In this explainer, we will learn how to describe the properties of enzymes and outline the lock-and-key theory of enzyme action.
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.
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!
An enzyme is a biological catalyst that speeds up the rate of reactions without being used up.
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.
Definition: Activation Energy
Activation energy is the minimum amount of energy required for a reaction to occur.
Example 1: Defining Enzymes
Which of the following statements correctly defines an enzyme?
- An enzyme is an inorganic catalyst.
- An enzyme is a biological catalyst.
- An enzyme is a molecule that has been broken down.
- An enzyme is a fast reaction.
- An enzyme is a product of digestion.
An enzyme is a catalyst that increases the rate of a reaction by lowering the activation energy required for a reaction to occur.
Option C, describing a molecule that has been broken down, is referring to a product and so is incorrect for our definition of an enzyme. Enzymes are used in digestion, but they are not the products of it, so option E is also incorrect. Enzymes make reactions occur faster, but they are not a reaction themselves, but a physical molecule, so D is incorrect, leaving options A and B. An inorganic molecule means it does not contain carbon, so it is not a biological matter.
As enzymes are proteins involved in biological reactions, they are organic molecules, so our correct definition is B: an enzyme is a biological catalyst.
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.
Definition: 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.
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.
|Name of the Substrate||Name of the Group of Enzymes||What the Enzymes Do|
|Lipid (fat)||Lipase||Break down fats into fatty acids and glycerol|
|Carbohydrate (sugar)||Carbohydrase||Break down larger carbohydrates (e.g., starch) into smaller carbohydrates (e.g., glucose)|
|Protein||Protease||Break down proteins into amino acids|
|DNA||DNases||Break apart DNA molecules|
|Lactose||Lactase||Break down lactose sugars into glucose and galactose sugars|
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.
Example 2: Describing How Enzymes Affect Biochemical Reactions Using a Graph
The graph provided shows how the addition of an enzyme affects a biochemical reaction. What key aspect of this reaction has changed with the addition of an enzyme?
- The time taken for the reaction to become complete has increased.
- The free energy of the reaction has increased.
- The activation energy has been lowered.
- The volume of products formed has decreased.
- There have been no obvious changes.
The question is asking us to determine how enzymes affect biochemical reactions. Let’s define what an enzyme is, so we can decipher the information given to us in the graph about this enzyme’s action. Enzymes are biological catalysts that increase the rate of a reaction by lowering the activation energy required for a reaction to occur.
The graph shows us that as the reaction proceeds, the amount of free energy stored in the molecules involved in the reaction changes. The red line represents the free energy in the reaction without an enzyme, while the green line shows us the same with an enzyme.
We can see that initially, the free energy contained within the reactants of both reactions is the same and remains constant, shown by the horizontal line on the far left. An input of energy into the reactants raises this level of energy in both reactions, which increases the free energy to a peak. The free energy in both reactions then decreases again, and the free energy stored in the products is lower than in the reactants, shown as the horizontal plateaus of both lines on the far right of the graph.
The red and green vertical arrows labeled as show us the difference between the energy stored in the reactants and that at the peak energy levels in both reactions. This value represents the activation energy needed for each individual reaction to occur. We can see that the activation energy for the reaction that occurs without an enzyme (red) is much higher than the reaction occurring with an enzyme (green).
Therefore, we can deduce that the aspect of this reaction that changes with the addition of an enzyme is that the activation energy has lowered.
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.
Key Term: Complementary Shape/Fit
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.
Example 3: Identifying the Enzymes, Substrates, and Products Involved in the Lock-and-Key Theory
The diagram provided shows a basic outline of the lock-and-key theory.
- Which letter represents the enzyme?
- Which letter represents the substrate?
- Which letter represents the products of this reaction?
Enzymes are biological catalysts 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 only one particular molecule can fit into it. After the substrate has been broken down or joined to another substrate by the enzyme, it is released from the active site of the enzyme. This frees the active site, which remains unchanged, for another substrate molecule to bind to it.
As the molecule labeled “X” contains a groove (the active site), this must be the enzyme. Furthermore, the shape of an enzyme remains unchanged after it has been used to catalyze a reaction. As molecule “X” remains the same shape throughout the reaction, the letter that represents the enzyme must be X.
The substrates will bind to the active site on the enzyme, which suggests that the substrate is the molecule labeled as “Y.” Molecule “Y” splits into two distinct subunits, showing that the enzyme “X” has catalyzed a reaction breaking down the substrate “Y” into its constituent parts. Therefore, the letter that represents the substrate must be Y.
As molecules “Z” are the constituent subunits that are released by the enzyme after the substrate “Y” is broken down, these must be the products of the reaction. So, the letter that represents the products of this reaction is Z.
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. 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.
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.
Example 4: Describing Changes in the Active Site
The enzyme in the diagram has had its active site irreversibly changed. What scientific term describes this change?
Enzymes are biological catalysts 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 only one particular molecule can fit into it. This active site changes shape in extreme pH or high temperatures, so the substrate is no longer a complementary and specific fit for that particular enzyme.
Enzymes are molecules that are not alive and therefore cannot be killed, so describing this change in the shape of the active site as dying would be inaccurate. Though enzymes can break down into their subsequent parts like most other macromolecules through decomposition, this is not the term used for a change in the shape of the active site. Although the active site can be described as deformed, a more accurate term for this change in shape of the active site is denaturation.
Our correct description of this change is therefore denaturation.
Let’s recap some of the key points we have covered in this explainer.
- Enzymes are biological catalysts that speed up the rate of reaction without being used up.
- They are proteins with a specifically shaped active site.
- The lock-and-key model proposes that each enzyme has an active site complementary to the shape of a substrate molecule, fitting together like a key in a lock to catalyze reactions.
- In extreme pH and high temperatures, enzymes denature as their active site changes shape.