Washington University Young Scientist Program
http://medicine.wustl.edu/ysp
Chemistry Teaching Team
Enzyme Catalysis Module
We will do two experiments with enzymes today. First, we will analyze the activity of
Lactaid on milk. Second, we will analyze the ability of a common food to dissolve
gelatin.
1. catalysis by lactase
2. catalysis by bromelain
Introduction
What is an enzyme?
An enzyme is like a tiny machine, and your body has thousands of kinds of enzymes.
Your body uses enzymes to break apart chemical bonds in your food so you can digest it.
An enzyme is a catalyst. A catalyst helps a reaction occur, but is not changed during the
course of the reaction.
Why do we care about enzymes? Well, all living organisms have them. And all living
organisms must be able to get energy by breaking down the chemicals that they have
obtained from other living things. So, enzymes are one of a few features that define an
organism as “alive”.
You may have learned that your body contains several kinds of biological molecules,
including carbohydrates and proteins. A carbohydrate is a long, chainlike molecule made
of carbon (C), hydrogen (H), and oxygen (O) atoms. A protein is similar but also contains
nitrogen (N) atoms. Both of these molecules are energy sources for your body. Your
body gets energy from carbohydrates and proteins by breaking them into smaller pieces.
Enzymes are required to do this.
Experiment 1: Enzymes break down sugars: catalysis by lactase
A sugar is a type of carbohydrate. Today we will be talking about the sugar lactose,
which is a disaccharide made of the monosaccharides galactose and glucose.
Lactose intolerant people produce too little of the enzyme lactase, which cleaves the
sugar lactose into the sugars galactose and glucose. Lactose is found in milk and other
dairy products. If a lactose intolerant person eats or drinks something containing large
amounts of lactose, he or she will experience stomach discomfort for several hours.
Washington University Young Scientist Program
http://medicine.wustl.edu/ysp
Chemistry Teaching Team
Materials
2-4 quarts milk
Dixie cups
Lactaid
Plastic dropper bottles containing Fehling’s solution
Test tubes
Sharpie markers
Hot plates and beakers filled with boiling water
Procedure
1. 1-2 days before the demonstration, add a capsule of Lactaid to a quart of milk, shake
vigorously, mark the container, and refrigerate.
2. At the beginning of the demonstration, pour ~100 mL water into a beaker and heat on
the hot plate.
3. Give each student a Dixie cup containing milk treated with Lactaid, and another
containing untreated milk. The students will note that the treated milk tastes slightly
sweeter.
4. Pass around the container of Lactaid tablets and explain that the active ingredient in the
tablets is the enzyme lactase.
5. Instruct the students to label test tubes “+” and “-“. Have them squirt 5 mL of
Fehling’s solution into the premarked test tubes.
6. Instruct the students to add several drops of the Lactaid-treated (“+”) or untreated (“-“)
milk to the test tubes.
7. Heat test tubes in boiling water on the hot plate. After a few minutes the “+” tubes
should be changing color at a noticeably faster rate, because the untreated milk will have
significantly less reactive reducing sugars present. After about 10-12 minutes, a redorange precipitate should fall.
Notes
Glucose and galactose taste sweeter than lactose, so you can taste the difference between
plain milk and milk treated with Lactaid.
Washington University Young Scientist Program
http://medicine.wustl.edu/ysp
Chemistry Teaching Team
Lactose, galactose, and glucose are all reducing sugars, so they will react with Fehling’s
solution to produce a red precipitate of copper (I) oxide. Lactase-treated milk will contain
a higher concentration of reducing sugars than plain milk, so it will react to a greater
degree with Fehling’s solution. [a reducing sugar has aldehyde or ketone groups that can
reduce alkaline solutions of metals such as copper. In the case of ring sugars, they will
react in water to break the ether linkages, which will then become reactive aldehydes.]
Questions
1. What differences do you observe between the two cups of milk?
The “+” cup tasted sweeter.
2. What was added to the “+” cup?
Lactaid, which is a dietary supplement. The active ingredient in Lactaid is the
enzyme lactase.
3. What did the additive do to the milk?
The additive (lactase) catalyzed the breakdown reaction of lactose to glucose and
galactose.
4. Which cup of milk has more sugar? Why?
The “+” cup has more sugar. The Lactaid-treated milk reacted more
extensively with the Fehling’s solution because of the higher concentration of
sugars. There is more sugar in the “+” cup because of the action of the lactase
enzyme.
5. How does Lactaid help people who are lactose intolerant?
People who are lactose-intolerant take Lactaid with a meal that contains lactose.
The Lactaid mixes with the lactose-containing food, breaking down the lactose
into glucose and galactose. Our digestive systems are able to digest glucose and
galactose, so the lactose-intolerant person can then digest their food.
6. We added only a few drops of Lactaid to a quart of milk, but the Lactaid made the milk
noticeably sweeter. How is this possible?
Each enzyme molecule was able to break apart numerous lactose molecules. The
ability to perform the same reaction numerous times is a characteristic of
enzymes.
Washington University Young Scientist Program
http://medicine.wustl.edu/ysp
Chemistry Teaching Team
Experiment 2: Enzymes break down proteins: catalysis by bromelain
Some enzymes break apart proteins into smaller chunks. Gelatin is a common substance
that is made up of proteins. Gelatin is added to various foods (Jello) to give them a
characteristic consistency.
Materials
Unflavored gelatin packets
Hot water
Test tubes
Fresh pineapple, cubed
Canned cubed pineapple
Procedure
1. For each group of students, heat 1 cup of water on the hot plate to boiling. Remove
from hot plate and add 1 packet gelatin powder.
2. Instruct the students to label three test tubes and do the following:
tube 1: pour in hot gelatin
tube 2: place one chunk of fresh pineapple in tube, pour hot gelatin over
tube 3: place one chunk of canned pineapple in tube, pour hot gelatin over
3. Place the three tubes in a large beaker filled with ice water to speed the gelatin
solidification. When tube 1 has solidified, take all three tubes out and compare them.
Notes
It may be useful to explain to the students that tube 1 is a control for the experiment, and
why controls are important for interpreting the results of the experiment.
Questions
1. What happened to the gelatin in the test tubes?
Tube 1: the gelatin solidified over several minutes
Tube 2: the gelatin did not solidify
Tube 3: the gelatin solidified over the canned pineapple
2. What is in the pineapple that could keep the gelatin from gelling?
The pineapple contains an enzyme called bromelain that breaks apart chemical
bonds in the proteins that make up gelatin.
Washington University Young Scientist Program
http://medicine.wustl.edu/ysp
Chemistry Teaching Team
3. Why didn’t the canned pineapple cause any change in the gelatin?
The canned pineapple also contains the bromelain enzyme, but the enzyme was
inactivated during the canning process, which involves high heat. Because the
enzyme is a protein, it is sensitive to heating and other physical changes.