So today, I'm going to talk to you about the induced fit
model of enzyme catalysis and how this concept can tell us
a lot about how enzymes work.
But before we do that, let's review the idea
that enzymes make reactions go faster.
And when you look at a reaction on a reaction
coordinate diagram, you'd see that the catalyzed reaction
would have a much smaller activation
energy than the uncatalyzed one.
Also remember that because of this,
the energy of the catalyzed reaction's transition state
is far lower than the energy of the uncatalyzed reaction's
transition state.
So what do enzymes look like?
Well, most enzymes are proteins, or at least partially
made up of protein.
And substrates are any molecule that an enzyme will act on.
And often, these substrates are the reactants
that the enzyme will ultimately help turn
into products through a reaction.
Now enzymes also have what is called
the active site, which is the location on the enzyme
where substrates bind.
And that's where the reaction ultimately happens.
And it's important to recognize that every enzyme has
a unique active site that will only
bind to certain substrates.
And just to clarify, I've referred to the active site
here as both of the notches found on the enzyme, and not
the space in between them.
So here, of the two substrates I've
drawn, the enzyme will only be able to bind to substrate 1,
since they fit together like puzzle pieces.
Whereas the shape of substrate 2 isn't
going to fit nicely in the enzyme's active site.
Now since enzymes have unique active sites,
we say that enzymes are specific to certain substrates,
and by extension certain reactions.
But let's dive a little deeper into what
happens when enzymes and substrates bind to each other
and how that binding pattern changes
as a reaction progresses.
So first you'll have your enzyme here
and your substrate over here.
And I'm just going to label this with the number 1,
since it'll be the first thing that
happens in the sequence of events to come.
And at this stage, nothing has happened yet.
And the enzyme and the substrate have yet to come in contact.
So next what will happen is the enzyme will bind
to the substrate.
But this binding won't be perfect.
So we'll call this initial binding,
which is stage 2 of the process.
And what that means is that the forces holding these two
together are strong, but they're not at their maximum strength
just yet.
And enzymes and substrates don't actually
fit together quite like puzzle pieces.
And they actually work a little bit more
like two pieces of clay that will both mold
together so that the fit is much tighter.
So in our next step, this is exactly what happens.
The enzyme and the substrate will both change
shape a little bit and bind to each other really strongly.
And we call this the induced fit because both the enzyme
and the substrate have changed their shape a little bit so
that they bind together really tightly.
And it's at this point where the reaction that the enzyme is
catalyzing is at full force.
And this would be stage 3.
So our next stage occurs after the reaction is completed
and the binding becomes similar to what it was in stage 2.
But the difference here is that there
was something different about the substrate.
So in this reaction, the enzyme is cutting our substrate
into two parts.
So now, the two parts have become separated.
And this would occur after the reaction is finished.
And we'll call this stage 4.
Now in our next and last stage, the products of the reaction
have been released from the enzyme.
And our enzyme is back in the same state
that it was in stage 1.
And we'll call this stage 5.
Now, let's look at this from a slightly different angle.
I'm going to label the enzyme as E, the substrate as S,
and our two products as P1 and P2.
And they're going to represent this series of events,
these different steps in the sequence of reactions.
So first we'll have E and S separate.
And this is stage 1.
And next, E and S will bind to each other to form
an enzyme substrate complex, which I have called ES.
And it corresponds to stage 2 from before.
Now what's really interesting is that in the next step, where
we had the induced fit of stage 3,
we're actually at the transition state of the entire reaction.
And this is the same as that really high energy point
that we saw at the beginning of this video.
And it's at the point of the transition state
where our enzyme is most tightly bound to its substrate.
Now, I've written the substrate out here with the letter X.
Because of the reaction's transition state
our substrate isn't quite a reactant
and it isn't quite our product either.
It's somewhere in between.
So that's why I've written it out as X instead of S.
And I've also written this double dagger symbol, which
is just a universal symbol for transition states.
Now in our next stage, which is after the reaction has
occurred, since it exists after the transition stage,
we have the enzyme bound to the two products P1 and P2.
And this was stage 4 from before.
And then finally in our last stage, stage 5,
we have our enzyme, which is now separated
from our two products, P1 and P2.
Now the big M away from this is that binding between enzyme
and substrate is strongest at the reaction's transition
state.
And this is because the enzyme and the substrate
have molded together.
And that's why we call it the induced fit.
Now, some enzymes will actually bind
to more than one substrate.
And if we look at a reaction that might be familiar,
which is lactic acid fermentation,
we can see that our enzyme, lactase dehydrogenase,
will have space to bind to two different substrates
in this reaction, one space being for NADH
and the other being for pyruvate.
So enzymes don't necessarily bind just to one substrate.
Now, sometimes things will bind to enzymes
at places other than their active sites.
And we call this allosteric binding.
So if we have an enzyme here with it's active site,
a regulating molecule like an inhibitor
made by the enzyme at a different location
than the enzyme's active site.
Now when something binds to an enzyme like this,
it usually has the effect of changing
the shape of an enzyme in some way
to affect its ability to catalyze reactions.
So in this case, when an inhibitor binds top the enzyme,
it might change the shape of the active site,
thereby inhibiting the enzyme, as it's
no longer able to bind to its intended substrate.
They don't quite fit together anymore.
So while enzymes bind to reactive groups
at their active sites, they can also
bind to regulators at their allosteric sites.
And allosteric sites just refer to any binding site
outside of the active site.
And remember allosterically binding molecules
can either be activators or inhibitors,
any regulating molecule.
So what did we learn?
Well, first we learned that enzymes are specific
and that they can each bind to only specific substrates
to catalyze specific reactions.
Next, we learned about the induced fit model
and how enzymes bind their substrates most tightly
in the middle of a reaction at the reaction's transition
state.
And finally, we learned that enzymes have both active sites
and allosteric sites, with active sites being where
the reaction takes place and allosteric sites being
where regulation takes place.