60 Enzymes
A substance that helps a chemical reaction to occur is called a catalyst, and the molecules that catalyze biochemical reactions are called enzymes. Most enzymes are proteins and perform the critical task of lowering the activation energies of chemical reactions inside the cell. Most of the reactions critical to a living cell happen too slowly at normal temperatures to be of any use to the cell. Without enzymes to speed up these reactions, life could not persist. Enzymes do this by binding to the reactant molecules and holding them in such a way as to make the chemical bond-breaking and -forming processes take place more easily.
It is important to remember that enzymes do not change whether a reaction is exergonic (produces energy, spontaneous) or endergonic (requires energy, non-spontaneous). This is because they do not change the free energy of the reactants or products (i.e. they do not change the amount of energy stored within a molecule of reactant or within a molecule of the product). They only reduce the activation energy required for the reaction to proceed (Figure 60.1). In addition, an enzyme itself is unchanged by the reaction it catalyzes. Once one reaction has been catalyzed, the enzyme is able to participate in other reactions.
Figure 60.1 Image Description
The image is a graph showing how enzymes affect activation energy during a chemical reaction. The vertical axis is labeled energy, and the horizontal axis is labeled reaction path. This represents the progress of the reaction over time. On the left side of the graph are the reactants, shown at a lower energy level. Two curves rise from the reactants to the products. A solid purple line represents a reaction without an enzyme and rises to a high peak, indicating a large activation energy. A red dashed line represents a reaction with an enzyme and rises to a much lower peak, showing that enzymes lower the activation energy required for the reaction to occur. Both curves end at the same energy level on the right, labeled products, indicating that the overall energy change of the reaction is the same with or without the enzyme. The diagram emphasizes that enzymes speed up reactions by lowering activation energy, not by changing the final energy of the products.
The chemical reactants to which an enzyme binds are called the enzyme’s substrates. There may be one or more substrates, depending on the particular chemical reaction. In some reactions, a single reactant substrate is broken down into multiple products. In others, two substrates may come together to create one larger molecule. Two reactants might also enter a reaction and both become modified, but they leave the reaction as two products. The location within the enzyme where the substrate binds is called the enzyme’s active site. The active site is where the “action” happens. Since enzymes are proteins, there is a unique combination of amino acid side chains within the active site. Each side chain is characterized by different properties. They can be large or small, weakly acidic or basic, hydrophilic or hydrophobic, positively or negatively charged, or neutral. The unique combination of side chains creates a very specific chemical environment within the active site. This specific environment is suited to bind to one specific chemical substrate (or substrates).
Video Transcript
Most chemical reactions do not occur spontaneously in a cell. Instead, cells rely on proteins called enzymes to kickstart chemical reactions and speed them up, enabling cells to get the most out of the energy sources available to them.
Enzymes have a unique way of kickstarting reactions. They work by binding to one or more specific molecules called reactants, or substrates. Binding occurs at a special region on the enzyme called the active site. Once the substrates bind to the active site, they form what’s called an enzyme substrate complex.
As the enzyme and substrates begin to react, some of the chemical bonds in the substrates begin to weaken, causing them to link together. Eventually the chemical reactions at the active site lead to the formation of a different molecule. This is referred to as the product.
Once the reaction has occurred, the product is released from the active site. The enzyme returns to its original state and is free to react again with another set of substrates.
For many years, scientists thought that enzyme-substrate binding took place in a simple “lock and key” fashion. This model asserted that the enzyme and substrate fit together perfectly in one instantaneous step. However, current research supports a model called induced fit (Figure 60.3). The induced-fit model expands on the lock-and-key model by describing a more dynamic binding between enzyme and substrate. As the enzyme and substrate come together, their interaction causes a slight shift in the enzyme’s structure that forms an ideal binding arrangement between enzyme and substrate.
