We have seen how enzymes flexibility enables
enzymes to accomplish what they accomplish.
But enzymes do have constraints
that they have to work in.
I have mentioned in this
presentation numerous times now
that enzymes and cells all are governed by their
rules of the universe. That is they can't
change the energies of reactions and so
those are true for cells and
those are also true for enzymes.
Now enzymes, as we will see,
are tricky little things. I have
mentioned how enzymes cheat
and enzymes are going to cheat
with respect to energy as well.
So let's consider a reaction of A going to B.
In A going to B, this is plotted from an energy
perspective on the screen what you see here.
On the left side of the
screen, we see a dot
placed on the graph showing free energy.
That is the energy that is associated with molecule A.
In the process of going from A to B
we see that there is a
change in the energy that the energy
is actually increased and we call this
increase in energy, the activation energy. This necessary
to get a reaction going.
The reaction proceeds and
as the reaction proceeds
we an see that the free energy
falls then we make a product B,
that down by the end has a lower free energy
than A had. That meant that energy was
released in the process of going from A to B
and this makes this reaction process favorable.
Now it's important to
note that this change in free energy, that's
shown right here, this change in free energy
can not be changed by an enzyme.
That is there is no change between the
starting and ending points of the enzyme.
The enzymes does some other things, however.
It's also important to note here that this
height of the peak is really a critical place.
The height of this peak is
the place where the reaction
can reverse and go backwards from where it came
that is A can start and then go back.
Or B if it got enough energy could climbed
that curve and then go back to A.
Otherwise A is gonna go forward to B
and the reaction is going to be occurring.
Now enzymes cheat, okay?
Enzymes can change the activation energy
There are no rules about activation energy, okay?
There are rules about beginning and ending energies.
But what activation energy changes do is they enable an enzyme
to make more molecules more easily go through that transition.
That is the magic of enzymes. How do they accomplish that?
Well, they accomplish this in a couple of ways.
One of the ways that they do is by the fact
that they have binding sites that are very
precisely oriented. So that the molecules are placed in the
close proximity that they randomly
would not be into close proximity so easily, right?
And that means that it takes less energy for them
to go through the next step in the process.
By doing this enzymes are able to
lower the transition energy.
Well, if the transition energy is lower it makes it much
more easy for A to go to B, as we have seen.
You notice again, enzymes have had no
change with overall free energy
the energy of A is still A.
The energy of B is still B, okay?
Only that transitional state has made a difference.
Now I wanna go through and spend some time talking
about the mechanism of an enzymatic reaction.
Mechanism is important to consider; because,
with mechanism we can begin to see
how enzymes are facilitating electronic changes
necessary for a chemical reaction to occur.
The example I will use is an
example of a serine protease.
A serine proteases are class of enzymes
that cut proteins. They break peptide bonds.
That's what they do. And they break
not every peptide bond they see.
But they break specific peptide bonds at specific places
within the proteins that they bind to.
Alright. So that means that they have binding specificity.
They don't cut everything that they see.
Serine proteases have flexibility so
we saw in the initial illustration
the flexibility of an enzyme and we
are going to see occurring again here
as we talked about the mechanism
of the serine protease.
The electronic environment is
very critical for a reaction.
In a chemical reaction, electrons are being
manipulated, electrons are being moved around.
And to be able to do that, one must have
the environment for those electrons
to readily be able move around.
And we will see that happening in
the active site of the serine protease.
Enzymes also use coenzymes.
Now in this example I won't show a coenzyme. But
I will say that the coenzymes actually
help an enzyme to accomplish
what it accomplishes.
Okay. Now, serine proteases, as I said, cleave peptide
bonds, that's the catalytic thing that they do.