- We've already seen that an enzyme helps
catalyze a reaction, so let's say this right over here,
this is our enzyme, and we have our substrate,
and it goes and it binds to the active site,
to the active site of the enzyme,
so let's say it binds right over there
on the enzyme we call it the active site
where the substrate binds and then the enzyme catalyzes
a reaction; maybe it breaks up this substrate
into two smaller molecules.
And so after the reaction,
the enzyme, whoops.
After the reaction, the enzyme is unchanged,
but a reaction has been catalyzed.
We now have the substrate being broken up,
in this case at least, into two smaller molecules.
Maybe I'll draw them. That's one of them.
And this is the other one right over here.
So they just came from the active site.
Once the reaction is catalyzed, they don't have
the affinity to the active site anymore and they break off.
So this enzyme has just catalyzed this reaction.
What I wanna talk about in this video
is how this might be inhibited, and specifically
how it might be inhibited competitively.
So we're gonna talk about competitive inhibition.
So competitive--
Let me write it over here.
Competitive
inhibition.
Inhibition.
So the classic case of competitive inhibition:
if there's some molecule that competes
for the substrate at the active site,
as we'll see this isn't the only form
of competitive inhibition, but this is the one
that you will most typically see in a textbook.
So that's our enzyme again.
So that's our enzyme.
And we've already seen that this is right over here
where I'm circling, that is the active site.
Active site.
And if the molecule, the intended substrate
I guess you could say, gets to it and we're gonna have
this first scenario up here.
But in classic competitive inhibition,
or at least the version I'm just gonna show you right now,
you could have another molecule
that let's say it looks something like this
that can compete for the active site
and if it gets to the active site first,
so if it gets there first,
let me show what's going to happen,
so then we have our enzyme,
we have the other molecule, not the intended substrate
binds to the active site first.
Well now the intended substrate,
the one for which the enzyme catalyzed the reaction,
isn't able to bind and the reaction isn't going to happen.
And you can see very clearly that they are competing
for the enzyme, and in this case,
they're competing for the active site.
Now this isn't the only form of competitive inhibition.
Another form of competitive inhibition
is allosteric competitive inhibition.
Let me write this down.
So you have allosteric.
Allosteric
competitive
inhibition.
Now I'm having trouble writing.
Inhibition.
And an allosteric site is a site other than the active site.
But in allosteric competitive inhibition
or competitive allosteric inhibition,
however you wanna say it, you have a scenario
where the competitor doesn't bind to the active site
but binds to a site that is not the active site,
an allosteric site you could say.
So in that one, the competitor, maybe might bind here,
so that's clearly not the active site.
So maybe the competitor looks something like that.
It didn't bind to the active site,
but by binding there, the active site can no longer bind
to the intended substrate, so you have the same effect.
You have the same effect right over here
where this thing isn't going to bind.
But if this thing binds first...
So let me draw that scenario.
So if the intended substrate binds first,
then the competitor can't bind.
So in this scenario, if the substrate is able
to get to the active site,
well then the competitor can't bind,
so once again, they're competing.
So I'll draw the competitor up here.
So then the competitor,
whoever gets to it first gets the enzyme.
So in this situation, the competitor's not going to bind.
So that's true of whether you're talking about
competitive inhibition where they're competing
for the active site, if the competitor gets there first,
the intended substrate isn't gonna get there,
the reaction isn't going to be catalyzed.
Or if the intended substrate gets first,
then the competitor's not going to be able to get there.
In fact it could have been this situation
where because the substrate got there first,
the competitor isn't going to be able to bind
to the active site.
When we're talking about allosteric competitive inhibition,
we're still competing for the enzyme.
Only one's gonna get it.
If one gets to the enzyme first,
then the other one's not going to be able to get there.
They are competing for the enzyme, but the competitor,
the non-substrate, is just acting at an allosteric site.
By binding to an allosteric site,
it changes the conformation of the enzyme
so that the active site no longer binds to the substrate.
And I wanna really emphasize this point
because when I first learned this, I said:
"Oh!"
And it's often sometimes confusing, even some things
you'll read on the Internet, that they'll say
that this allosteric type of inhibition, they'll call this
non-competitive 'cause you're not competing
for the active site, but that is actually not the case.
In non-competitive inhibition, and I'm gonna do
the whole next video on non-competitive inhibition,
in non-competitive inhibition, the inhibitor right over here
can bind regardless of whether the substrate
has bound or not, but when the inhibitor does bind,
it prevents the reaction from moving forward,
it changes the conformation of the protein
so it no longer catalyzes the reaction.
So non-competitive, they both can bind,
but if the inhibitor is there,
the reaction isn't going to proceed.
In competitive inhibition, whether we're talking about
allosteric or non-allosteric competitive inhibition,
only one of the substrate or the inhibitor
is going to be able to bind.
They are competing for the enzyme.