Figure 60.3 Image Description
The image illustrates the induced fit model of enzyme action, which shows how an enzyme and its substrate interact to catalyze a chemical reaction. On the left, a large shape with a cavity in the top represents the enzyme’s active site. A smaller shape labeled substrate approaches the active site. Instead of fitting perfectly right away, the enzyme’s active site changes shape slightly as the substrate binds, molding itself around the substrate. After the substrate binds, the diagram shows a chemical reaction occurring and products being released from the active site. Labels indicate the enzyme, substrate, and products, and arrows show the direction of the interaction from binding to reaction to product release.
Video Transcript
Enzymes are protein molecules that speed up chemical reactions in the cell. A special region in the enzyme, called the active site, is the area where enzyme activity takes place. An enzyme works by binding to molecules, called substrates, at its active site.
The job of the enzyme is to convert the substrate into a different product or products, through a series of chemical reactions. Following the reactions, the products are released and the enzyme is free to act on another substrate. At the active site, the substrate should fit into the enzyme almost like a key fits into a lock. Because of this, most enzymes can fit only one substrate.
To accomplish this lock and key fit, the active site undergoes a slight change in shape, in order to better accommodate the substrate. This is called the induced fit model, because the enzyme is induced to undergo small changes, so that the substrate can achieve optimum fit.
All enzymes have optimal environmental conditions that are correlated with maximum enzyme activity. Environmental conditions, such as pH and temperature, play a role in how efficient the enzyme is in conducting its reactions. When conditions are less than optimal, an enzyme will lose its configuration and slow its activity. This is due to a change in the three-dimensional shape of the enzyme, and is called denaturation.
When an enzyme binds its substrate, an enzyme-substrate complex is formed. This complex lowers the activation energy of the reaction and promotes its rapid progression in one of multiple possible ways.
- On a basic level, enzymes promote chemical reactions that involve more than one substrate by bringing the substrates together in an optimal orientation for reaction.
- Enzymes promote the reaction of their substrates is by creating an optimal environment within the active site for the reaction to occur. The chemical properties that emerge from the particular arrangement of amino acid R groups (side chains) within an active site create the perfect environment for an enzyme’s specific substrates to react.
- The enzyme-substrate complex can also lower activation energy by compromising the bond structure so that it is easier to break.
- Finally, enzymes can also lower activation energies by taking part in the chemical reaction itself. In these cases, it is important to remember that the enzyme will always return to its original state by the completion of the reaction.
One of the hallmark properties of enzymes is that they remain ultimately unchanged by the reactions they catalyze. After an enzyme has catalyzed a reaction, it releases its product(s) and can catalyze a new reaction.
Video Transcript
Most of the chemical reactions that occur in your cells do not occur spontaneously. Instead, cells rely on proteins, called enzymes, to kick start chemical reactions and speed them up, enabling cells to get the most out of the energy sources available to them. In fact, if it weren’t for enzymes, chemical reactions would proceed too slowly to support life.
This graph illustrates how enzymes speed up chemical reactions. Remember that in a chemical reaction, the reactants interact to form a product. The chemical reactants for this example are shown on the left, and the products on the right. The wall that separates them represents the activation energy. You can think of this wall as an energy “speed bump.” The larger the bump, the slower the reaction.
The yellow speed bump represents a chemical reaction without an enzyme, and the orange speed bump represents the same reaction with an enzyme. As you can see, the orange speed bump is a lot lower. This is because the enzyme acts to physically bring the reactants together. By doing so, it increases the efficiency of the reaction, and lowers the amount of energy needed for the reaction to occur. Since less energy is required, the reaction occurs at a faster rate.
Notice that the enzyme does not influence the energy level of the reactants or the products, but only the amount of energy that is required during the process of the chemical reaction. Without the use of enzymes, many of our body’s processes, such as digestion, and the processing of nerve impulses, would simply occur too slowly.
References
Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.
Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016. http://cnx.org/contents/b3c1e1d2-839c-42b0-a314-e119a8aafbdd@9.10
