Lesson video

This lesson is called enzymes: function, structure, and specificity, and is from the unit, biological molecules and enzymes.

Hi there, my name is Mrs. McCready and I'm here to guide you through today's lesson.

So thank you very much for joining me.

In our lesson today, we're going to explain what an enzyme is and why the shape of an enzyme is important for how it works.

Now we're gonna cover a number of key words in today's lesson, and they're shown with their definitions up on the screen now.

You may wish to pause the video and make a note of them, but I will introduce each word as we come across them throughout the lesson.

Now in our lesson today, we're going to firstly look at enzyme function before we look at the structure of an enzyme, and then we'll look at enzyme specificity to finish.

So I hope you're ready to go.

I certainly am.

Let's get started.

So we know that all organisms are made from one or more cells.

So let's look at the tiger in the jungle in the picture.

We can see that the tiger is made of animal cells and the plants are made from plant cells.

Now all cells contain biological molecules called enzymes.

And enzymes are absolutely vitally critical to life on Earth.

That is because they are essential for all biological processes.

There is not a single biological process that happens without the use of enzymes.

Because without the use of enzymes, life would not be able to exist.

So why is that? Enzymes are critical because they speed up the rate of chemical reactions.

They are biological catalysts.

So you can see in the graph there, the rate of a reaction happening without an enzyme is significantly lower than the rate of a reaction with an enzyme.

Now chemical reactions occur within cells, and because of an enzyme being present to catalyse that reaction, they happen millions of times faster with the enzyme than they would if the enzyme were not present.

And there are many chemical reactions that simply would not happen fast enough in order to keep an organism alive.

And so enzymes are critical to life on Earth.

So what is an enzyme? Is it a chemical reaction, a biological catalyst, or a lock and key? I'll give you five seconds to think about it.

So hopefully you've decided that an enzyme is a biological catalyst.

Well done.

Now some industrial chemical processes use catalysts to speed them up.

So let's look at the Haber process, for instance.

The Haber process uses nitrogen and hydrogen to form ammonia, and it uses an iron catalyst in order to speed that reaction up and make it go fast enough to be worth doing at an industrial scale.

But the iron catalyst is not an enzyme, and that's because the iron catalyst is not a biological molecule.

So although the iron is a catalyst, it's speeding up the rate of the reaction, it is not an enzyme because an enzyme is a biological molecule which speeds up the rate of a reaction.

Now some processes happen within cells without enzymes.

For instance, diffusion happens because particles move randomly, and therefore move about the organism.

So diffusion is about the movement of particles and substances into around and out of cells.

This does not require enzymes.

But enzymes are essential to control a vast range of chemical reactions that happen within and outside of cells.

For example, respiration requires a huge number of enzymes in order to make it function properly.

Photosynthesis also requires many different enzymes to function effectively.

Muscle contraction requires enzymes.

Digestion requires enzymes, so does our immune response, and so does protein synthesis.

These are just a few examples of where enzymes are critical to life processes within cells.

So life is able to exist because of enzymes, and it's able to exist with complexity and diversity because of enzymes.

So let's just quickly check our understanding.

Which of these reactions are controlled by enzymes? Is it the Haber process, diffusion, or photosynthesis? Okay, so hopefully you've decided that photosynthesis is the example reaction controlled by enzymes, well done.

So what I'd like you to do now is to firstly write a definition for this term enzyme; then I would like you to list three biological processes that use enzymes; then I'd like you to draw a bar chart, which shows the change in rate of a reaction without and with an enzyme.

I've already shown you an example of that.

And then finally, I'd like you to have a go at describing what type of biological enzymes must be knowing that they are made from amino acids.

So pause the video, have a good go at those tasks, and come back to me when you are ready.

Okay, let's check our understanding of these.

So your definition for the term enzyme should be that an enzyme is a biological catalyst that speeds up the rate of a chemical reaction without being used up.

Three biological processes that use enzymes might include respiration, photosynthesis, movement, digestion, immune responses, protein synthesis, cell division, or any other biological process which requires enzymes that you might have listed.

Well done, by the way.

The bar chart that you should have drawn to show the difference of the reaction rate with and without an enzyme should look something along the lines of this with a very slow rate of reaction when the enzyme isn't present and a much, much higher rate of reaction when the enzyme is there.

And then finally, this question that enzymes are made from amino acids.

So what type of biological molecules must they be? Well, they must be proteins because amino acids are joined together to make proteins.

So well done for all of that.

That was quite challenging, good job.

So let's move on to how enzymes are structured.

