In this explainer, we will learn how to describe the effect of temperature, pH, and substrate concentration on enzyme activity.
Did you know that the typical human cell makes over 1 000 different enzymes? Enzymes are vitally important for all life on Earth. They allow our cells to carry out the many different chemical reactions they are responsible for, quickly and efficiently. They are biological catalysts that work by lowering the activation energy required for a reaction to occur. This means that more reactions can occur over a set time, increasing the rate of reaction. Not only do enzymes catalyze most of the reactions occurring within our bodies that are essential to our survival such as respiration and digestion, but they are also used commercially, for example, in synthesizing antibiotics.
Enzymes are made up of protein molecules and have a region on their surface called the active site. Each enzyme has a different, specifically shaped active site. This is because each type of enzyme is complementary to one particular molecule that will bind to it, called the substrate. In Figure 1, the substrate binds to the enzyme’s active site, and the whole structure is called an enzyme–substrate complex. When the enzyme has done its job, it releases the molecules (which are now called products) from its active site. The active site is now free to have more substrate molecules bind to it. In chemical reactions, enzymes are not “used up”—this means they can continue catalyzing reactions, even after several reactions have occurred.
Key Term: Active Site
The active site is the region on the surface of an enzyme molecule to which a specific substrate will bind and where it will undergo a chemical reaction.
Key Term: Substrate
The substrate is the molecule, or combination of molecules, that has a specific and complementary shape for a particular enzyme’s active site.
Key Term: Product
The product is the molecule, or combination of molecules, that is released from the enzyme’s active site following an enzyme-controlled reaction.
Though enzymes are reusable, they are not indestructible. As you can see in Figure 2, when exposed to conditions such as a high temperature or an extremely high or low pH, the protein structure of the enzyme changes. This means that its active site changes shape, and the enzyme is said to have denatured. The active site no longer has a complementary shape to its specific substrate molecule, and it will no longer be able to function to catalyze a biological reaction. Enzymes all have an optimum temperature and optimum pH at which they function to catalyze a reaction faster. If these optimum conditions are exceeded, the enzymes involved will start to denature and the rate of reaction will drop. This change is irreversible, so when the active site has changed shape, it will not be able to change back.
Key Term: Optimum (Temperature/pH)
The optimum is the favorable level of a certain condition for a reaction to occur at its highest rate. For example, an enzyme’s optimum temperature or pH is that at which it works to catalyze a reaction at the fastest rate.
Denaturation occurs when an enzyme’s active site irreversibly changes shape so that it no longer has a complementary shape to its specific substrate molecule.
Example 1: Explaining the Process of Denaturing
Which of the following best explains what happens when an enzyme denatures?
- A change in the lipid structure of an enzyme causes a change in the shape of the active site.
- A change in the structure of an enzyme causes the active site to become permanently bound to a substrate.
- A change in the carbohydrate structure of an enzyme causes a change in the shape of the active site.
- A change in the protein structure of an enzyme causes a change in the shape of the active site.
Enzymes are proteins that increase the rate of a reaction by lowering the activation energy required for a reaction to occur. They work by having an active site that is complementary to the shape of a substrate molecule so that only one particular molecule can fit into it. When exposed to high temperatures and extreme pHs, the protein structure of an enzyme changes, which means the active site also changes shape irreversibly. This is called denaturing and it means that the active site is no longer a complementary shape to the substrate molecule. The substrate molecule can no longer bind to the active site and enzyme-controlled reactions can no longer occur, therefore decreasing the rate of reaction.
As enzymes are proteins and not carbohydrates or lipids, the two answer options that state otherwise are incorrect.
As denaturation causes a change in the active site of the enzyme so that the substrate cannot bind at all, it definitely will not become permanently bound to the enzyme. Therefore, the answer that states otherwise is also incorrect.
The following, therefore, is the correct explanation of what happens when an enzyme denatures: a change in the protein structure of an enzyme causes a change in the shape of the active site.
