Introduction to enzymes and catalysis | Chemical Processes | MCAT | Khan Academy

Introduction to enzymes and catalysis | Chemical Processes | MCAT | Khan Academy

December 3, 2019 17 By Kody Olson


So today I want to talk
to you about enzymes and how they’re critically
important pieces of cellular machinery. But first, let’s review the
idea that biochemical reactions happen in the body all the time. Almost every cellular process
involves a biochemical reaction at one point or another. You know, the TCA
cycle is actually just a series of different
biochemical reactions in carbon metabolism. DNA replication, which needs
to happen before a cell to go through mitosis, is also
just a series of reactions. And this also applies to
the expression of genes, going from DNA to
RNA to protein. And we need enzymes
because enzymes make all of these
reactions go much faster. And let’s look at this
idea little more deeply, and how a reaction
will go on differently when it has an enzyme versus
not having one at all. So you may be familiar
with the reaction where water and
carbon dioxide can combine to form carbonic acid. And this is a
reversible reaction, so it can go backwards
and forwards. Now, when people make soda
or any carbonated beverage, they’ll start by pumping
that soda can full of CO2. And while some of
that CO2 will dissolve in the water and the can,
the soda making companies are able to get a lot
more CO2 in the water by using this reaction. The abundant CO2 will
react with the water to form some carbonic
acid in the can. And when you go to
open the can, you’ll hear a pop sound,
which is really just a bunch of CO2 escaping. But after that, the soda will
start to fizz really slowly. And what’s happening
here is the carbonic acid that was made before is
slowly dissociating back to carbon dioxide in
water as CO2 escapes. And that extra CO2
that’s being made will come out of
the soda solution, and you’ll see it as little
bubbles floating around. But what happens if you then
take this person over here, and he’ll pick up a can
of soda and take a drink? That person might
notice the soda will start fizzing a lot more once
it hits his or her tongue. And this is because humans
have an enzyme in their blood and saliva called
carbonic anhydrase. And this makes the
carbonic acid turn into carbon dioxide in
water much more quickly. So more CO2 will come out of
the can, and it will fizz more. And this is just one
of the many examples of how enzymes make
reactions go faster. So how exactly do the enzymes
make the reactions go faster, though? Well, they use a bunch of
different catalytic strategies to push reactions along
a little more quickly. And I’m going to talk about
a few those strategies just to give you an idea of
what enzymes are doing. So first I’ll mention
acid/base catalysis, which happens when enzymes act
like either acids or bases. Now, remember that
acids and bases are proton donors and acceptors. And if you look at this type of
reaction, which if you remember from organic chemistry is
a keto-enol tautomerization reaction. We have a proton moving from a
carbon atom to an oxygen atom. And since acids and bases are
pretty good proton carriers, they could both help
with this reaction, make it go a little
more quickly, by helping to move
that proton around, instead of this molecule
of doing it by itself. Our next catalytic strategy
is covalent catalysis, which happens when enzymes
form a covalent bond with another molecule,
usually their target molecule. Remember that
covalent bonds involve two molecules sharing electrons. And looking at
this reaction here, we have a decarboxylation
reaction going on. Which, if you remember
for organic chemistry, is when a carboxy or CO2 group
is being taken off a molecule. And, if you remember,
these reactions usually have a lot of electrons
moving around. So if we had
covalently bound enzyme that could hold on
to some electrons, be an electron carrier,
or what some people like to call an electron
sink, then that would definitely help this type
of reaction move a little more quickly. Next, we have
electrostatic catalysis. Now, if you remember,
DNA is a very negatively charged
polymer because of all the negatively charged phosphate
groups that we find in DNA. So if an enzyme had a metal
cation on it, like magnesium, we could use it to stabilize
the negative charge found in DNA and make it a little
easier to work with. And DNA polymerase,
which is the enzyme that allows DNA replication to
occur, does exactly this. And in order for it to
help with DNA replication, it needs to find a
way to counteract all of the negative
charge on DNA. The magnesium ions totally
come in handy there. So the last catalytic
strategy I want to mention is a little more general. And it has to do with proximity
and orientation effects. Remember that in order
for two molecules to react with each other, which
is usually what enzymes help out with, they need
to physically collide at some point. If we have molecule
A and molecule B, they’ll only react once
they crash into each other. And a lot of enzymes are
able to bring two molecules close together, so that
these types of collisions happen more often, making
the two molecules react more quickly. Also remember that
the orientation of the two colliding
molecules in space is also really important. If molecule A and molecule
B collide, but one of them is upside down or not
in the correct position, then the collision
may not result in a successful reaction. So enzymes also make sure
that the two molecules will collide in the
right orientation. And all of this increases
the frequency of collision in general, but also
helps to make sure that those collisions
are successful and result in a reaction. So what did we learn? Well, first we learned
that the role enzymes play is to make biochemical
reactions happen more quickly. And the next thing
we talked about were four of the many
different catalytic strategies that enzymes can use. We talked about
acid base catalysis which helps with
proton transfer. We talked about
covalent catalysis, which helps with
electron transfer. We mentioned
electrostatic catalysis, which deals with
stabilizing charge. And finally, proximity
in orientation effects, which increase the frequency
of successful collisions between molecules that we
want reacting together.