Lesson Video: Factors Affecting Enzyme Action Biology

In this video, we will learn how to describe the effect of temperature, pH and substrate concentration on enzyme activity.


Video Transcript

In this video, we’re gonna learn how enzymes are affected by certain conditions. And you probably remember how important enzymes are, the specialized proteins that catalyze or speed up the chemical reactions in our bodies. Now, we’re gonna see how enzymes deal or don’t deal with changing conditions in their little biological microenvironments.

We’ll begin with a quick review of enzymes. Enzymes catalyze or speed up chemical reactions. So to understand them, we need to explain them in the context of chemical reactions. Chemical reactions begin with one or more reactants, which rearrange by breaking and reforming chemical bonds to produce one or more products. Without an enzyme, chemical reactions typically need an input of energy to get started. So, here, we have the reactants in a test tube being heated with a flame that provides that necessary input of energy to get the reaction going. The bonds of the reactants rearrange in the chemical reaction, producing the product or products. But if an enzyme catalyzes this reaction, less of an energy input is required. And here’s how that works.

Enzymes have a complementary shape to their substrates. That means they fit together sort of like a lock and key. And the term substrate replaces the term reactant when the chemical reaction is controlled by an enzyme. This allows the bonds of the substrate or substrates to rearrange to those of the product or products while reducing the energy requirement for the reaction.

The vast majority of enzymes are proteins. And proteins form specific shapes based on their sequence of amino acids. And the most important shape on an enzyme is its active site. Because that’s where the substrate or substrates need to attach or bond. Enzymes are very specific. Each type of enzyme catalyzes only one particular reaction and bind only certain molecules as substrates. Enzymes aren’t used up during chemical reactions either. After releasing the product, enzymes catalyze the same reaction over and over again.

We all have fabulously huge numbers of chemical reactions being helped along by enzymes every second of our lives. And that takes us from the quick enzyme review and into the topic of factors that affect enzyme action. Beginning with, why do enzymes need a specific range of conditions? We’ll use this graph here, sort of like a really easy racing video game where we have to keep the car or functional enzyme moving forward along the 𝑥-axis within the range of some kind of condition, such as temperature or pH.

So here we go. We have to keep that car in the range of conditions where this enzyme is gonna keep working. Looks like we’re doing a bad job; we’re getting too high. But oh yeah, we saved it, alright. We’re not really straightening out though. We’re getting too low, uh-oh, and splat! Our enzyme is denatured, which means game over for our enzyme.

A denatured enzyme is one that’s been permanently changed so that it doesn’t function anymore. So, why do enzymes need a specific range of conditions? Because outside those conditions, enzymes can be denatured and then stop catalyzing our all-important chemical reactions of life.

Next, let’s take a look at what might happen to an enzyme that gets denatured. Okay, so we know that enzymes reduce the energy input needed to start a chemical reaction by binding the substrate or substrates along the complementary fit of their active site. And here’s a little memory check. What will still be present after the product or products are formed? Yep, the enzyme, good as new and ready to catalyze a whole bunch more chemical reactions. Now, let’s see what happens though when the enzyme is denatured.

Here’s a cartoon diagram of a denatured enzyme. And the key change here is the misshapen active site. This can occur when environmental conditions are outside the enzyme’s optimum range, for example, temperature being too high or pH being too low. The active site of the enzyme does not have a complementary fit for the substrate anymore, so they can’t bind. And then there’s no chemical reaction and no product or products. This is why high fevers can be fatal. Once the fever gets high enough to start denaturing enzymes, those life-sustaining chemical reactions just don’t happen enough anymore.

Here’s an example to help you better visualize a denatured enzyme. Like enzymes, egg whites are made out of protein. And once it gets too hot, those proteins denature. So, think what happens to the egg white when it’s fried. It’s a big change, went from clear and gooey to white and solid. And that’s not going back; it’s never gonna be a raw egg white again. So once denatured, always denatured; it’s irreversible. So, denature, it looks like a permanent change to an enzyme’s active site that prevents it from functioning.

So then, just what are the optimum conditions for an enzyme? Optimum conditions for an enzyme can be identified by measuring the enzyme’s activity level and catalyzing its particular reaction while varying some factor. The highest level of enzyme activity over a range of values for the factor or factors is the optimum condition for that enzyme. If the level of the factor is too low, the enzyme’s activity will also be low. And if the level of the factor is too high, the enzyme activity will also be low. But when the level of the factor is just right for the enzyme, it’s as busy as it can be, catalyzing one reaction after another. And that’s the optimum condition for an enzyme.

