Enzymes: Videos & Practice Problems

Enzymes, primarily globular proteins, catalyze chemical reactions by forming an enzyme-substrate complex at their active site, speeding up reactions without being consumed. They do not convert all substrates into products but help achieve equilibrium concentrations faster. Enzymes lower the energy of activation, facilitating reactions without altering thermodynamic favorability or equilibrium constants. Understanding these mechanisms is crucial for grasping enzyme kinetics and their role in metabolic pathways, including anabolic and catabolic reactions.

Enzymes

So now that we're practically experts on amino acids, protein structure, and even protein techniques, in this video we're going to begin our discussions on protein function by starting to talk about enzymes. Most of you guys already know from your previous courses that most enzymes are globular proteins that catalyze or speed up the rate of chemical reactions without being consumed by the reaction. What we really mean by "without being consumed" is that by the end of the reaction, the enzyme takes on its original state that it had before the reaction took place. So, the enzyme is not chemically or permanently altered by the end of the reaction. It's important to note that not all enzymes are proteins, and that's because ribozymes are actually enzymes that are RNA catalysts, and they are made up of RNA nucleotides instead of amino acids. But moving forward in our course, most of the enzymes that we're going to talk about are going to be made up of proteins. So that's important to keep in mind.

Also, recall from your previous courses that the reactants of enzymes are called substrates, and the substrates will specifically bind to the enzyme at the enzyme's active site. Down below in our example, what you'll see is that this green structure here represents our enzyme, and the specific region of the enzyme that binds the substrate is referred to as the active site. In this example, it's this yellow structure here that is representing our substrate, which is the reactant of the reaction. What you'll notice is that the substrate will bind to the enzyme specifically at the enzyme's active site, and that's what we're seeing here. That forms what's known as an enzyme-substrate complex, which we'll talk more about later on in our course. But what you'll notice is that the enzyme is able to interact with the substrate in such a way that it can speed up or catalyze the chemical reaction, and it can produce the products here at a much faster rate than what the products would be produced if the enzyme were not present. Really, that's the main takeaway of this video: enzymes are able to speed up the rate of chemical reactions. Now in our next lesson video, we'll explain exactly how enzymes speed up the rate of chemical reactions. So I'll see you guys in our next video.

Hexokinase uses ATP to convert glucose into glucose-6-phosphate during glycolysis. Which option is true?

• A. Hexokinase is a fibrous protein.
• B. Hexokinase is permanently altered in the enzymatic reaction.
• C. Hexokinase is a molecular catalyst.
• D. Hexokinase & all enzymes are proteins.

Answer: (C)

Enzymes

So in our last lesson video, we said that enzymes catalyze or speed up the rates of chemical reactions. But, there's a common misconception about enzymes that they always convert 100%, or all of the substrate into product, so that there's no more substrate remaining. So in this video, I just want to clear up this idea and make sure that you guys know that this is actually not true. Instead, what enzymes do is they only help reactions get substrates and products to their equilibrium concentrations. We know that equilibrium does not necessarily mean 0. What we can say then is that enzymes really help reactions get to equilibrium at a faster rate.

Down below what we have is a typical enzyme-catalyzed reaction, very similar to the one from our previous lesson video. You can see we have the substrate here in orange, the enzyme in green; it forms an enzyme-substrate complex where the enzyme can convert the substrate into the product. In our graph here, you can see the concentration on the y-axis and the time as the reaction progresses on the x-axis. This orange curve right here represents the concentrations of the substrate over time, and the blue curve right here represents the concentrations of the product over time. Typically, when we're studying an enzyme-catalyzed reaction in a lab, we only add the substrate and some enzyme but we do not add any product. Therefore, the product concentration is 0 initially. On the time axis where the time is 0 initially, notice that the product concentration is also 0 because we don't add any product initially. But, we do have a high concentration of substrate, and we also add some enzyme as well. This allows our reaction to proceed in only one direction here initially.

As the reaction proceeds over time, the concentrations of substrate are decreasing and those of the product are increasing, because the substrate is being converted into the product. Here's where the common misconception comes into play. A lot of students think that enzymes will convert all of the substrate into the product, and so that would mean that the substrate concentration would go down to 0. And then that would also mean that the product concentration would always go as high as the initial substrate concentration. But as we can see from this graph here, this is actually not true. At the end of an enzyme-catalyzed reaction, notice that we have some substrate concentration here, and the product concentration doesn't necessarily go all the way up to the initial substrate concentration.

