The amount of energy necessary to start a biochemical reaction is called activation energy. Special proteins called enzymes lower activation energy, allowing life to exist. In this lesson, explore how enzymes work and see what conditions affect them.
Enzymes and Reactions
Have you ever been out for a walk or jog and had to face a big hill or a long flight of steps? If so, you know that it takes a lot of energy (and oxygen) to get up over that hill or up those steps! Wouldn’t it be great if there were something that could flatten out that hill or cut the number of steps in half?We may not have that ability, but there are substances in nature that can change the amount of energy needed to accomplish something. Enzymes are biological catalysts, or molecules that lower the amount of energy needed to start a biochemical reaction.
You can visualize this energy, called activation energy, as that high hill. Enzymes lower that hill. Without the lowering of this energy, most of the processes required for life wouldn’t be possible.
For example, consider the turnip. As part of its normal, everyday life, this root vegetable’s cells produce hydrogen peroxide. This compound is highly toxic to cells. The turnip uses an enzyme called peroxidase that breaks hydrogen peroxide down into water and oxygen, which are harmless. Peroxidase has a specific area, called the active site, to which the hydrogen peroxide (called the substrate) binds. The water and oxygen are the products of this reaction; the enzyme itself is free to tear apart another molecule of hydrogen peroxide. The active site of peroxidase is picky – because of its shape, it can’t bind to other substrates.
Likewise, if something happens to the active site to change its shape, it won’t be able to bind to the hydrogen peroxide. You can think of it sort of as puzzle pieces that fit together.Peroxidase is called a catabolic enzyme, since it functions in breaking down substrate. You might remember this by thinking that catastrophes break down landforms, like hurricanes pounding into sand dunes. Enzymes that combine substrate molecules to build new products are called anabolic enzymes.
Also, the names of enzymes usually end in -ase, and the first part of the term sometimes refers to the enzyme’s substrate. So, peroxidase is an enzyme that breaks down peroxide.
Investigating the Peroxidase Reaction
The peroxidase reaction can be summarized in the following equation: 2H2O2 –> 2H2O + O2.
But how can we tell if this reaction is occurring? To tell for sure, we need to either measure if the substrate is disappearing or if the products are appearing. We can qualitatively (that is, without numbers) tell that oxygen is appearing by using a chemical called an indicator. For this reaction, we can use an indicator called guaiacol, which changes from clear to brownish-gold in the presence of oxygen without being involved in the reaction itself. As the product oxygen is made, it reacts with the indicator: the darker the indicator, the more oxygen present.First, we need to test the activity of the enzyme by running a baseline. To do this, we’ll create two tubes: one that has hydrogen peroxide, neutral buffer, and guaiacol, and one that has neutral buffer and peroxidase. To be clear – that means one tube has substrate, neutral buffer and indicator, and one contains neutral buffer and enzyme.
At this point there is no reaction – why? The answer is that the substrate and enzyme are in separate tubes. We’ll mix the two tubes, and monitor the color change over a period of about two minutes. Once the substrate and enzyme are brought together, oxygen begins to form as a product. This oxygen combines with the indicator, turning it from clear to amber/gold.But what would happen if we changed the pH of the buffer? Let’s consider what happens to proteins when pH is changed.
Recall that the shapes of proteins are due in part to the charges on the amino acids in their sequences. When placed in an environment that interferes with those charges, such as a change away from protein’s normal pH, the proteins will denature, or lose their shape. This means the substrate can’t properly bind to the active site. As you might imagine, not being able to bind to the active site means that the rate of reaction is going to slow down – or, in more extreme cases, stop entirely.Let’s choose a slightly acidic pH and a very basic pH.
Notice that the reaction occurs in the slightly acidic pH conditions, but does not appear to occur at all in the strongly basic condition. Again, this is due to a partial loss of conformation of the active site in the first condition and a total loss of the active site in the second. It’s very important to note that the effect of changes in pH depends on the value of the enzyme’s normal or optimal pH. For example, a pH of two may denature the peroxidase enzyme, but it’s actually the optimal pH for some of the enzymes present in stomach acid.
What have we covered in this lesson? First, enzymes are biological catalysts, or molecules that lower the amount of energy needed to start a biochemical reaction. Their ability to lower this energy, called activation energy, makes life possible.
In this lesson, we looked at an example reaction in which the enzyme peroxidase catalyzes the breakdown of the substrate hydrogen peroxide into water and oxygen. Oxygen, in a separate reaction, interacts with an indicator, guaiacol. As this chemical is exposed to oxygen, it turns from clear to brown. Peroxidase is a catabolic enzyme, because it breaks down its substrate. Other enzymes, which help to assemble molecules into products, are called anabolic enzymes.
Changing the environment of an enzyme may have profound effects on its ability to catalyze a reaction. For example, altering the environmental pH from an enzyme’s ‘normal,’ or optimal, pH, will likely denature the protein, altering the shape of the active site. A misshapen or lost active site will cause a decrease in the reaction rate, since the substrate and the enzyme cannot bind together optimally.
Once you are finished, you should be able to:
- Recall what an enzyme is and its purpose
- Describe the two types of enzymes
- Explain the effect of pH on an enzyme’s ability to react