Lab : Enzymes : Life's Chemical Machines
Introduction:
The Scientific Method:
Enzymes allow many chemical reactions to occur within the
homeostasis constraints of a living system. Enzymes function
as organic catalysts. A catalyst is a chemical involved in, but
not changed by, a chemical reaction. Many enzymes function
by bringing the reactants closer together, chemical bonds may
be weakened and reactions will proceed faster than without the
catalyst.
As you go through the lab, the
scientific method will be your
guide to the process. Click here
for more information about the
scientific method.
Enzymes can act rapidly, as in the case of the enzyme carbonic anhydrase (enzymes typically
end in the -ase suffix), which causes the chemicals to react 107 times faster than without the
enzyme present. Carbonic anhydrase speeds up the transfer of carbon dioxide from cells to the
blood. There are over 2000 known enzymes, each of which is involved with one specific
chemical reaction. Enzymes are substrate specific. The enzyme peptidase (which breaks peptide
bonds in proteins) will not work on starch (which is broken down by the human-produced
enzyme amylase in our mouths).
Enzymes are proteins. Whether or not an enzyme can act on a given substrate depends on the
shape of both the enzyme active site as well as the shape of the substrate. The arrangement of
molecules on the enzyme produces an area known as the active site within which the specific
substrate(s) will "fit". The active site recognizes, confines and orients the substrate in a particular
direction.
The induced fit hypothesis suggests that the binding of the substrate to the enzyme alters the
structure of the enzyme, placing some strain on the substrate and further facilitating the reaction.
Temperature: Increases in temperature will speed up the rate of non-enzyme-mediated
reactions, and so increased temperature will also speed up enzyme-mediated reactions, but only
to a point. When heated too much, enzymes (since they are proteins dependent on their shape)
become denatured. When the temperature drops, the enzyme regains its shape. Thermolabile
enzymes, such as those responsible for the color distribution in Siamese cats and color
camouflage of the Arctic fox, work better (or work at all) at lower temperatures.
Concentration of substrate and product also control the rate of reaction, providing a
biofeedback mechanism.
Activation, as in the case of the enzyme chymotrypsin, protects a cell from the hazards or
damage the enzyme might cause.
Changes in pH will also denature the enzyme by changing the shape of the enzyme. Enzymes
are also adapted to operate at a specific pH or pH range.
Allosteric interactions may allow an enzyme to be temporarily inactivated. Binding of an
allosteric effect or changes the shape of the enzyme, inactivating it while the effector is still
bound.
Competitive inhibition works by the competition of the regulatory compound and substrate for
the binding site. If enough regulatory compound molecules bind to enough enzymes, the
pathway is shut down or at least slowed down. PABA, a chemical essential to a bacterium that
infects animals, resembles a drug, sulfanilamide, which competes with PABA, shutting down an
essential bacterial (but not animal) pathway.
Noncompetitive inhibition occurs when the inhibitory chemical, which does not have to
resemble the substrate, binds to the enzyme other than at the active site. Lead binds to SH groups
in this fashion. Irreversible Inhibition occurs when the chemical either permanently binds to or
massively denatures the enzyme so that the tertiary structure cannot be restored. Nerve gas
permanently blocks pathways involved in nerve message transmission, resulting in death.
Penicillin, the first of the "wonder drug" antibiotics, permanently blocks the pathways certain
bacteria use to assemble their cell wall components.
Objectives:
1. Study the effects of temperature, pH, substrate concentration, enzyme concentration, and
enzyme specificity on the function of several types of enzymes.
2. Be able to discuss the role of enzymes in the functioning of the human body.
3. Be able to discuss the effects of altered enzymes on the human body.
4. Understand and be able to relate
the protein nature of enzymes to
their function.
Lab : Enzymes : Life's Chemical Machines : Part One
Time Requirements
This lab should take 2-3 hours to complete.
Recording Your Observations:
Click here to download the lab report for this lab, where you’ll record your hypotheses,
observations, and conclusions.
Procedures:
Your liver is very important to you. A list of its functions is quite long. One important function
is the liver’s ability to produce a chemical that destroys a very caustic substance, hydrogen
peroxide (H2O2). This chemical is produced in small amounts in cells throughout your body. All
your blood eventually flows through your liver, delivering any hydrogen peroxide to where
specialized cell structures destroy it. An enzyme in the liver cells, known as catalase,
accomplishes destruction of hydrogen peroxide. Catalase is an important antioxidant.
Notebook Activity:
What does an antioxidant do for the human body? Use the Internet or a book to answer this.
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
Materials










Commercially available bottle of hydrogen peroxide, from drugstore
Liver, either chicken or beef
An onion
A small potato
An Apple (peeled)
Wooden sticks (can be the ones used for shish kebobs)
Matches
Sharpie marker pen or other pen that can write on a glass surface
Tapwater
Several glass kitchen glasses (so you can observe the inside of the glass)
Lab : Enzymes : Life's Chemical Machines : Part Two
Procedure
1. Cut two small (1/4 to 1/2 inch long) pieces of liver.
2. Place the liver into two small glasses, labeled “C” and “E”.
3. Pour enough hydrogen peroxide from the bottle to cover the liver in glass “E”, add a little
more. Pour the same amount of water into tube “C”.
Notebook activity:
What happened to each test tube?
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
What was the purpose of the water in tube “C”?
_____________________________________________________________
_____________________________________________________________
Since hydrogen peroxide (H2O2) is a chemical made up of hydrogen (H) and oxygen (O), and
since enzymes are not SUPPOSED to be used up, what might be formed in the chemical reaction
shown below?
H2O2 -----------------------> _____________ + ____________
_____________________________________________________________
_____________________________________________________________
4. A simple test for the presence of hydrogen gas is to hold a glowing wooden applicator stick at
the mouth of the test tube or in the bubbles forming at the surface of the liquid. Hydrogen gas
produces a popping sound. A simple test for the presence of oxygen is to hold a glowing
applicator stick in one or more of the bubbles inside the tube. If oxygen is present, the glowing
end of the stick will flare up and glow brighter. Light a match and burn the end of one of the
wooden sticks. The stick should catch fire. Allow the stick to burn for a few seconds, and then
blow out the flame, leaving a glowing tip. Perform the tests described above.
Notebook activity:
Which gas, oxygen or hydrogen, was produced by the reaction? State your evidence.
_____________________________________________________________
_____________________________________________________________
5. Since enzymes are not supposed to be used up, extract the piece of liver from the glass and
add it to a second glass to which you then add more hydrogen peroxide.
Notebook activity:
What happened in the second tube?
_____________________________________________________________
_____________________________________________________________
6. Pour the liquid from the first glass into a third glass containing fresh hydrogen peroxide.
Notebook activity:
Was there a reaction? Explain.
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
Lab : Enzymes : Life's Chemical Machines : Part Three
Test and Record
Since the enzyme catalase is important to humans (or in this case cattle or chickens, the source of
the liver we are using in this lab), how might you test for the presence of the enzyme in some
common materials such as apples, potatoes, and onions? Design and record some tests you have
performed in the Enzyme Observation form to detect the presence of catalase in these items.
Notebook activity:
What is hydrogen peroxide used for around the house?
_____________________________________________________________
_____________________________________________________________
Explain its use and action in view of the activities in this lab
_____________________________________________________________
_____________________________________________________________