So we know that enzymes are proteins, and you can see an example of a protein molecule on the board there.

Now proteins are made as we know of amino acids, and the amino acids are built into a chain and then folded into this really complicated 3D shape.

And you can see that folded single chain into this complex shape in that diagram there.

Now as part of that shape, an enzyme includes an active site.

So you can see in the diagram of this simplified enzyme, you've got the main body part of the enzyme and then this section called the active site.

So what is the active site then? Well, the active site is the part where the chemical reaction takes place.

Now enzymes aren't used up in the reaction that they are catalysing.

So they merge with the substrate that they're catalysing the reaction for, catalyse the reaction, then let it go, and are ready to do the same reaction again on a different substrate.

And they can go over and over and over again making them extremely efficient.

So let's look at that in a bit more detail.

So during an enzyme reaction, the substrate or the substrates bind to the active site.

So you can see there's one substrate here and it's about to bind into the active site.

And by doing so, it forms what is called an enzyme substrate complex.

The enzyme substrate complex is the combination of the enzyme and the substrate.

And this is when the chemical reaction is catalysed, and by catalysing the reaction, the product or the products can then be released.

So you can see there are two products made as part of this chemical reaction, and the enzyme is free then to be reused and catalyse the same reaction again.

So we've got substrate binding to form an enzyme substrate complex, and then the products being released and the enzyme being reused.

And the point at which the substrate binds the enzyme is called the active site.

So what I'd like you to do is to put the phases of the enzyme reaction into the correct order.

So which goes first, second, or third? So the phases are the enzyme substrate complex is formed, the product is released, and the substrates bind to the active site.

I'll give you a few seconds to think about it.

Okay, so hopefully you've started off with the substrate binding to the active site, that's part c; then you've listed a, the enzyme substrate complex is formed; before concluding with the product is released.

Well done if you got that right, good job.

So the enzyme substrate complex forms, and by doing so, this lowers the activation energy of the chemical reaction required to make the chemical reaction take place.

And this means that the reaction requires less energy, and therefore the reaction rate can increase.

So you can see on the graph there that there's an an energy hump required to make the reaction take place.

And this activation energy hump is really quite large when an enzyme is not in use in the reaction.

But if we introduce an enzyme into the reaction, we can see that that activation energy, the energy required to convert the substrates into the products reduces significantly.

So the amount of extra energy required to make the substrates turn into products really reduces quite substantially.

And it's by doing that, that causes the reaction to increase in speed or rate.

So let's quickly check that then.

When the substrate binds to the active site, an enzyme substrate complex forms. Is this true or false? I'll give you a few seconds to decide.

Well, hopefully you decided that that is true, but why is it true? Can you justify your answer with one of those two statements? Again, I'll give you a few more seconds to decide.

Okay, hopefully you've decided that it's true because it lowers the activation energy, so the reaction can happen faster.

Well done.

So what I'd like you to do now is to draw a labelled diagram to show the key steps in an enzyme controlled reaction.

So your diagram must include drawings and labels of the substrate, the active site, the enzyme, the enzyme substrate complex, and the products.

So make sure you've included all those details and showing those key steps that are taken as the enzyme reaction is carried out.

Then I'd like you to explain what happens to the enzyme at the end of the reaction and why this is beneficial.

So take your time and come back to me when you are ready.

Okay, so hopefully in answer to the first task, you've included a diagram which looks roughly like this with the active site and the substrate and the enzyme shown, and then how they combine together to form the enzyme substrate complex before the products are released and the enzyme is free again.

Then in answer to the explanation about what happens to the enzyme at the end of the reaction, why is this beneficial? Hopefully you've included notes of the form that the enzyme is not used up, and therefore it is available to catalyse another reaction.

And this is beneficial because one enzyme can catalyse a reaction many, many times.

So you don't need just one enzyme for one reaction.

That's it, it's done.

It can go on and on and on many times over, and that is really useful.

And so that means that there are fewer enzymes required, and therefore the reaction can be much more efficient.

So well done.

Just check over your work, make sure you've got all the really important parts, and then let's move on.

So let's have a look at enzyme specificity now.

So we've considered what an enzyme is.

We can consider it as if it were a lock, and therefore use this lock and key model to explain how enzymes work.

So in the lock and key model, the lock is the enzyme, the key hole is the active site, and the key is a substrate.

Now if you think about this in a bit of detail, we know that locks are specific for their key.

Otherwise it'd be pointless having a lock, wouldn't it? We could just close the door and be done with it, but we lock a door because only one key can unlock that door, and that makes that closing the door much safer.

