Video Transcript
In this video, we’ll learn how
enzymes catalyze biological reactions, how lock and key is related to enzyme action,
and what it means to denature an active site. We’ll also take a look at why
enzymes are important for life and find out what they’re made of. Then, we’ll work some practice
problems about enzymes as well.
So why is it that we study
enzymes? Well, enzymes catalyze biological
reactions. And if we take a look at the big
picture first, we’ll gain an understanding of why this is important. We can use a human as an example of
a biological or living thing because we, like all living things, are made of
cells. Cells are the basic unit of life
because they organize the chemical reactions that underlie biological processes,
biological processes being things like respiration, excretion, moving, thinking,
digestion, transporting things within cells between cells or across your body,
sensory information processing, and many more.
Humans are made up of about 37
trillion cells, each busy doing many, many chemical reactions every second. And that means that chemical
reactions aren’t just important for life; there’re the essence of life. And biological reactions, which are
chemical reactions mediated by an enzyme, are really important. Let’s take a look at the term
“catalyze” next. The term that’s synonymous with
catalyze is speed up. And enzymes, amazingly, speed up
chemical reactions that they catalyze. Comparing the speed of a typical
biochemical reaction with an enzyme to one without an enzyme is certainly like
comparing a rocket to a rock. So that’s why we study enzymes. They speed up really important
chemical reactions that underlie our biological processes.
What is it about enzymes that
enables them to speed up or catalyze chemical reactions? Let’s start by taking a look at
energy. And this graph will help us make
sense of what’s happening to the amount of energy during a chemical reaction with or
without an enzyme. Okay, so energy is represented on
the 𝑦-axis. The 𝑥-axis represents the progress
of the reaction as time goes by. So we start with the reactant or
reactants on the left and the product or products on the right. In our example, we have two
reactants, A and B, and one product, C. You can see that our reactants have
to go up an energy hill. Imagine the excitement at the top,
so high up, and about to go screaming down, forming the lower-energy product C.
Let’s see what happens to the shape
of the graph when we add a catalyst such as an enzyme to the reaction. The roller coaster car actually
helps to bond the reactants together, allowing for the product to form without
having to go over such a high energy hill. And we use the term substrate
instead of reactant in reactions involving enzymes. The smaller amount of energy
required in enzyme-mediated reactions allows them to happen where they otherwise
wouldn’t, such as a reaction in a test tube that requires a large input of
energy. That happens in a cell with an
enzyme rather than a flame. So now we know that enzymes reduce
the amount of energy required for a reaction to occur.
Next, let’s look at how a roller
coaster car can change its own tracks or, rather, how enzyme speed the production of
products from substrates. Okay, so we just learned that
enzymes reduce the amount of energy that’s required for a reaction to take
place. But now we’re gonna learn how they
do it. Let’s take a look at a very
simplified cell with just two reactants floating in it, A and B, and no enzyme. In order for a reaction to occur
between the reactants A and B, they need to collide with enough force and be
oriented in the right way. So if this particular reactant A
moves towards B while this particular reactant B moves towards A and they collide,
there still may not be a reaction.
So, let’s try again with more
force, which, of course, requires more energy. Okay, so we’ve got A hurtling
towards B and B hurtling towards A with more force this time. And they still may not react. So next time, let’s try orienting
them as if they were actual molecules that need to face each other in a certain way
before they can form a bond. Now, reactants A and B hurtle
towards each other with sufficient force and oriented so the legs of the A puncture
into the loops of the B, producing a chemical reaction that forms a product which,
in this case, is C.
The low probability of specific
chemicals colliding with enough force and the correct orientation seems to allow
reaction rates that are just fine for general universe building. But life requires mind-boggling
numbers of chemical reactions per second without added energy from things like
Bunsen burners or hot plates. So, without enzymes, reactions just
require too much energy and have to slow a reaction rate to support life. Now, let’s take a look at how this
reaction might happen with an enzyme, which I have shown here in green except for
the pink portion, which is called the active site. And that’s where the substrates A
and B will bind.
Now the substrates are bound to the
enzyme in a very particular way that’s determined by how their shapes fit
together. The closely matching fit between
the enzyme and the substrates is called complementary, since they tend to complete
each other. The interaction within the complex
of enzyme and aligned substrates reduces the energy or force otherwise required for
the chemical reaction to occur, producing the product C, which is then released,
leaving the enzyme free to catalyze more reactions. So reactions catalyzed by an enzyme
require less energy and have an increased reaction rate due to the complementary fit
in interaction between enzyme and substrate.
