So today, we're going to
talk about the effects of the environment on enzymes
and how a changing environment can affect an enzyme's ability
to catalyze a reaction. But first, let's review
the idea that enzymes make reactions go faster. And looking at a reaction
coordinate diagram, you'd notice that
enzymes speed things up by lowering a reaction's
activation energy. Now, it's important to
recognize that enzymes work best in specific
environments. And when I say environment,
I can be really referring to many different aspects
of an enzyme's surroundings. But right now, we're
really only going to be focusing on pH
and temperature values. So let's take
another look at this by imagining that we have
this person over here. And he's hungry, so
he's eating some food. Now, there are a bunch of
different digestive enzymes in this guy's body that are
going to help him break down all the food he's eating
into tiny usable parts. So first the food
will be in his mouth. And one of the enzymes
found inside a human's mouth is called alpha amylase, which
is responsible for breaking down complex carbohydrates
like starches into small simple carbohydrates
like individual sugars. And alpha amylase
is able to work well in your mouth
since it functions best at a pH of around 7, which is
about the same pH as a human's mouth. Now moving along, the
food that our guy ate is going to go all the way
down to his stomach, where a whole different
group of enzymes will start breaking
down the food. Now, one enzyme that humans
have in their stomachs is called pepsin, which breaks
down big proteins into smaller peptides. Now, pepsin will be most
active at a pH of around 2, which is also the pH
of your stomach, which is so low because of
all the stomach acid that you'd find there. Now in terms of temperature,
both of these enzymes typically work at a temperature
of around 37 degrees, which is the same as body temperature. But you can see that these
two different enzymes are functioning at different
environmental conditions. So what would happen
if we took an enzyme and moved it into a
different environment? Well, let's first
look at the effects of changing the pH of an
enzyme's environment and jump right in with an example. So remember that DNA is a very
negatively charged molecule because of all the negatively
charged phosphate groups that you'd find on DNA. And in order for the
enzyme DNA polymerase to help out with
DNA replication, it binds a magnesium
ion cofactor, which it uses to stabilize all
the negative charge on DNA. Now under normal pH
conditions, the DNA polymerase hold onto that
magnesium ion through an electrostatic interaction
between magnesium and one of its aspartate residues, which
would be deprotonated and thus negatively charged
at neutral pH values. Now, if we were to
take DNA polymerase and put it into an
environment with a reduced pH, then that aspartate residue
will become protonated since the pH has
dropped so much. And in its protonated
form, aspartate no longer has a negative
charge and can't hold on to that
magnesium ion anymore. And overall, this means
that DNA polymerase won't be able to
do its job properly in a low pH environment. And keeping this enzyme
at an appropriate pH is essential to its
normal function. So now, let's take a
look at the effects of temperature changes
on enzyme function. So remember that
proteins need to fold into their secondary, tertiary,
and possibly quaternary structures in order to be
in their functional form. And significant changes
to a protein's temperature can disrupt a protein's
folded geometry and cause it to lose
its functionality. If we have our same person from
before, who was really hungry and really wants to
eat, but now this person get sick with a fever,
his temperature will rise. And a bunch of the digestive
enzymes in his body will get all
jumbled up and won't be properly folded anymore. And this is why you might have
a hard time eating and digesting food when you have a fever. And this can sometimes
lead you to throwing up anything you'll eat since
all of the digestive enzymes in your body won't
work anymore because of the increase in your
body's temperature. And the food you eat will
just sit there, sit there in your body and
make you feel sick. So what did we learn? Well, first we learned that
enzymes generally function only under very specific
environmental conditions. And different enzymes will
often function ideally in different environments
from other enzymes. And next, we learned that
changes to an enzyme's environment, like changes to the
surrounding pH or temperature, can lead to a loss of
enzyme functionality.