What we can see here is that enzymes are only helping the reaction get to equilibrium at a faster rate. Equilibrium here is being represented by this dotted line, and we know that it's at equilibrium because notice that the substrate concentration has horizontal lines. Recall from our previous lesson videos on equilibrium, that the reason the product and substrate concentrations don't change is because the rate of the forward reaction is exactly equal to the rate of the reverse reaction. Because when we're at equilibrium, we need to consider the reverse reaction, that means that it's better to sometimes consider the reverse reaction as well in an enzyme-catalyzed reaction. Even though we don't typically think of enzymes being able to catalyze the reaction in the reverse direction, they can in many cases. It's important to sometimes consider these reverse arrows.

Later in our course, as we move forward, we'll be able to talk even more about these equilibrium arrows that exist in a typical enzyme-catalyzed reaction. But, for now, all I want you guys to realize is that the substrate concentration does not necessarily go down to 0, and instead, all enzymes do is they help reactions get to equilibrium faster. That concludes this lesson, and I'll see you guys in our next video.

Which graph below corresponds with a reaction that has an equilibrium constant less than 1 (Keq < 1)?

• A.
• B.
• C.

Answer: (C)

The enzyme catalase converts hydrogen peroxide into water and oxygen gas. Which of the following is true?

• A. All enzymes utilize ATP to catalyze reactions.
• B. All hydrogen peroxide is converted to products.
• C. Catalase stops catalyzing when reaction reaches equilibrium.
• D. Catalase catalyzes forward & reverse reactions at equilibrium.

Answer: (D)

Enzymes

So the way that enzymes speed up chemical reactions is by lowering their energy of activation, which can be abbreviated with either of these two symbols shown here. And so recall from your previous chemistry courses that the energy of activation is literally just the energy difference between the substrates of the reaction and the transition state. And the energy of activation is required to initiate a reaction. So the transition state, which can be abbreviated with this double dagger symbol shown here, is an unstable transient entity at the local maximum peak energy point of a reaction. And so because it's symbolized with a double dagger symbol, it makes a lot more sense why the energy of activation is symbolized with Δ_G^‡, because it's the difference in energy between the substrate and the transition state.

So down below in our image, we have two different scenarios. We have a scenario where there's no enzyme that's present on the left and a scenario where there is an enzyme that's present on the right. And so over here on the left, what you'll notice is that it's showing the same exact reaction where we're converting substrate A into products B and C. And because there's no enzyme that's present, this is the uncatalyzed reaction. And down below in our energy diagram where we have free energy on the y-axis and the reaction progress on the x-axis, we can see that the local maximum peak energy point of this reaction is at the very highest point of this curve, which corresponds with the transition state. So we can go ahead and put our double dagger symbol here for the transition state, which is right at this point, the local maximum peak. And so we know that the difference in energy between the substrates, or the reactants here, and the transition state, which is this symbol up here, so essentially the difference in energy would be, symbolized by this blue box here vertically, corresponds with the energy of activation, which corresponds to the kinetics, and the kinetics is all about the speed of the reaction. So the greater this energy of activation is, the slower the reaction will take place.

And so what you'll notice is that in the presence of an enzyme, you can see that our enzyme is this red structure shown here in this example, that the reaction will take place at a much faster rate than if the enzyme were not present. And so down below, what you'll notice is in the energy diagram, the red curve represents the enzyme catalyzed, reaction. And you can see that we have the same exact curve except we have a smaller energy of activation for the catalyzed reaction and the transition state is much lower in energy. And so, because the energy of activation for the enzyme catalyzed reaction is smaller, that allows the reaction to occur faster. And again, that's exactly what we say enzymes are responsible for doing, speeding up chemical reactions. And so the enzyme catalyzed energy of activation is, being represented by this yellow region right here, the difference in energy between the enzyme catalyzed transition state and the reactant. And so one thing that's really important to note is that enzymes do not affect either the thermodynamic favorability of the reaction, meaning that there's no change to the ΔG of the reaction. So notice that the ΔG, which is the difference in energy between the reactant and the product, does not change even in the presence of an enzyme. So notice that the ΔG is represented by this pink box. And the pink box does not change in size from the left side, where there is no enzyme that's present, and, from the right side, where there is an enzyme that's present. So it's very important to keep in mind that enzymes do not affect this thermodynamic favorability of ΔG. And the second thing that enzymes do not affect is the equilibrium constant. So there's no change to the K_eq, which recall from our previous lesson videos, is the ratio of the concentration of products over the concentration of reactants at equilibrium. And so these are two very important things to note that enzymes do not affect. Enzymes only affect the energy of activation. They only affect the kinetics and the speed of the reaction. So that's important to note moving forward, and in our next couple of videos, we'll be able to get some practice utilizing these basic concepts. So I'll see you guys there.