We are now going to look in more detail at how temperature, pH, and substrate concentration affect the rate of enzyme activity. We will practice distinguishing between describing and explaining what happens as these factors change. Describing means that we state what we can observe happening. This usually means looking at data on a graph, for example, and stating how the gradient, or slope, of the line changes according to the two variables. It is good to quote the values you are using as evidence when you are describing a graph or table. Explaining gives the science behind what is occurring. We explain “why” we observed the change we have just described.
Let’s take a look at how temperature affects enzyme activity.
We can describe the graph in Figure 3 by stating that it shows that as the temperature increases from to , the rate of reaction also increases. From to , the rate of reaction decreases. The thermal range of the enzyme, which is the range between the temperature at which enzyme activity starts and stops, is . This is the entire range of temperatures within which this enzyme can function.
Let’s explain why we see these changes.
Most chemical reactions are sped up by heating. This is because increasing temperature increases the thermal (heat) energy supplied to the particles involved. This thermal energy is converted into kinetic energy, and more kinetic energy means that the particles move faster and collide with each other more frequently.
In the case of enzyme-controlled reactions, the increase in kinetic energy means that the substrates will collide with the enzyme’s active site more frequently and form more enzyme–substrate complexes and therefore more products. If more products are formed with higher temperatures in the same amount of time as with a lower temperature, this means that the rate of reaction has increased. Remember that though the enzymes are “reusable” and so can bind with multiple substrates, each substrate can only bind to an enzyme once.
This is visible on the graph in Figure 3, where at point 1, from to , as the temperature increases, so does the rate of reaction. Point 2 at is described as the enzyme’s optimum temperature. This is the temperature at which the rate of reaction is highest, so the most products are being formed per unit of time. Once the temperature has passed at point 3, however, the enzymes start to denature as the temperature is too high. This decreases the rate of reaction because fewer complementary active sites are available for the substrates to bind to.
It can be helpful to know the optimum temperatures for specific enzymes due to the useful functions they provide us with. For example, detergents used to wash clothes often include enzymes. By knowing the optimum temperatures of these enzymes, the manufacturers know the temperature that will most effectively clean the clothes.
Example 2: Interpreting Graphs of the Effect of Temperature on the Rate of Enzyme-Controlled Reactions
The graph provided shows the rate of an enzyme-controlled reaction compared to temperature.
- State the number on the graph that shows that the rate of collisions between enzymes and substrates is increasing with an increase in temperature.
- State the number on the graph that shows that the enzymes have begun to denature.
As temperature increases, the kinetic energy contained within substrate and enzyme molecules also increases. This means that they move faster, and the substrates are more likely to successfully collide with the enzyme’s active site. Remember that though the enzymes are “reusable” and so can bind with multiple substrates, each substrate can only bind to an enzyme once.
On this graph, that occurs between and . Therefore, the number on the graph that shows that the rate of collisions between enzymes and substrates is increasing with an increase in temperature is 1.
The optimum temperature of a particular enzyme is the temperature at which it works at its fastest rate to catalyze a reaction and produce products, therefore giving the highest rate of reaction. On this graph, the optimum temperature for this enzyme is , as this is the temperature at which the rate of reaction is highest.
Once an enzyme has passed its optimum temperature, the active site begins to change shape so it is no longer complementary to the shape of its specific substrate molecule. The enzyme is said to denature, and this change is irreversible. It means that the enzyme will no longer be able to catalyze the reaction, and the rate of reaction will fall. The point at which the enzymes begin to denature on this graph is beyond .
Therefore, the number on the graph that shows that the enzymes have begun to denature is 3.
It is important to remember that not all enzymes will have denatured when the optimum temperature is passed, which is why the rate of reaction does not immediately drop to zero. There are billions of tiny enzymes and substrates involved in reactions, and not all of them will be supplied with the same thermal energy to cause them to all denature immediately. Furthermore, many enzymes can tolerate slight increases in temperature above their optimum.
Eventually, when the temperature reaches , all the enzymes have denatured and the reaction cannot occur at all. Not all enzymes have the same optimum temperature, but most that work within the human body have an optimum temperature of , which is why our bodies work so hard to keep ourselves at that temperature consistently! A rise in temperature of just a few degrees can make us seriously unwell as our enzymes simply stop working efficiently as some of them begin to denature.
