Lesson Video: Enzymes | Nagwa Lesson Video: Enzymes | Nagwa

Lesson Video: Enzymes Biology • Second Year of Secondary School

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In this video, we will learn how to describe the action of enzymes in catalyzing biological reactions.


Video Transcript

In this video, we’ll find out what enzymes are. We’ll explore their important role in catalyzing biological reactions. And we’ll also see how their activity is affected by changes in their environment. So let’s channel our inner enzyme and get this reaction started.

Proteins are biological molecules which take a wide variety of forms. In the human body, proteins can be found as hormones which help regulate biological processes, as structural components which are responsible for holding cells or tissues in place, or as enzymes which are the subject of this video. We often refer to enzymes as biological catalysts because they increase the rate of chemical reactions inside the body without being changed or used up. Enzymes are therefore really important, as many of our cellular reactions are too slow to happen by themselves.

For example, if all the enzymes involved in cellular respiration suddenly stopped working, the breakdown of glucose would not occur as frequently. Our cells would not be able to release as much energy. And we would therefore not receive enough energy to move, digest our food, or eventually even to breathe. Basically if we didn’t have enzymes, we wouldn’t be able to survive.

Some human disorders are caused by the absence of a particular enzyme. For example, people suffering from phenylketonuria are missing the enzyme that breaks down a particular amino acid called phenylalanine. This means they have to be really careful about what they eat as many foods contain the artificial sweetener aspartame, which is made of phenylalanine. If someone with phenylketonuria consumes too much aspartame, it can build up particularly in the brain and can have dangerous toxic effects.

Enzymes have a distinct structure which is closely related to their function. Enzymes are typically globular proteins, which means they have a round shape and are formed from multiple polypeptide chains interacting with each other. Within this structure, enzymes have a specific region called an active site. This is usually a groove or pocket formed by the three-dimensional structure of the protein, and it’s where the reactant for whatever reaction the enzyme is catalyzing binds. When we’re talking about enzyme-catalyzed reactions, the reactant is known as the substrate, and the shape of the active site is adapted to fit the shape of the substrate as you can see in this diagram. We call this a complementary fit. And it’s the mechanism behind enzyme specificity.

Enzyme specificity is the phenomenon whereby a particular enzyme only catalyzes a specific chemical reaction. For instance, the enzyme amylase will only catalyze the breakdown of starch into its component sugars. It can’t catalyze the breakdown of other food molecules such as proteins into amino acids or fats into fatty acids and glycerol. When the substrate molecule binds to the enzyme active site, it forms an enzyme–substrate complex. The enzyme can then catalyze the reaction to convert the substrate into the products. Once the products have been formed, they’re released from the active site, leaving the enzyme free to catalyze further reactions.

Some enzymes catalyze reversible reactions. As the name suggests, reversible reactions are chemical reactions which can happen in both directions. This is the symbol that we use to represent reversible reactions. The conversion of substrate into product is known as the forward reaction, while the conversion of product back into substrate is known as the reverse reaction, and they can both happen at the same time. Enzymes will increase the rate of both these reactions until they reach equilibrium. This is the point at which the forward reaction is happening at the same rate as the reverse reaction.

A good example of an enzyme which catalyzes a reversible reaction is carbonic anhydrase. When we breathe, we take in oxygen, which is used for cellular respiration as represented by this equation. A product of cellular respiration is carbon dioxide, which is a waste product and therefore must be removed from the body. In the cells, carbonic anhydrase converts this carbon dioxide into carbonic acid and bicarbonate ions, and the bicarbonate ions are then transported to the lungs in the blood. Once the blood reaches the lungs, the reverse reaction happens and the bicarbonate ions are converted back into carbon dioxide, which can then be breathed out.

Not all enzymes are found in an active state. Some enzymes exist as a precursor molecule known as a proenzyme. Proenzymes will only become activated under certain conditions or in the presence of certain substances. Let’s have a look at an example. Pepsin is an enzyme which is found in the stomach of humans and is responsible for digesting protein. The stomach’s gastric juice contains hydrochloric acid, giving it a pH of around two and making the stomach an extremely acidic environment. If cells of the stomach produce pepsin in its active form for secretion, it would digest the cell’s own proteins, meaning they could no longer function.

