Today, we're going to talk
about how enzymes can influence a reaction's activation energy. But first, let's review
the idea that enzymes make biochemical
reactions go faster. And in order to do
that, they use a bunch of different
catalytic strategies. Now, there are lots of
different catalytic strategies that enzymes use. But a couple of the key ones
are acid/base catalysis, where enzymes use their
acidic or basic properties to make reactions go
faster by helping out with proton transfer. There's also covalent catalysis
where enzymes covalently bind to a reacting molecule to help
with the electron transfer. There's electrostatic catalysis
where enzymes use charged molecules or metal
ions to stabilize big positive or
negative charges. And we also have proximity
and orientation effects, where enzymes make collisions
between reacting molecules happen a little more often. So what effect do these
catalytic strategies actually have on a reaction? Well, let's look at
a sample reaction where we're having molecule A
being converted to molecule B. Now, we can look at the
process of this reaction using something called a reaction
coordinate diagram. And here, we'll plot the
energy state of our molecules against the progress
of the reaction. So essentially,
using this graph, we'll follow the energy
level of molecule A as it's converted to molecule
B. Remember that a molecule's energy level is related
to its stability. And something that has a lower
energy state is more stable. And for something to transform
to a more unstable form, it needs an input of
energy to get there. So looking at this
graph, you'll notice that the energy of molecule
A will rise up pretty high and then drop all the way down
to the energy of molecule B. And we can actually
define a couple of values from this graph. The transition
state of a reaction, which is represented by
this double dagger symbol, is the highest energy point
on the path from A to B. And it's where you'll
find the most instability throughout the entire reaction. Now the difference between the
energy level where we start and the top of our graph
at our transition state is what we call the
delta G double dagger or the free energy
of activation. And this is the amount of energy
that A needs to have in order to break the reaction barrier
to ultimately get to point B. You'll also notice that there
is a difference in energy between point A
and point B. And we call this the
standard free energy change for the entire reaction. And it represents the net
change in energy levels between our reactant
and our product. And it's also the energy that
is released into the environment once the reaction is over. Reactions you typically look
at will have their products at a lower energy state
than their reactants since that makes the
reaction spontaneous. Now, it's important
to recognize that it is the free energy of
activation energy value, which is the difference
between point A and the transition state,
that usually determines how quickly a reaction will go. And usually this energy
value is much higher than the free energy change
for the reaction, which is why enzymes
speed up a reaction by lowering the reaction's
activation energy. Now, I want to quickly
point out that you may see delta G double
dagger written out as EA in some textbooks. And you may see the
standard free energy change for the reaction
written out as E reaction. And I'm just letting
you know that might see both sets of terms
used from time to time. Now, let's look at an
analogy to get a closer look at how this all works. And let's say
there's a giant hill that you're trying to climb. And it's a pretty steep hill,
that goes up really high. But you need to get to the
other side of the hill. Now, this would be a pretty
scary thing on its own since you would need to
go all the way up and then all the way down the mountain
to get to the finish line. But if I were to
give you a shovel, then now you could dig your
way through the mountain and not have to
climb up so high. In this example, the
shovel represents an enzyme and the
hill represents the activation energy
barrier that prevents you from getting to start to finish. By using the shovel,
you're able to lower the height of the hill
you have to climb. But in both cases, it's
important to recognize that you still started and
finished at the same points. So let's go back to our example
from before with our reaction coordinate diagram. But now, let's say that the
reaction has a catalyst. So with the catalyst, the
activation energy barrier that molecule A has to overcome
in order to get to point B is much smaller. And this will mean that your
reaction will have a transition state with a much
lower energy, meaning that it's more stable
with the enzyme and also that the
reaction as a whole have a much lower
activation energy. Now it's really
important to recognize that like our
example where you're trying to climb the
hill, the enzyme will not be changing the starting and
ending points of the reaction. It doesn't change
molecule A or molecule B. Your starting and ending
points are always the same. And the only thing that
changes is the path that you take to
get from A to B. Now since our starting and
ending points aren't changing, it follows that
the enzymes are not used up when they
catalyze a reaction. And there is no permanent
change to the enzyme following a reaction. So what did we learn? Well, first we
learned that enzymes work by lowering the free energy
of activation of a reaction, making it much easier
for the reactants to transition and form products. And we also learned that the
free energy of the reaction doesn't really change when
you use an enzyme and when you don't. Second, we learned that,
despite the change in pathway to get from A to B, the
reactants and products do not change when using an enzyme
versus when not using an enzyme. And finally, we learned
that enzymes are not consumed when they catalyze a
reaction and the same enzyme can catalyze reactions
over and over again.