The next one we’re going to
go through is our ATPases.
The prototype that we’re going to
use for this is a V-type calcium
or V-type sodium-potassium
ATPase or a pump.
Now, why are these
pumps so important?
Well, this is important
because the last two
things we went through,
pores and ion channels,
needed to have a
concentration gradient for
there to be any transport
across the membrane.
Here, we are creating the gradient or
we are pumping against the gradient.
So we don’t need to rely on a
gradient, we can do it ourselves
by using energy to get us across
that particular cell membrane.
So that’s why this is
called active transport.
And this is used to either
establish a gradient
or move something
against other gradient.
Okay, besides having ATPases in
places like the plasma membrane,
we can also use ATPases in other places
such as the endoplasmic reticulum.
There’s a certain kind of calcium
pump located on the endoplasmic
reticulum of muscle cells that
allows us to pump in calcium.
And why this is so important is this
is going to help us relax a muscle
by removing calcium from the
cytosol of that muscle cell.
Now, what are the important aspects
of having these particular ATPases?
Well, the first thing to think about is
there needs to be ATP hydrolysis occur.
Now, this occurs both on our pumps,
as well as on our ABC transporters
and these are not pumps in
themselves but still require ATP
hydrolysis or energy to cause
the transport process to work.
But let’s get back
to our prototype.
Usually, for our prototype,
what we’re trying to do
is pump a certain number
of ions across the cell.
For this sodium-potassium
ATPase, we’re going to
be pumping an unequal
number across the membrane.
So, three get kicked out of the cell
and two potassium move into the cell.
This unequal number though creates
an electrogenic response,
which means that there’s going to
be an electrical gradient formed.
Because you’re losing three positives
and gaining two positives,
that means there’s a
positive difference there.
As that difference occurs, you’re going
to create an electrical gradient.
So let’s go through that
prototype in more detail.
There’s a number of cyclical processes
involved here and we’re going to
take these one by one to explain
to you how this process works.
The first thing we’re going to
talk about is to think about
the number of different
ions that are exchanged.
So this, again,
is a V-type pump,
extrudes or pushes out three sodium
and brings in two potassium.
The the other things to think about
is where this is primarily located.
Usually, this is going to be on the
basolateral side of most epithelial cells,
and we’ll keep talking about epithelial
cells throughout this particular course.
But these cells are usually
located along on a junction
in which you’re moving something
from one side of that cell
across one membrane into the
cytosol then across that membrane
on the other side to be moved
into somewhere like the blood.
It involves a number of steps
in which here have eight
and we’ll try to animate
these for you so you really
understand how this
sodium-potassium ATPase works.
Okay. Now, that we have the basics
of the sodium-potassium ATPase down,
let’s now take each one
of those steps in turn.
So let’s first start off with
the basic portion of the pump.
So we have ATP bound to it,
we have the outer gate closed,
and we have nothing in the pump yet.
We now have sodium that enters into the
pump, in fact, three sodium to be precise.
The next thing that happens
is the hydrolysis of ATP.
Hydrolysis means we break down ATP into
an ADP and an inorganic phosphate.
The inorganic phosphate
What this also does is
close the inner gate.
Now, the outer gate opens
through a conformational change
and that allows sodium to leave.
This then allows the
potassium, in fact, two
of them to be precise,
entering into the pump.
Then, inorganic phosphate leaves
the pump, closing the outer door.
And now, ATP binds back to the pump
which opens up that inner gate,
to go into the cell.
And that sets up the
sodium-potassium ATPase pump cycle,
where you need to have ATP bound,
broken down, sodium entering,
sodium to and exit the cell,
potassium to enter the pump, and then,
finally, potassium to enter the cell.
And it’s very important to have
that process working correctly
to open and close the various
gates at the appropriate time.
It’s a great example of a
sodium-potassium ATPase V-type pump.