The methods by which enzymes catalyse biological reactions are examined in this area of the AP Biology curriculum. We’ll begin by looking at how reactions occur in the absence of enzymes and why a specific level of activation energy is necessary for the reaction to occur. Then we’ll look at what happens to reactants in terms of energy as they move through an enzyme-catalyzed reaction. We’ll examine the structure of enzymes and the method by which a substrate binds to the active site of an enzyme to get a better understanding of how the enzyme actually reduces the required initiation energy and accelerates a reaction. Finally, we’ll investigate how enzymes function in both endothermic and exothermic processes.
Overview of Enzyme Catalysis
Chemical reactions have the potential to be explosive! However, most compounds do not react with one another immediately. In order to begin, the majority of reactions need the addition of a quantity of energy, referred to as activation energy.
This would be a major issue for cells. Without enzymes, cells would be stuck for hundreds or thousands of years waiting for specific metabolic processes to occur. So, how do enzymes reduce the reaction’s activation energy? This question will very certainly be on the AP exam in some form. Therefore, stick with us as we go through all you need to understand regarding enzyme catalysis!
We must first comprehend how an enzyme functions and why it is a fundamental component of all cells before we can comprehend how a normal chemical reaction occurs. Because practically every biological reaction occurs in an aquatic medium, let’s look at what really happens at the molecular level in a solution.
In an aqueous solution, billions and trillions of water molecules are constantly moving and pulling at one another. Once we add two reactants to this solution, they will not immediately react with one another. The two molecules must first discover each other. This is no simple feat, given the fact that the water molecules are tugging them around. Second, the two molecules must really collide at the exact correct spot. If they don’t, they could merely bounce off each other. Finally, they must travel quickly enough in the desired direction to actually cohere the molecules, causing the reaction to occur. Then, and only then, will there be a response.
These are all referred to as “activation energy” processes. In the laboratory, we may generate the requisite activation energy by increasing the temperature, the concentration of reactant molecules in a solution, and also by electrifying the solution. All of these strategies increase the probability that two reactant molecules will collide at the optimal speed and position. Cells, on the other hand, lack these sophisticated laboratory tools and must depend on enzymes. Enzymes supply the activation energy required for critical biological events to occur quickly.
A catalyst is whatever substance that accelerates a process, often by providing the activation energy necessary for the reaction to occur. A biological catalyst, often referred to as an enzyme, is a protein or RNA molecule that contributes to the reduction of the activation energy required for a particular activity. Let’s look at it more closely.
Consider the dissolution of a large macromolecule into micro molecules known as monomers. Due to the strength of the covalent bonds between monomers, this reaction takes a considerable amount of activation energy to initiate. This reactant would need high temperatures or a lengthy period without a catalyst before the circumstances were exactly correct and enough energy was given for the reaction to occur. This is beyond the patience of the cells!
Cells have evolved into the ultimate biological machinery to speed up and regulate critical biochemical events throughout billions of years of evolution. Enzymes grasp the reactant, precisely arrange the reactants, and significantly reduce the activation energy necessary to initiate a reaction.
For a variety of reasons, enzymes are regarded as catalysts rather than reactants. To begin with, when we examine the outcome of a process, we can see that there is no variation in liberated energy between an enzyme-catalyzed reaction and one that occurs without an enzyme. In other words, each approach produces the same amount of energy in the final product. The change in Gibbs Free Energy (abbreviated delta-G) is the same in both processes, suggesting that an identical quantity of energy was liberated overall. This illustrates that the enzyme did not provide substance or energy to the reactants or products; rather, it lowered the energy necessary to initiate the reaction.
Let’s take a pause from the complexity and do a little thought experiment to get a better understanding of how enzymes work. Take these twenty toothpicks as substrate molecules, and your fingers as an enzyme that has developed to break toothpicks in half. You reach down and break an intact toothpick in two without even looking at the toothpicks. If you do this for a while and graph the data, you can see exactly how an enzyme acts in a solution containing substrate.
There are no broken toothpicks at 0 seconds. But you go straight to work, and your fingers discover intact toothpicks with ease. Because you may easily come across a full toothpick each time you reach down, your task is simple and goes swiftly for a while. After that, when you run out of undamaged toothpicks, progress becomes more difficult. Your fingers take longer to locate the unbroken toothpicks, and your enzyme rate slows. You eventually run out of toothpicks to shatter, and the flow of new items slows to a halt. In solution, most enzymes function similarly, they operate at rapid speed until they run out of substrate molecules to change into products!
Thus far, we’ve examined the functions of enzymes in biological processes. Consider how enzymes actually lower activation energy!
A substrate would penetrate the active centre, and if it fit, a reaction would happen miraculously, according to an obsolete idea of enzyme operation known as the “lock and key model.” If the improper substrate was introduced, the enzyme would not work and no reaction would occur. This concept, however, could not explain how the enzyme really reduced the activation energy necessary for the process to occur.
The induced fit method is a method that more accurately describes how an enzyme decreases the activation energy needed to initiate a process. This model is based on the same concept as the last one: that the substrate and active centre should be physically and chemically similar in order to attract one another. However, in this method, the substrate does not have the precise form, size, or charge that the active centre requires. When the substrate is brought into the active site, the whole enzyme needs to significantly modify its shape to accommodate it. This conformational shift provides energy to the substrate molecule, increasing the likelihood of a reaction. Consider how this works in two unique ways.
Consider an enzyme that breaks down a substrate into tiny pieces during a catabolic process. When a single substrate molecule binds to an enzyme, the enzyme undergoes a shape change and undergoes stress. One of the substrate molecules’ bonds. This tension causes the link to dissolve, which causes the precise response that the enzyme is designed to help with.
The reverse occurs in an anabolic process that combines two substrates into a single molecule. The enzyme changes form when substrates connect to it, forcing the two molecules together. Because the enzyme is really driving the molecules together, they do not require to bind at the correct speed or direction, the activation energy needed is reduced.
As a result, enzymes minimize the activation energy required for processes by precisely arranging substrate molecules and slightly altering their shapes to provide pressure where it is needed.
Enzymes, as previously stated, may carry out both catabolic and anabolic responses. Catabolic reactions often release energy because they disrupt bonds. Because these reactions produce bonds, anabolic bonds usually need energy. Exothermic processes are those that produce energy, whereas endothermic reactions are those that demand energy from the environment.
This topic might be perplexing for AP students. Since it seems that if a process produces energy, zero activation energy is required to initiate the reaction. However, this is a misunderstanding. Consider an exothermic reaction similar to a bomb. Even though the bomb releases a large amount of energy, it still requires an initial spark (similar to a lit fuse) to exceed the needed activation energy. An enzyme serves as the “fuse” in the majority of catabolic and exothermic activities in the body.
Similarly, students often make the distinction between activation energy and the overall energy change in a process. If the products of a reaction have a higher energy level than the reactants, you may be tempted to include both the activation energy and the difference in the energy levels of the products and reactants when calculating the total energy change. However, the difference between the reactants and products is all that matters in terms of total energy change. The activation energy may be readily reduced by employing an enzyme, although the total energy change remains unchanged.
This is significant because it demonstrates that enzymes are primarily responsible for bringing the proper chemicals together, positioning them correctly, and stressing the creation or breakdown of the correct connections. Otherwise, enzymes have little impact on the entire chemical process. This allows enzymes to fully reset after each reaction, enabling them to conduct hundreds or thousands of reaction cycles with a single enzyme.