Which of the following is the best description of what an enzyme does?

• A. It allows a chemical reaction to proceed extremely fast.
• B. It increases the rate at which a chemical reaction reaches equilibrium relative to its uncatalyzed rate.
• C. It makes a reaction thermodynamically favorable to allow it to proceed faster.

Answer: (B)

Which of the following best describes how enzymes catalyze reactions?

• A. Alter the equilibrium constant (K_eq) of a reaction.
• B. Decrease the thermodynamic free energy (ΔG) of a reaction.
• C. Change nonspontaneous reactions into spontaneous reactions.
• D. Force reactions to proceed in only one direction.
• E. Stabilizing the transition state.
• F. c & d.

Answer: (E)

Here’s what students ask on this topic:

What are enzymes and how do they function in chemical reactions?

Enzymes are primarily globular proteins that act as catalysts in chemical reactions. They speed up reactions by lowering the energy of activation, which is the energy required to initiate a reaction. Enzymes achieve this by forming an enzyme-substrate complex at their active site, where the substrate binds specifically. This interaction stabilizes the transition state, reducing the energy needed for the reaction to proceed. Importantly, enzymes are not consumed in the reaction; they return to their original state after the reaction is complete. This allows them to catalyze multiple reactions. Enzymes do not alter the thermodynamic favorability (ΔG) or the equilibrium constant (K_eq) of the reaction; they only affect the reaction rate.

Do enzymes convert all substrates into products?

No, enzymes do not convert all substrates into products. Instead, they help reactions reach equilibrium faster. At equilibrium, the concentrations of substrates and products remain constant because the rate of the forward reaction equals the rate of the reverse reaction. This means that some substrate will always remain, and not all of it will be converted into product. The common misconception is that enzymes convert 100% of the substrate into product, but in reality, they only facilitate the reaction to reach its equilibrium state more quickly.

How do enzymes lower the energy of activation in a chemical reaction?

Enzymes lower the energy of activation by stabilizing the transition state of the reaction. The energy of activation is the energy difference between the substrates and the transition state, which is an unstable, high-energy intermediate. By binding to the substrate at their active site, enzymes provide an alternative reaction pathway with a lower energy transition state. This reduces the energy required to initiate the reaction, allowing it to proceed faster. The enzyme-substrate complex facilitates the proper orientation and interaction of reactants, further lowering the energy barrier.

What is the difference between enzyme kinetics and thermodynamics?

Enzyme kinetics refers to the study of the rates of enzyme-catalyzed reactions and how they change in response to various factors such as substrate concentration, enzyme concentration, and the presence of inhibitors or activators. It focuses on the speed of the reaction and the energy of activation. Thermodynamics, on the other hand, deals with the energy changes associated with chemical reactions, specifically the Gibbs free energy (ΔG) and the equilibrium constant (K_eq). While enzymes affect the kinetics by lowering the energy of activation, they do not alter the thermodynamic properties of the reaction, such as ΔG or K_eq.

What are ribozymes and how do they differ from protein enzymes?

Ribozymes are RNA molecules that act as catalysts in biochemical reactions, similar to protein enzymes. Unlike protein enzymes, which are made up of amino acids, ribozymes are composed of RNA nucleotides. Ribozymes catalyze reactions by forming specific three-dimensional structures that facilitate the chemical transformation of substrates. They play crucial roles in various biological processes, including RNA splicing and the catalysis of peptide bond formation in ribosomes. While most enzymes discussed in biochemistry are proteins, ribozymes highlight the versatility of RNA in catalysis.