Let’s use pH as an example of a factor. pH values vary widely along our digestive tract, from low acidic values in our stomach to high or more basic values in our intestines. Will enzymes in these areas have the same optimum pH? No, enzymes in our stomach have optimum pH values that align with those of the stomach. And the same thing goes for the enzymes in the more basic or alkaline, high-pH areas of the intestines. But if the enzyme activity drops to zero because the active sites of the enzymes have permanently lost their shape, then they’re denatured. So, the optimum conditions for an enzyme are the conditions that allow the highest level of enzyme activity.

You may already know that reaction rates generally increase with increasing temperature, since the molecules involved have more kinetic energy, meaning they move faster when they heat up. But it gets a little more complicated when enzymes are involved. Here’s a generalized curve of how temperature can affect enzyme activity. And notice that the drop in enzyme activity is steeper at high temperatures than at low temperatures. This is because high temperatures denature enzymes, while low temperatures inactivate enzymes. The difference is that inactive enzymes can become active again, while denatured enzymes cannot regain their ability to catalyze reactions. And what’s the optimum temperature for this enzyme? The optimum temperature for this enzyme will correspond to the highest level of enzyme activity.

Next, let’s take a look at how substrate concentration affects enzyme activity. So, substrates again are the reactants in a chemical reaction that’s catalyzed by an enzyme. And concentration refers to the given amount of a substance, such as a substrate in a given space. Now, the graph has a concentration gradient of substrate molecules from low to high. Let’s see how an enzyme’s activity is affected by this.

First, if we start with no substrates, we’re not gonna have any chemical reactions. But as there’s more and more substrate, the enzyme’s gonna find more and more chemical reactions to catalyze. But it’s gonna get to a certain point where like when you walk in a bakery and you’re really hungry and you feel like you could eat everything right now, but it just can’t happen. You can only eat so much so fast. So, the level of enzyme activity levels off at that point. At this point, we say that enzyme activity has reached a plateau. Those enzymes are working as fast as they can, turning over one chemical reaction after another. Their active sites are just as full as possible. Even if you had more substrate, they just can’t work faster.

Let’s take what we’ve learned so far and apply it to a practice problem.

The graph provided shows how the rate of an enzyme-controlled reaction changes with the pH. What is the optimum pH of this enzyme?

Key knowledge required to answer this question correctly is an understanding of factors that affect enzyme action. And a good place to start is gonna be a review of how enzymes control chemical reactions. In chemical reactions, a reactant or reactants are converted into a product or products. And if the chemical reaction is controlled by an enzyme, then we call the reactant a substrate.

An enzyme’s active site has a complimentary shape to the substrate, meaning that they fit together sort of like a lock and key. The binding of the substrate to the enzyme reduces the amount of energy required to start the reaction. And reactions that occur more easily can also occur more frequently. So, enzymes increase the rate of reactions that they control. And rate of reaction is an important term in this question.

The function of an enzyme is to speed up or catalyze chemical reactions, and that’s another way of saying increasing the rate of reaction. And we wanna know what the optimum or highest rate of reaction is over a range of pH values for a certain enzyme. Well, this point here represents the highest or optimum rate of reaction. So, we just need to find its corresponding pH value, which we can see is seven.

And if you’re not familiar with the pH scale, here’s a quick review. Acids are on the lower end of the pH scale. Seven is neutral and above seven is a basic or alkaline pH. Therefore, the answer to the question, what is the optimum pH of this enzyme? is pH seven.

And here’s some key points from the video. First of all, the function of enzymes is to speed up or catalyze chemical reactions. Enzyme function is based on the complementary fit between the substrate and active site of the enzyme. Denatured enzymes have permanently lost their complementary shape and their ability to catalyze reactions.

Factors that can lead to the denaturation of an enzyme include temperatures that are too high or pHs that are either too high or too low. pH and temperature can also affect the rate of a reaction. Optimum conditions and pH or temperature occur when they maximize the rate of the chemical reaction. The rate of a chemical reaction will continue to increase as the substrate concentration increases to a point. That’s when the enzymes are catalyzing the reactions as fast as they can and the rate levels off; that’s called a plateau.

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