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All chemical reactions require an input of energy to get started, called the activation energy. Catalysts speed up the rate of reactions without being used up themselves. Organisms need to expend their energy wisely and efficiently, so they use biological catalysts called enzymes to reduce the activation energy required for a reaction to occur. This lowers the overall energy consumption of organisms.
Most enzymes are protein molecules that have a huge range of helpful functions in living organisms. Biological catalysts are used in every biological process that occurs within our cells from respiration to digestion to the immune response.
Figure 1. With the enzyme, the activation energy is much lower, and so the reaction can occur at a faster rate.
Using enzymes means that more chemical reactions can occur over a set period of time than without the enzyme, increasing the rate of reaction. You can see in Figure 1 below how the energy that must be supplied for a reaction to occur is much higher without an enzyme than with the enzyme. Imagine this one reaction occurring thousands of times over, and you can see why it is so much cheaper energetically to use an enzyme in biological reactions. Many essential reactions occurring within our cells are simply too slow to occur by themselves. For example, if the enzymes involved in respiration did not function properly, we would not be able to release enough energy in our cells to survive. Without enzymes we would be dead!
Vocab Check
Enzyme
An enzyme is a biological catalyst that speeds up the rate of reactions without being used up.
Catalyst
A catalyst is a substance that lowers the activation energy required for a chemical reaction to occur without being used up itself, so the overall reaction occurs at a faster rate.
Activation Energy
Activation energy is the minimum amount of energy required for a reaction to occur.
Enzymes are proteins. Enzymes generally have a globular shape, and on their surface, there is a region called the active site. Each enzyme has a different, specifically shaped active site. This is because each type of enzyme is suited to one, or a few, particular molecules that will bind to it, called the substrates. When the substrate has bound to the enzyme’s active site, it is called an enzyme–substrate complex, as can be seen in Figure 2 below. Some substrates can bind to a few different enzymes, but they must all have an active site specific to that particular substrate.
Vocab Check
Active Site
The active site is the region on the surface of an enzyme molecule to which a specific substrate will bind and undergo a chemical reaction.
Substrate
The substrate is the molecule, or combination of molecules, that are specific and complementary in shape for a particular enzyme’s active site.
For example, the substrate might be lactose, a sugar molecule found in milk that gives it its sweetness. The enzyme lactase, an example of a carbohydrase, has an active site that is a specific shape for lactose molecules only. Lactase would break down the lactose substrate into smaller glucose and galactose sugar molecules, which are then released from lactase’s active site and are called the products. If a person is lactose intolerant, it means they do not produce enough of the lactase enzyme to break down lactose, which leaves it sitting in their digestive system to be broken down instead by bacteria, which creates some nasty digestive problems. Luckily for them, lactase enzymes can now be purchased as a food supplement, and these enzymes are added to milk to make it “lactose free.”
You might have noticed that lactase and carbohydrase both end in the letters “ase.” This is an easy way of spotting if something is an enzyme, as almost all biological words that end in -ase are enzymes. Typically, the substrate is also found in the name of the group of enzymes, with some examples shown in Table 1 below.
Depending on the enzyme, it can be used to either join substrate molecules together or to break them apart. The top image in Figure 3 displays an enzyme being used to break apart one substrate into two products, while the bottom image displays a different enzyme being used to join together two substrates to form one product. Figure 3 shows that when a substrate molecule binds to a specific enzyme’s active site, it forms the 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, which is why we say that enzymes are not used up themselves as they can continue catalyzing reactions even after several reactions have occurred.
Each individual type of enzyme has a specifically shaped active site. Only the substrate that is involved in the reaction that the enzyme catalyzes will fit into its specific active site. We therefore say that an enzyme’s active site is a complementary shape to the substrate that fits into it.
A scientific model called the “lock-and-key” model displays this specificity of enzymes nicely. In this model, the key is the substrate, while the lock is the enzyme. The space within the lock, which is the correct shape for one specific key, is the enzyme’s active site. The top image in Figure 4 shows a substrate as a “key” that is a specific complementary shape to fit into the enzyme’s active site or “lock.” Therefore, an enzyme–substrate complex is formed and the enzyme-controlled reaction occurs. The bottom image in Figure 4 shows that when the incorrect substrate or “key” is used, it will not fit into the enzyme’s active site or “lock.” This means that no enzyme–substrate complex will form and no enzyme-controlled reaction will occur
Vocab Check
Complementary Shape
The complementary shape of an enzyme’s active site to a specific substrate molecule means that only that substrate will be able to fit with that enzyme so that it only catalyzes one specific reaction.
Though enzymes are reusable, they are not indestructible. As you can see in Figure 5, when exposed to conditions such as a high temperature or an extremely high or low pH, the active site of the enzyme changes shape (denatured). This means that it is no longer 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. Once these optimal conditions are exceeded, the enzymes involved will start to denature and the rate of reaction will drop.
In the normal enzyme on the left in Figure 5, the active site is a complementary shape to the substrate molecule and so forms an enzyme–substrate complex, and an enzyme–catalyzed reaction will occur. At high temperatures or in extreme pH, the enzyme will denature and change shape. The changed shape of the active site seen in the bottom-right image in Figure 5 means that it is no longer a complementary shape to the substrate molecule, and so no enzyme–substrate complexes will form, and the enzyme–catalyzed reaction cannot occur.
Vocab Check
Denatured
An enzyme is said to denature when its active site irreversibly changes shape so that it is no longer a complementary fit for its specific substrate molecule.
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