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.
