Chemical reactions inside living cells

BIOLOGY 211 LA-4 Enzymes Page 1 of 5


· Understand how an essay works.

· Compare the rate of peroxidase activity in varying conditions.

· Create a graph of the results and draw conclusions to support or reject your hypothesis



(Background adapted from Biology 160 Lab Manual at Seattle Central College)


A wide variety of chemical reactions must occur all the time inside living cells in order to keep them functioning properly. In very general terms, a chemical reaction can be described as:


reactants à products example 1: A + B à C example 2: E à F + G + H


In example 1, two different reactant molecules join together to form a single product. In example 2, one reactant breaks down into three separate products. Chemical reactions do not occur instantaneously, however. Without “help,” most of the chemical reactions that occur inside cells would not proceed rapidly enough to maintain life. Enzymes are proteins (produced by cells) that catalyze chemical reactions. This means that they speed up the rate at which reactions occur, without being used up or transformed into something else in the process.


An enzyme capable of catalyzing the reaction in Example 1 above might do its job by attaching to substances A and B, and holding them in position so that a bond can easily form between them. Without an enzyme, the reaction would not occur until a molecule of A and molecule of B just happened to bump into each other at precisely the right orientation for the bond to form. In example 2, the enzyme’s job might be to hold molecule E in such a way that the bonds attaching its monomers become stressed, and are therefore more likely to break.


Notice that in both of the situations described above, the enzyme must physically hold the reactant molecule(s) in order to work. Because enzymes are proteins, hydrogen and sometimes ionic bonds contribute to their specific and often quite elaborate shapes. In order for an enzyme to catalyze a reaction, it must fit together precisely with the reactant(s), like a lock and key. For enzymes, the reactants that physically touch the enzyme are referred to as the “substrate”. The particular part of the enzyme where the substrate fits in is called the active site. Because their shapes are so specific, most enzymes are capable of catalyzing only one or a few specific reactions. We can now describe a chemical reaction catalyzed by an enzyme more specifically than we did above.


substrate(s) + enzyme à substrate-enzyme complex à product(s) + enzyme


The substrate-enzyme complex exists only briefly before the substrate(s) is changed into the product(s), which separate from the enzyme. The enzyme is now available to join with new substrate molecules and repeat the process. A single enzyme molecule may catalyze thousands of reactions each second!

However, enzymes can be inactivated by factors that either cause changes in their shapes or block their active sites. If the substrate and enzyme no longer fit together, the reaction will not proceed rapidly. The hydrogen bonds that contribute to the unique shape of each different protein are relatively weak, and they can be disrupted. Some enzymes function well over a broad range of temperature and pH values, however, many enzymes can only hold their proper shapes under specific temperature or pH conditions.

The figure below shows how enzyme activity might increase with increasing temperature. Recall that as temperature rises, molecule vibration becomes faster. As molecules vibrate faster, there is a greater likelihood that substrate and enzyme molecules will encounter each other and a reaction will occur. However, note that the reaction rate abruptly decreases after a certain temperature. At this temperature, the enzyme likely denatured.

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