Example 3: Explaining the Effect of Low Temperature on the Rate of Enzyme-Controlled Reactions
Why will most enzyme-controlled reactions occur slowly at a low temperature?
- A lower temperature means the molecules have less elastic energy, so they do not bind together as frequently.
- A lower temperature means the molecules have more kinetic energy, so they collide frequently.
- This is incorrect; most enzyme-controlled reactions will occur quickly at low temperatures.
- A lower temperature means the molecules have less kinetic energy, so they do not collide as frequently.
As temperature increases, the kinetic energy contained within substrate and enzyme molecules also increases. This means that they move faster and the substrates successfully collide with the enzyme’s active site more frequently. This results in more enzyme–substrate complexes forming and more products being released per unit of time, so the rate of enzyme–controlled reaction increases.
If the temperature decreases, the kinetic energy of the substrate and enzyme molecules also decreases. This means that both will move more slowly and that successful collisions between an enzyme’s active site and a substrate molecule will be less frequent.
Therefore, the reason why most enzyme-controlled reactions occur slowly at a low temperature is that a lower temperature means the molecules have less kinetic energy, so they do not collide as frequently.
Let’s take a look at the effect of pH on enzyme activity and describe the changes in the graph in Figure 4.
pH, otherwise known as the power or potential of hydrogen, is a representation of the concentration of hydrogen ions in a solution. A higher hydrogen ion concentration indicates a more acidic solution and a lower pH. A lower concentration of hydrogen ions indicates that the solution is more basic, or alkaline, and so has a higher pH.
The graph shows how the activity of an enzyme changes with a change in pH. As the pH increases from 2 to 7, the rate of reaction increases. From pH 7 to pH 12, the rate of reaction decreases. In very acidic or alkaline conditions, the rates of enzyme-controlled reactions are low.
Let’s explain why we see these changes.
For example, for the enzyme in Figure 4, the optimum pH is 7 as this is the pH at which the rate of reaction is highest. Each individual enzyme has a specific optimum pH. The rate of reaction is low in acidic conditions from pH 2 to pH 6, and in alkaline conditions it is from pH 8 to pH 12. This is because above and below the optimum pH, many of the enzymes involved in this reaction will start to denature, and their active sites will change to no longer be complementary to the shape of their substrate molecule.
Though many enzymes in the human body have an optimum pH of 7 like the one in Figure 4, not all enzymes have the same optimum pH. It depends on where they are located. The stomach, for example, is very acidic, which protects us against disease-causing microorganisms. All enzymes that work in the stomach therefore need a very low optimum pH as they will be exposed to these acidic conditions constantly.
Example 4: Interpreting Graphs of the Effect of pH on the Rate of Enzyme-Controlled Reactions
The graph provided shows how the rate of an enzyme-controlled reaction changes with the pH.
What is the optimum pH of this enzyme?
Each individual enzyme has a specific optimum pH. This is the pH at which its active site is most likely to successfully collide with a substrate and therefore catalyze a reaction at the fastest rate. When the pH is changed significantly above or below this optimum pH, the enzymes begin to denature. This changes the shape of the active site of the enzyme, so it is no longer complementary to the substrate molecule and the enzyme-controlled reaction will not occur.
The optimum pH is when the rate of reaction is highest, so look for the highest point on the graph and read the pH value at which this occurs.
Therefore, the optimum pH of this enzyme is 7.
Finally, let’s look at the effects of substrate concentration on enzyme activity.
The concentration of a substrate describes how much substrate there is compared to the volume of space it occupies. For example, if we have 50 substrate molecules in a cell and then increase it to 100 substrate molecules in the same cell, the concentration of substrate will have increased as the volume of the cell has not changed.
Key Term: Concentration
The concentration of a substance is how much of it there is relative to the volume it occupies. For example, if the same amount of a substance takes up a smaller space, it has a higher concentration.
Let’s describe the graph in Figure 5.
As substrate concentration increases from 0, the rate of reaction increases. This occurs until point 2 where the rate of reaction levels off. Past the vertical dashed line where the graph is shaded in purple at point 3, the line begins to plateau. When a reaction reaches a plateau, it means that the line becomes flat and horizontal as the rate of reaction is no longer increasing.
