#31 Biochemistry Membrane Transport Lecture for Kevin Ahern's BB 451/551


Uploaded by oharow on 18.01.2012

Transcript:
Professor Kevin Ahern: You get all caught up on all
your studying for this class?
People are smiling.
That's good.
Breaks are good.
I want to finish up talking about membranes in general,
and then I will move on to our next topic,
which is transport across membranes
which is a very relevant topic for us this term.
Last time when I finished, I talked about proteins
that could cross the membrane,
and I described to you that they would have
some characteristic chemical properties
based on the fact that they are interacting
with very nonpolar portions of the membrane
when they're on the inside of the membrane.
What I want to talk about today to start with
are to characterize membranes themselves
in a little bit different way.
And part of this is very general just sorts of information.
And one of the things that I want you to keep in mind
with respect to a biological membrane
is that a biological membrane is a very fluid substance.
It's fluid.
It behaves like a fluid.
We see that there's flexibility to it.
There's movement within it.
And that movement we can actually study very carefully
or very nicely if we're careful.
What you see on the screen is an experiment demonstrating
the fluidity of a membrane, and that fluidity of a membrane
is studied by embedding dyes that are sensitive
to a laser in the molecules of the membrane itself.
And what happens is that the dye when it's exposed
to a laser will actually get bleached.
So you actually have a color change associated
with that blasting that hits it with a laser beam.
If you studied that over time and you watch the rate
with which that bleaching disappears, that is the rate
with which that color disappears, you can get an idea
about how fast things in that membrane
are literally diffusing around.
And what you discover is things are moving fairly rapidly.
You can see here's the event where we have the measurement
of the color that is in that particular cell,
a bleaching event with a laser beam.
And then on the order of less than seconds
where we see this recovery that's happening
where this recovery that's happening is because
unbleached molecules are in fact diffusing into that place
where we had in the membrane the bleached portion.
So we can actually measure that rate of diffusion
by doing an experiment like this.
And so we can actually see that recovery
happening as a result.
No big point.
The main point just being that we have flexibility
and that is that there is fluidity associated
with these membranes.
It turns out that for membranes to be functional for a cell,
they really have to be fluid.
That's a very, very important consideration.
Fluidity is very, very important
for the proper functioning of a membrane.
One of the ways in which we can see fluidity
or two of the ways in which we can see fluidity
are known as lateral versus transverse diffusion.
So they're both diffusion type processes that are happening.
One occurs much more commonly than the other.
So the top figure we can see a lipid bilayer
that has a specifically marked molecule.
You can see it in red there.
And you see that in this lateral diffusion,
what's happening is the red dye is shifting places
on the same side of the membrane.
In this case it's going from position number two
to position number three.
And that movement that happens on one side of the membrane,
it could happen down here as well, is very, very rapid.
So lateral diffusion happens very quickly across a membrane.
However, if we look at that same molecule here in red
and we measure the rate with which it now appears
on the other side of the membrane,
that process is called transverse diffusion,
that process occurs very, very slowly.
So molecules tend to stay on one side of a membrane.
And there are in fact enzymes that are known
to cause a molecule to flip from one side to the other.
They have a very complicated name.
They're know as flippases.
Which I always think of flipping somebody off
when you're doing that, but it's not quite what it is.
But a flippase flips a molecule
from one side of the membrane to another.
And they facilitate transverse diffusion.
They have nothing to do with lateral diffusion.
Lateral diffusion happens quite readily.
Now as I said, the fluidity of a membrane
is a very, very important consideration.
We can think of a membrane as behaving
something like a liquid or a solid.
I'm using those terms a little bit loosely here.
But we can imagine a more ordered structure
in the membrane would be more consistent with a solid
and a less ordered structure in the membrane
would be more consistent with a liquid.
That's kind of what we see at the molecular level
that happens when we take a substance
through its melting point.
Now this shows the transition that's happening
for a biological membrane in going from a solid-like state
to a liquid-like state.
And not surprisingly, that transition happens
over a range of temperature,
kind of like it does when we thaw ice.
So when we thaw ice,
there's a temperature at which that transition occurs.
For a biological membrane, it's actually
not a specific point, but we see
a sort of a range of temperature
over which that transition occurs.
The midpoint of that transition has a name.
It's called the Tm.
You can think of it as the melting temperature if you want,
but we'll call it Tm.
And it's the midpoint of that transition.
Now I talked about cholesterol last week,
and one of the things that cholesterol does
is it widens that transition temperature.
It doesn't change the transition temperature,
but it may make it start way over here
and make it go all the way up over here, something like that.
So cholesterol widens the range of the transition
without changing the actual Tm.
Now as I said, having biological membranes be fluid is very,
very important for their proper function.