So if we think about it, there are many, many, many different keys and they all come in different shapes.

We also know that there is only one key that fits a specific lock that can fit into the keyhole, but not just fit into it, but actually turn to make the lock operate.

So this is specificity.

The lock and the key are specific to each other.

The keyhole has a very specific shape, and only one key can both fit into that lock and also operate it.

This is called specificity.

Now we can apply the idea to enzymes and their substrates.

So we know that just like keys, there are many different shaped substrates.

So substrate molecules in many different shapes and sizes.

Some are really small, some are really complex and large, some are multi parted.

You get the idea.

But only one substrate has a specific shape that can fit into the active site of the enzyme and fit it well enough for the enzyme to catalyse the reaction to turn it into products.

So despite there being lots of different enzymes and lots of different substrates, only one substrate and one enzyme can come together to be effective at catalysing that reaction.

So because an enzyme is specific for its substrate, one enzyme can only catalyse one chemical reaction.

So enzyme X can only catalyse the reaction for substrate one, enzyme Y can only catalyse the reaction for substrate two, and enzyme Z can only catalyse the reaction for substrate three.

Different chemical reactions require different enzymes, and this is called specificity.

So let's just double check this.

Can you match the enzyme on the left to its correct substrate on the right? I'll give you a few seconds to think about it.

Okay, so let's see.

So enzyme a matches up with substrate f, enzyme b matches up with substrate d, and enzyme c matches up with substrate e.

But I hope you can see from those active sites that d would not fit into enzymes a or c, for instance.

They are specific to each other.

Now each enzyme is highly specific.

I've said that they're specific, but they really, really are.

Only one enzyme can catalyse a particular chemical reaction.

It is only able to catalyse that chemical reaction.

And we can see that biological enzymes, most of them have the ending dash-A-S-E, ase.

So if we look at the enzymes, which catalyse carbohydrate reactions, they're called carbohydrases, whereas enzymes involved in breaking down proteins are called proteases.

You can see that A-S-E indicates that the protein is or the molecule is an enzyme.

So bearing in mind that carbohydrases break down carbohydrates and proteases break down proteins, what do you think the enzyme might be called, which break down fats, which are scientifically called lipids? What do you think those enzymes might be called? Well, well done if you've remembered or you've guessed that they're called lipases.

So lipases is the enzyme term for enzymes which break down lipids.

Well done.

So just to summarise that process then, we've got carbohydrates being broken down by carbohydrases into sugars, proteins being broken down by proteases into amino acids and lipids, also more commonly known as fats, being broken down by lipases into fatty acids and glycerol.

So let's check.

What is the name of the group of enzymes that break down fats, carbohydrases, proteases, or lipases? What do you think? I'll give you a few seconds to decide.

Okay, so I hope you've chosen lipases as the group of enzymes that break down fats.

Well done.

What I'd like you to do now is to use the words shown on the screen to describe how an enzyme catalyses a biological reaction.

So use those words, not necessarily in that order to help you with your description.

And then once you've done that, use the lock and key model to explain enzyme specificity.

So pause the video and come back to me when you are ready.

Okay, so let's see what you might have written.

So using the words to help describe how enzymes catalyse biological reaction, you might have said that enzymes catalyse biological reactions but are not used up themselves.

The substrate fits the enzymes active site.

It is a specific shape, like a lock with a key; and when the substrate binds, the activation energy is lowered, which increases the rate of the reaction.

So hopefully you've got something along those lines and that you've used all of those words correctly.

So do make sure that your work is accurate.

Then I asked you to use the lock and key model to explain enzyme specificity, and your answer should have included ideas along the lines that enzymes are a specific shape for their substrate, and they can therefore only catalyse one type of chemical reaction.

A substrate fits into the active site like a key fits into a lock.

And all of this can be explained using the lock and key model.

So I hope you've enjoyed our lesson today.

In our lesson, we have seen that enzymes are proteins that catalyse chemical reactions, but are not used up during them.

That the substrate binds to the enzyme at its active site and forms an enzymes substrate complex before the products are then released.

And that when the substrate binds to the enzyme, it lowers the activation energy of the chemical reaction, and that enables the reaction to happen faster because it requires less energy.

We've also seen that critically each enzyme is specific to its substrate, like a lock and a key.

And that if it weren't for enzymes, life on Earth simply would not exist.

So thank you very much for joining me today.

I hope you've enjoyed the lesson.

Well done for all your hard work today, and I hope to see you again soon, bye.