To avoid this, stomach cells produce and secrete an inactive form of the enzyme called pepsinogen. The presence of hydrochloric acid in the stomach initiates a reaction that converts pepsinogen into active pepsin. Pepsin is now ready to catalyze the reactions which break down proteins ingested in our food into amino acids.

What about other environmental conditions that can affect the function of enzymes? For enzymes to react with their complementary substrates, the two molecules must collide. This just means they must physically bump into each other so the substrate can successfully bind to the enzyme’s active site. The faster the enzyme and substrate molecules are moving, the more they will collide with each other and the higher the rate of reaction. There are several environmental conditions that affect the rate of enzyme-catalyzed reactions. And the two we’re going to concentrate on in this video are temperature and pH. Let’s look at temperature first.

We can see on this graph that as the temperature increases, the rate of the enzyme-controlled reaction also increases. This is because as the environment gets hotter, the enzyme and substrate molecules get more kinetic, or movement, energy, so they collide with each other more often. Once we go past a certain temperature, however, the rate of reaction quickly decreases. This is because at high temperatures, enzymes denature. Denaturing is when the active site of the enzyme irreversibly changes shape, meaning the complementary substrate will no longer be able to bind and the enzyme will no longer be able to function in catalyzing the reaction.

We see a similar trend when we observe how the rate of an enzyme-controlled reaction changes with pH. All enzymes have an optimum pH at which they work best, and it depends on what the conditions are like in the part of the body where they’re active. Although the enzyme and substrate molecules are unlikely to be moving faster at the optimum pH, there will be far more successful collisions between them. This graph shows the rate of a reaction catalyzed by a particular enzyme as the pH changes. As we can see, the rate of this reaction peaks around pH two, which suggests that this enzyme works best in acidic environments. Either side of the optimum pH, as the environment either becomes more acidic or more alkaline, the rate of reaction drops because the enzymes denature.

The rate of enzyme-controlled reactions can also be affected by changes in either the enzyme concentration or the substrate concentration. This graph shows that as the concentration of enzymes in a reaction increases, the rate of the reaction also increases. This only happens up to a point, however, after which the rate of reaction levels off. Let’s explain these trends on the graph in terms of what’s actually going on in the reaction. When the concentration of enzymes in a reaction increases, this initially means that there are more active sites available for substrates to bind to. If there is more binding of substrates to enzymes, more products will form in a shorter period of time and the rate of reaction will increase.

However, once all of the substrate molecules have bound to an active site and are reacting, even if we increase the number of enzyme molecules further, the rate of reaction won’t increase anymore. Because all the substrate molecules will have been used up. To increase the rate of reaction beyond this point, more substrate would need to be added. We see the same pattern with an increase in substrate concentration. Eventually, all the enzyme active sites become full, so the only way to increase the rate of reaction further would be to add more enzymes. Now we’ve learned all about enzymes. Let’s have a go at a couple of practice questions.

Enzymes act as catalysts. What does a catalyst do? (A) A catalyst ensures a reaction never ends. (B) A catalyst always maintains a constant rate of reaction. (C) A catalyst slows down the rate of a reaction. (D) A catalyst speeds up the rate of a reaction. Or (E) a catalyst increases the number of reactants in a reaction.

Enzymes are globular proteins because they generally have a round shape and are formed from multiple polypeptide chains joined together. Within their three-dimensional protein structure, they have a groove or pocket known as the active site. This is where the reactant or substrate for a particular chemical reaction combine to the enzyme. We often refer to enzymes as biological catalysts because they catalyze chemical reactions that take place inside the body. A good example of a chemical reaction that’s catalyzed by enzymes would be cellular respiration.