Key Term: Plateau
A plateau is when the gradient of a line on a graph is zero, so the line appears flat and horizontal.
Let’s explain these changes.
As substrate concentration increases, there are more substrate molecules occupying the same volume area. This can be seen in Figure 6, with a low substrate concentration on the left occupying the same area as the high substrate concentration on the right. A high substrate concentration means that a successful collision is more likely to occur between a substrate and an enzyme’s active site. This means that more products will be formed over the same time, increasing the rate of reaction.
Immediately as the substrates begin binding with an enzyme’s active site, more active sites are occupied. This means that, temporarily, fewer active sites are available for other substrates to bind to. Points 1 and 2 on the graph in Figure 5 will therefore be occurring simultaneously, though the effects will increase as substrate concentration increases further.
Once all of the active sites of the enzyme are in use, there is no benefit to the rate of reaction from increasing the substrate concentration as there are no free active sites for the substrates to occupy. This means that after the substrate concentration has increased by a certain amount, the rate of reaction will stay the same, or plateau, which can be seen in Figure 5 at point 3, where the graph has been shaded in purple. Unless the enzyme concentration increases to allow more active sites to become available for the excess substrate molecules, the rate of reaction will remain constant.
Example 5: Explaining the Effect of Substrate Concentration on the Rate of Enzyme-Controlled Reactions
The graph provided shows the rate of an enzyme-controlled reaction compared to the concentration of substrates.
- Why does the graph plateau?
- There are no available enzymes to bind to more substrates, so the rate of reaction has reached its maximum.
- The enzymes have denatured, and so the reaction has stopped.
- There are no more substrates to be broken down, so the rate of reaction has reached its maximum.
- The products made bind to the active sites, so no more substrates can bind.
- Which of the following changes would allow the rate of reaction
to keep increasing?
- Decreasing the substrate concentration
- Increasing the concentration of the enzymes
- Decreasing the concentration of the enzymes
- Decreasing the temperature rapidly
As substrate concentration increases, there are more substrates available to bind with the enzymes’ active sites in the same area of space. This means that the rate of reaction will increase as more products will be formed per unit of time.
However, once the substrate concentration has increased so much that all of the enzyme’s active sites are occupied, increasing the concentration of substrate further will have no effect. This is because there are no freely available enzyme active sites for them to bind to. For this reason, the rate of reaction will remain the same, or plateau, as it cannot increase any further.
Therefore, the graph plateaus as there are no available enzymes to bind to more substrates, so the rate of reaction has reached its maximum.
If more enzyme concentration increases after the graph has plateaued, more active sites become available for the excess substrates to bind to. This will allow the rate of reaction to increase further as more enzyme-controlled reactions will be free to occur. Increasing the concentration of enzyme therefore will allow the rate of reaction to continue to increase.
Therefore, increasing the concentration of the enzymes would allow the rate of reaction to continue to increase.
It is worthwhile noting that temperature, pH, substrate, and enzyme concentration are not the only factors that affect the rate of enzyme activity.
For example, molecules called inhibitors can also bind to an enzyme, either to their active site or other regions. If an inhibitor binds to an enzyme’s active site, they occupy it so that a substrate cannot bind to it. Sometimes this binding is permanent and irreversible. Inhibitors that bind to other regions of the enzyme can cause a change in the shape of the enzyme’s active site, much like extreme temperatures. A change in the shape of the active site means that the enzyme is no longer a complementary shape to that of the substrate.
If these inhibitor effects occur on a large scale to many of the enzymes involved in a reaction, this can severely decrease the rate of enzyme-controlled reactions. Therefore, an increase in the concentration of enzyme inhibitors will also decrease the rate of reaction.
Let’s recap some of the key points we have covered in this explainer.
- As temperature increases, the rate of enzyme-controlled reactions increases until the optimum temperature is reached, beyond which the enzymes denature and the rate of reaction decreases.
- At a pH above or below their optimum pH, enzymes denature causing a decrease in the rate of reaction.
- Substrate concentration causes enzyme activity to increase until there are no more active sites available, at which point the rate of reaction plateaus unless enzyme concentration increases.