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.
