The next one we’re going to go through is our ATPases.
The prototype that we’re going to use for this is a P-type calcium
or a P-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.
ATPases can be on any membrane.
It can be on a plasma membrane or an intracellular membrane.
And a good example of an intracellular membrane pump or ATPase
is the calcium ATPase on the sarcoplasmic endoplasmic reticulum.
Now if a pump moves, an unequal number of ions
either in or out of the membrane is referred to as electrogenic.
What electrogenic means is it creates
an electrical chemical gradient.
And this gradient can be used to help in voltage exchanges.
It's important to know that an ABC transporter
is not the same thing as an ATPase.
This ATP binding casette or ABC transporter
needs ATP to undergo hydrolysis but it is not a ATPase.
A good example of ABC binding transporter is the
Cystic Fibrosis Transmembrane Regulator, or also known as the CFTR.
This CFTR, if ATP binds to an appropriate
nucleotide binding domain, will open up the channel.
And therefore, it really is acting like a ligand type of a channel.
Something is activating it.
But instead of that activation portion being
on the outside of the membrane,
it's internal or it's being activated from the inside.
A good picture of the CFTR can be seen over here
where you have two nucleotide binding domains.
ATP can bind in these two domains, undergo hydrolysis which
opens up the pore, for in this case chloride to travel through.
And chloride is gonna travel through based upon
its electrochemical gradient.
They'll determine if it flows through into the cell
or they go out of the cell.
So let's go through that prototype in more detail.
There's a number of cyclical processes and volunteer and we're gonna
take these one by one to explain to you how this process works.
The first thing we're gonna talk about is to think about
the number of different ions that are exchanged.
So this again is a P-type pump extrudes or
pushes out three sodium and brings in two potassium.
The other things to think about is where this is primarily located.
Usually this is going to be on the
basaolateral side of most epithelial cells.
And we'll keep talking about epithelial cells
throughout this particular course
that these cells are usually located along our junction
in which we're moving something from one side of that cell,
across one membrane, into the cytosol, then across that membrane,
on the other side to make move into somewhere like the blood.
It involves a number of steps, in which here we have eight,
and we'll try to animate these for you
so you really understand how the 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 sodiums to be precise.
The next thing that happen is the hydrolysis of ATP. Hydrolysis
means we breakdown ATP into an ADP and an inorganic phosphate.
The inorganic phosphate stays bound.
What this also does is close the inner gate.
Now the outer gate opens through a confirmational change
and that allow 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,
allowing potassium 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 then 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 P-type pump.