by Tami Port MS

Created on: January 28, 2011

Enzymes are biological catalysts; proteins that help speed up reactions that are necessary for life. An enzyme is primarily made of protein (amino acids linked by peptide bonds), but also contains a cofactor—a non-protein portion that is required for the enzyme to work properly.

In summary, enzymes are…

    * protein molecules
    * catalysts that speed up cellular reactions

    * not permanently changed in the reactions they mediate
    * specific to what they will catalyze
    * reusable
    * usually named with an –ase suffix

* 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, which fits its target substrate much like a key fits in a lock. Other substances that don’t fit can't enter the active site and no reaction occurs. If the shape of an enzyme’s active site is altered, or the cofactor removed, an enzyme can no longer work.

* Enzymes and the Browning of Fruits and Vegetables *

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. The product of this reaction is polyphenol, which is 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. 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.

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:

* apple
* lemon wedge
* paper plate
* napkin

1. Have each student take a bite of their 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.
3. While the reaction is occurring, the teacher can lecture on 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 the low pH of the juice 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 at all.

* Sources *

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.

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