The fit between the active site of
the enzyme is so close that we use a lock and a key as an analogy. Here’s an analogy problem for a
page title. A lock is to a key as a blank is to
a blank. And our word choices are either
enzyme or substrate. Here’s a locking door knob and a
couple of keys. Will either key open the door? No, of course not. That’s the whole point of keys. You need the right key to get in
the door. Here’s tyrosine hydroxylase, and
its name says a lot about what it does. First, most all enzymes end in this
suffix, A-S-E. So we have a pretty good idea it’s
an enzyme. And the rest of its name comes from
its substrates, tyrosine and a hydroxyl group.
Now that the substrates are bonded
to the enzyme, you can probably tell which is represented by the lock and which by
the key. A lock is to a key as an enzyme is
to a substrate. And here’s another question: what
is the name for this closely matching fit? It’s called a complementary fit,
since their shapes tend to complete each other. But to continue the story about our
enzyme here, the reaction is catalyzed by the well-aligned complex of enzyme and
substrates, producing the product, which in this case is L-dopa. And after a couple more
enzyme-mediated reactions, L-dopa will become dopamine, a neurotransmitter that can
make you feel like you’ve just gotten a reward. And the enzyme will continue to
catalyze more of the same reaction.
Enzymes can have problems
catalyzing reactions though, especially if something happens to their active
site. If the complementary fit between
enzyme and substrate is lost, the reaction won’t be catalyzed, and the enzyme is
said to be denatured. Since enzymes are usually proteins
and the shape of a protein results from a DNA gene, mutations can affect the shape
of an enzyme, causing it to be denatured. Up next, let’s work a couple
example problems.
Which of the following
statements correctly defines an enzyme? (A) An enzyme is a product of
digestion. (B) An enzyme is a molecule
that has been broken down. (C) An enzyme is a biological
catalyst. (D) An enzyme is an inorganic
catalyst. (E) An enzyme is a fast
reaction.
Key knowledge required to
select the correct solution option is knowing what enzymes are and what they
do. Let’s review by taking a look
at the enzyme ATP synthase while we review key terms in the question. Like all enzymes, ATP synthase
catalyzes or speeds up a chemical reaction. And that makes it a
catalyst. Here’s the reaction that ATP
synthase catalyzes. ADP plus a phosphate groups
yields ATP. The two parts of a chemical
reaction that you’re probably used to seeing are reactant or reactants that are
converted through the chemical reaction into a product or products. But during a reaction with an
enzyme, we say that the substrate or substrates are converted into products or a
product. So that means we know that a
product is a chemical that’s made in a reaction.
Now let’s take a look at what
ATP synthase is gonna do with the substrates, ADP and a phosphate group. The substrates bond to the
active site of the enzyme. And that’s where the chemical
reaction takes place, forming the product ATP. And the active site of the
enzyme is now free to catalyze more reactions. Okay, just a few key terms
left. Let’s take a look at “broken
down.” If we have a larger molecule that we separate into smaller parts, we say we’re breaking it down. If we go in the opposite
direction, we take some smaller molecules and put them together into a larger
molecule, we call that building up.
And, of course, the term
biological means “from a living system.” And enzymes are proteins
produced in living systems. And since proteins are organic
molecules, we know that enzymes are not inorganic. And we’re ready to answer our
question. Which of the following
correctly defines an enzyme? Let’s start with option
(A). An enzyme is a product of
digestion. Well, we saw that enzymes are
catalysts. They’re not products, so option
(A) can’t be correct. Option (B) says an enzyme is a
molecule that has been broken down, but we saw that enzymes are not broken
down. They have to stay intact
because they’re reused over and over and over again.
Option (C) says an enzyme is a
biological catalyst, and that sounds pretty good. Option (D), an enzyme is an
inorganic catalyst, but we already know that can’t be true because proteins are
not inorganic. Option (E) says an enzyme is a
fast reaction, but we know that enzymes speed up or catalyze chemical
reactions. They’re not the reactions
themselves. So option (E) is also
incorrect. Therefore, option (C), an
enzyme is a biological catalyst, is the answer to our question.
We’re running a little bit short on
time. So instead of doing another
practice problem, let’s go to reviewing the key points from the lesson. First, an enzyme is a biological
molecule and usually a protein that speeds up chemical reactions. The active site is the part of the
enzyme where the substrate will bind. And substrate is another word for
reactant in an enzyme-mediated reaction. The closely matching shapes of
active site and the substrate is called a complementary fit. The complementary fit lowers the
energy requirement for the chemical reaction to take place. The resulting products are released
from the enzyme. And active site is open for more
and more reactions.
Catalysts speed up chemical
reactions, and enzymes are biological catalysts. When an enzyme doesn’t work anymore
because the shape of its active site is changed, it’s said to be denatured. Here’s just a couple more facts
about enzymes: they lower the energy requirement of chemical reactions, and the
complementary fit between the enzyme’s active site and the substrate is the basis
for the lock and key analogy we discussed.
