D
H2O2 + Crushed Liver
H2O2 + Crushed Potato
Part E
H2O2 + Liver + Hot Water Bath
H2O2 + Liver + Cold Bath
Questions: answer in complete sentences in your logbook
1.
2.
3.
4.
5.
What is catalase and what does it help our bodies do?
Why do you think we chose liver to extract the catalase from?
What did the Hydrogen Peroxide represent in this lab?
Were there any results that surprised you?
With the various temperatures (cold, room temp, body temp, hot), which one had the greatest activity? Propose a reason
why this would be.
6. For two situations where there was no reaction (or very little), propose reasons for these results. Think about how an
enzyme’s shape plays a role in its ability to function.
7. Using your results, describe the effect of temperature and particle size on the rate of enzyme action.