The ability of enzymes to speed up
reaction is actually mind-boggling.
The subject of this presentation is to
talk about the ways in which we study
the kinetics or the ways that
enzymes speed up reactions.
In this presentation, I will give a little
bit of background about the process of catalysis,
talk about the flexibility of enzymes and
how that enables them to do what they do.
Talk about activation energy which is another
consideration for enzymatic catalysis.
I will talk about the mechanism of a specific
reaction for an enzyme called a serine protease.
And then I will give the kinetic considerations
that we have during that analytical process.
And finally talk about the overall overview, of
using what are called Michaelis Menten Kinetics.
Now, when we think about enzymatic
reactions there is actually a
series of different ways that molecules can react
in interacting with an enzyme.
We can have for example a reaction that is a single
substrate reaction and a single product,
A is converted into B.
We can have a reaction in which a single substrate
is converted into multiple products
so for example if I took A and
I split it into two molecules
I would make B and C.
I could take multiple substrates and make single products
which is the opposite which would mean I will be
putting two things together to make a third.
That third being C, as shown here.
And last I could have multiple substrates
and multiple products in which
A and B are converted into two
different things, C and D.
Now enzymes are, as I said,
magical in their ability to
catalyze reactions and they are so
much faster than a chemical catalyst
that it's important to
think about the ways
in which they are able to accomplish
what they accomplish. And so
this illustration of an enzymatic reaction goes
step by step into some of the considerations
for the ways that enzymes
accomplish what they do.
Chemical catalysts, I want you to remember,
are things that are very fixed.
A platinum catalyst for example, has
no breathing. It has no
movement to it, it simply is a surface
on which something can happen.
And enzymes are fundamentally different from that.
In this illustration we see
an enzyme shown in green
and we see the active site
of the enzyme, that is the
place where the reaction is
catalyzed, shown in light green.
Now the enzyme in this
reaction I am showing you
is a reaction of multiple substrates, multiple
products. So we will have A and B,
as you can see here, that will be converted
into two other molecules.
We start with the enzyme unloaded.
No products on the enzyme
and of course no substrates.
The substrates are the molecules
that bind to the enzyme
and they will bind so as to position
be positioned at the place
where the reaction occurs, the active side.
We can see here there are substrates
have started to bind to the enzyme.
We see the enzyme again in green. We see substrate A
that has bound at the top portion of the enzyme
and substrate B that has bound the bottom.
Now the interaction of the substrates
with the enzymes will actually
cause the enzyme to start to change.
This is the Koshland Induced Fit model of the enzymes.
And the Koshland Induced Fit it says
that not only does the enzyme change
the substrates into products,
but transiently during the catalytic process,
the substrates change the enzyme. And as we will
see that's essential for this reaction to occur.
So the substrate binding has happened.
We have formed at this point what we call
the ES Complex, enzyme substrate complex.
Now in the next step you see right here, what has
happened is we see the reaction going on.
And the enzyme has actually changed its
shape slightly from the initial binding
to bring A and B into closer proximity.
Well, of course, for a chemical reaction closeness
is an absolutely essential requirement
for the reaction to occur. So the slight
change in the shape of the enzyme
has converted A and B from being apart
to being slightly closer together.
These changes of shape can be very large
on enzyme terms, or it can be very very startle,
but nonetheless the change happens with every reaction.
Now the reaction is occurring
again, as we can see, because
they have been brought
into close proximity.
At this point as the
reaction is going on
we have something called the ES* complex.
And we can just simply think about this as the
place where the reaction is now able to occur.
As we look at this reaction closer, we see
during the reaction a part of A,
has moved from A to B.
And this has been a transfer of a
part of one substrate to another.
A is no longer A
and B is no longer B, at this point.
We have made what we call the EP complex.
We have made the products
but we haven't released the products yet.
So A has become C
and B has become D.
Now the products are still contained within the enzyme.
But the products are different than A and B were.
So just as A and B
cause the enzyme to change shape
so too will C and D cause the enzyme to change shape.
And you can probably see where this is headed.
The enzyme is going to back to where it was.
And that's what happens right here.
We see in this reaction now that the enzyme
has been changed and it's changed back to its initial state.
In the initial state, we can think of its
finger being open like my hand is open
and C and D are ready to go flying away.
The enzyme now being back in its original state
is able to go and bind more substrate.
It's ready for the next process. Now if
we think about this, our definition of a
catalyst that everybody learns in
freshmen chemistry, is a molecule
or an entity that catalyzes a reaction
but it's unchanged in the process.
That's a principle that is hammered into
every freshmen chemistry student.
Now we see that enzymes are actually
slightly violating that principle.
They are being changed transiently during the process
but they ended up in the end
in the same way they started.
So overall they are not violating
it but they cheat a bit.
We see in this slide then a summary of all the reactions
or the steps and the process that you have seen before.
And I don't wanna go through those
again but I do wanna make the point
that you notice that the
arrows are going both ways.
And that means that this reaction and every
step in this reaction is reversible.
Now reversibility of a
reaction is a very important thing to
keep in mind when we are talking about
metabolic processes. For that matter even
non-metabolic processes. But especially
for metabolic processes; because,
we have to think that is "What are the conditions
that would make something go backwards?"
We have seen how enzymes flexibility enables
enzymes to accomplish what they accomplish.