How Enzymes Work
Unlike inorganic catalysts, enzymes are very specific organic molecules. Each type of enzyme acts on only one particular compound, known as its substrate. The substrate briefly binds with the enzyme, and, in the process, is changed.
Each enzyme has a unique three dimensional shape, including a surface groove called an active site. The active site fits its target substrate much like a key fits in a lock. Other substances that don’t fit can't enter the active site, so no reaction occurs. If the shape of an enzyme’s active site is altered, the enzyme can no longer work, and therefore is considered to be denatured.
How to Prevent Fruits & Vegetables From Browning
Catecholase is an enzyme present in most fruits and vegetables. It facilitates the browning of cut or bruised produce by catalyzing a reaction between the substrate molecule catechol and atmospheric oxygen (O2). The product of this reaction is polyphenol, a brown substance that accumulates when fruits and vegetables are exposed to air. This color change is especially apparent in produce that has white flesh, such as apples and potatoes.
There are a few ways to prevent this enzymatic reaction from occurring. One is to submerge the fruit or vegetable in water. Water doesn’t affect the enzyme catecholase, but does reduce the availability of oxygen, a reactant required for the formation of polyphenol. This is why putting cut fresh potatoes in water delays browning.
A cofactor is the non-protein portion of an enzyme that also required for the enzyme to work. The cofactor of catecholase is a copper ion, which can be removed by changing the pH surrounding the enzyme. When the copper cofactor is removed from catecholase, the enzyme no longer works.
Easy Enzyme Experiment Using Apples
This experiment is so simple that it can be done in any classroom, and requires no special scientific equipment. The supplies for each student include one of each of the following:
1. Have each student take a bite of apple, and then immediately rub juice from the lemon over the exposed flesh. Be careful not to let the juice touch other areas of the apple.
2. Each student then takes another bite, on the opposite side of the apple. Do not put any lemon juice on this area of the apple.
3. While the reaction is occurring, the teacher can explain what enzymes are, how they work, etc.
4. After at least a half hour has passed, have the students compare the two bite marks on the apple.
The apple flesh that had been exposed to lemon juice should still be bright white, while the other bite mark, that was not exposed to lemon juice, will have browned. Browning is prevented with lemon juice because its low pH causes the copper cofactor to separate from the catecholase enzyme, after which the enzyme will no longer work. The low pH of citric and ascorbic acids is what prevents the flesh of citrus fruit from browning.
Sources and Helpful Enzyme Links
- Campbell, NA and Reece JB (2005) Biology. Seventh Edition.Pearson Education, Inc.
- Thorpe, P. ed. (2007) Biology 120 General Biology I: Laboratory Experiments & Exercises. Grand Valley State University.