But what do we actually mean by this word “catalyze”? As this question is asking us, what does a catalyst do? This graph shows the amount of product the substance produce during a chemical reaction over time when there’s no enzyme present. We can see that the reaction progresses at a constant rate as the product is gradually made. However, when we add an enzyme, we can see that the product is made much more quickly over the course of the reaction. In other words, the reaction is happening at a faster rate when the enzyme is present. This is because catalysts such as enzymes reduce the amount of energy that’s needed for a reaction to take place, meaning it can happen much more quickly.

We have therefore demonstrated that the correct answer to the question is (D), a catalyst speeds up the rate of a reaction.

Let’s try another question.

A student is completing an experiment studying the rate at which trypsin, an enzyme found in the human body, breaks down proteins in a beaker of milk. They are running the experiment at 20 degrees Celsius with a pH buffer of pH nine. What changing conditions would most likely speed up the rate of reaction? (A) Reducing the amount of light the reaction is exposed to. (B) Decreasing the concentration of trypsin. (C) Increasing the temperature to 37 degrees Celsius. Or (D) increasing the pH to 14.

For an enzyme-catalyzed reactions such as this to take place, the substrate, in this case proteins, must physically collide with the enzyme. Therefore, any condition which increases the chances of the enzyme and substrate colliding will increase the rate of the reaction. There are four main changes in conditions that can do this. So let’s consider each one in the context of this reaction.

The first is to increase the concentration of the enzyme, which in this case is trypsin. The more trypsin molecules there are in the reaction, the more likely each protein molecule is to collide with one and therefore the higher the rate of reaction. This means we can rule out option (B) because it’s talking about decreasing the concentration of trypsin, which would actually have the opposite effect and decrease the rate of reaction.

The second change is to increase the concentration of the substrate, which in this case is protein. This works in the same way as increasing the trypsin concentration. The more protein molecules there are in a reaction, the more they will collide with trypsin molecules and the higher the rate of reaction. The third is to change the pH, which is how acidic or alkaline the conditions are. You may recall that pH is measured on a scale from one to 14, where pH one represents a very acidic environment and pH 14 represents a very alkaline environment. pH seven represents a neutral environment, which is neither acidic nor alkaline.

All enzymes have an optimum pH. This is the pH at which the enzyme catalyzes its chemical reaction at the highest rate. The further away the conditions are from the optimum pH, the slower the rate of reaction. Because most body cells have a neutral pH of around seven, most enzymes have an optimum pH of around seven too. For the experiment carried out in the question, we’re told that a pH buffer of pH nine is used. We can therefore also rule out option (D) because it suggests increasing the pH to the most alkaline value of 14. Any substantial change in pH like this is likely to inhibit the enzyme’s activity and would therefore decrease the rate of reaction.

We can also rule out option (A) because although there are a few examples of enzymes in the human body which are sensitive to light, it’s very dark inside the digestive system where trypsin is active. So it’s highly unlikely to respond to changes in light intensity.

The final changing conditions which will increase the rate of an enzyme-catalyzed reaction is to increase the temperature. At higher temperatures, both enzyme and substrate molecules have more kinetic, or movement, energy. Meaning they will collide more often because they’ll be moving around more and the rate of reaction will therefore be higher. Humans have an average body temperature of 37 degrees, so most enzymes in the human body, including trypsin, will be adapted to work best at this temperature. Therefore, increasing the temperature from 20 degrees up to 37 degrees will be very likely to speed up the rate of the reaction. We can therefore conclude that the correct answer is (C). Increasing the temperature to 37 degrees would be most likely to speed up the rate of reaction.

Let’s summarize what we’ve learnt in this video by reviewing the key points. Enzymes are globular proteins because they have a round shape and are formed from multiple polypeptide chains interacting with each other. They are often known as biological catalysts because they speed up chemical reactions in the body without being changed or used up. Enzymes have a region called an active site, which has a specific shape that is complementary to a particular substrate. Some enzymes, such as pepsin, need to be activated before they can catalyze their particular reaction. And finally, the rate of enzyme-controlled reactions is affected by changes in conditions such as temperature and pH.

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