Enzymes as I noted at the beginning can bind
reactions in different ways and I talked about
one substrate going to one product or
two substrates going to one product.
Or the case I am going to describe here,
two substrates going to two products.
Now when we think of two substrates
that can bind to an enzyme
we realize that there is different
ways that they could bind.
For example if we have the reaction
A plus B goes to C plus D.
We could imagine that maybe
A would have to bind first
and then B. Or maybe B
binds first and then A.
But what we find is that for some enzymes it
really doesn't matter which one binds first.
This is called random binding as it shown in
first example that I have on the screen.
Random binding means it doesn't matter.
Now some enzymes bind substrates randomly as I
am showing you here. But a lot of enzymes
do what's called ordered binding.
That means that either A must
bind first or B must bind first.
Now that model or that mechanism is significant
and the reason it's significant is
it's probably the best illustration that I can give you
for the Koshland Induced Fit model.
Because what order binding tells us is
that if one of these must bind first
before the other one does. That
means then that the binding of first
one is actually changing the shape of
the binding side for the second one.
Because, if the second one tries to bind first,
the change hasn't already happened
and that's why the second one
can't bind first. So ordered binding
reinforces the Koshland Induced Fit model.
Now that might seem to cover all the territory but there
is actually a third model that enzymes use
to catalyze reactions and this one is
kinda interesting and it has a fun name.
We called it the Ping-Pong
mechanism and it's also called
double displacement reaction.
But the point is the same,
the Ping-Pong mechanism
is an enzyme that actually exists
in two covalently different states.
It means that the enzyme is actually
physically binding to something
and causing a change. We will
see this happen in the next slide.
Now this illustrates a
reaction of A plus C going to B plus D,
and we are seeing it split into two reactions.
Alright. In this reaction what's happening
is A is starting out with an oxygen on it.
And in the reaction of A to B
the oxygen is being replaced by an amine.
So, we see this happening and where
is the amine coming from?
The amine is coming from the enzyme.
So the enzyme is carrying the
amine and it's carrying it to A.
So when A interacts with the enzyme
the enzyme swaps the amine that it is carrying
for the oxygen that's on A.
So on the right side of the top equation
we see that A has become B;
because, it now has an amine and the enzyme
has grabbed the oxygen. It no longer has an amine.
So I have colored it with green so you can see that.
In the second part of the
reaction, C which has an amine
is interacting with the enzyme
that now has an oxygen.
And when that happens they trade places
C becomes D, where D has a
double bond to the oxygen,
and the enzyme has become linked to an amine.
Alright? So the enzyme has
returned to its original state.
So by this Ping-Pong mechanism the enzyme
is continually going from amine
to oxygen, to amine to oxygen, and depending upon
which states it is in, it determines
which of the substrate
it binds and swaps with.
Now this type of reaction that I just described to you is a
common reaction that is used by enzymes called transaminases.
Transaminases are enzymes that
do just what I have described.
They swap oxygens for amines
and this is a very important reaction
in the metabolism of amino acids;
because, amino acids get their amines, in some
cases, by the reaction that you see on the top.
They start out with a double bonded
oxygen and they become an amine.
A really good example of this is the molecule
alpha ketoglutarate in the citric acid cycle.
Alpha ketoglutarate can become glutamic acid
if the oxygen on it is swapped for an amine.
In this way the cell can make an amino acid
that it might need; because of this mechanism.
On the other hand we might have the
situation where glutamic acid
or even another amino acid is needed for energy.
People that go on low carb diets for example,
don't have a lot of carbohydrates.
But they are not starving to death; because, they are
eating plenty of protein. Proteins providing amino acids.
And amino acids provide energy as a
result of what I am showing you here.
The lower left reaction has an amino acid
that has the amine replaced by a
double bonded oxygen. So imagine
if amino acid on the lower left
side is actually aspartic acid.
Aspartic acid can be converted by swapping its
amine with an oxygen into oxaloacetate.
And oxaloacetate can be oxidized
in the citric acid cycle.
So this transaminase reaction is
important to both for making amino acids
and also for metabolizing
amino acids for energy.