And cells have ways of controlling
what the Tm of a membrane is.
It turns out that what they do
is they vary the amount of unsaturated fatty acids
that they have in their glycerophospholipids
and in their sphingolipids.
The more unsaturated fatty acids they have,
the lower their Tm will be.
I'll repeat that.
The more unsaturated fatty acids they have
in their membrane lipids, the lower the Tm will be.
That means that at a lower temperature,
something that has a lot of unsaturated fatty acids in it
will be more likely to be fluid than something
that has a high amount of, I'm sorry,
having a low amount of unsaturated fatty acids.
On the other hand, if I want to have
a higher melting temperature, I would tend
to have more saturated fatty acids in my membranes.
Now a prime place where we see evidence of this
is if we look in the ocean.
In the ocean we discover that the temperature
of the ocean is, especially in these parts of the world,
fairly chilly and that fish
that are swimming around in that water
are exposed to very cold temperatures.
If we want them to have fluid membranes,
we might imagine that they would have a need
for a greater amount of unsaturated fatty acids
in their membranes than we would have living out here
in the nicely climate-controlled environments.
And in fact, that's exactly what we see.
Fish oil, everybody knows,
is very high in polyunsaturated fatty acids.
And part of the reason that they're that
is because of this phenomenon with respect
to the fluidity of membranes.
So fish oil is high in polyunsaturates
to help keep the membranes fluid at a lower temperature.
Yes, sir.
Male student: So the colder the native habitat of the fish
the higher that proportion would be.
Professor Kevin Ahern: In general, that's the case.
In general, that's the case.
So his question was the lower the temperature
in which the fish lives, the higher the amount
of unsaturated fatty acids; and in general, that's true.
Yes, sir?
Male student: [Inaudible]
Professor Kevin Ahern: So his question is, you know,
there's a lot of variation in fish
with respect to fatty acids and so forth.
How does this all apply.
I'm talking about general phenomenon
now with respect to fatty acids.
And yes, you will see ranges of fatty acids
present in fish for different reasons besides membranes.
But from a membrane perspective,
what I'm talking about is pretty much the case.
Good?
So that's the case with fluidity with respect to membranes.
There is simply an illustration of this phenomenon
that I'm describing to you.
Here we see something that has 18 carbons, no double bonds,
and its transition temperature is 58.
Here's something with 18 carbons, and one double bond,
and it's transition temperature is -22.
So that single double bond made a heck of a difference
in the melting temperature.
You'll also notice that the shorter the fatty acids are,
the lower that melting temperature is also.
Here's something that's 22 versus something that's 14.
We go from 75 down to 24.
So shorter, more unsaturated
will lower the melting temperature.
Longer, more saturated will raise that melting temperature.
Part of the reason let that happens
is the more saturated the fatty acids are,
the more easily they are to get ordered.
And as I said, ordered is a model for freezing as it were.
And so this depicts on the screen three unsaturated
fatty acids as we would find in a, I'm sorry,
three saturated fatty acids as we would find in a fat.
And here's the same thing with an unsaturated fatty acid,
and you can imagine that this would be a lot harder to order.
And to order something that's physically different like this,
it takes a lower temperature to do that.
And that's why we see the lower melting temperature
for the membranes that contain the unsaturated fatty acids.
Cholesterol fits into membranes not surprisingly like this.
And you can see that they're arranged in this way.
This little round thing here is used to depict
that single hydroxyl group that's on there.
And so you can see it's kind of hanging out up here
near the hydrophilic end, but everything else
is pretty much buried in that tail of the lipid bilayer.
Now, one of the things I'm gonna talk about,
and I'll talk about it in a bit again to remind you,
is that substances have to move across membranes.
Cells have a very, very important thing that they have to do.
And I want to spend a couple minutes
talking about this to kind of give you an idea
of the importance of what you see on the screen here.
What you see on the screen is a membrane protein.
It's an integral membrane protein, and it performs a function
for the cell that is gonna be the topic
of most of the rest of today's lecture,
and that is active transport.
This guy is transporting ions across a membrane,
and we call it active transport because energy
is required for the movement.
Energy is required for the movement.
And the reason energy is required for the movement,
and this is something you're gonna wanna write down,
is because at least one molecule
is being moved against a concentration gradient.
So active transport is a phenomenon
that occurs across membranes
when at least one molecule is moved
against a concentration gradient.
Now because at least one molecule is being moved
against a concentration gradient, energy is required.
But the definition of active transport
is not that energy is required.
The definition is that at least one molecule
is being moved against a concentration gradient.
Well, what's going on with this?
Why do we even bother
with moving things across a concentration gradient?
Well, when I say across a concentration gradient,
what I'm talking about is moving something
from a low concentration to a higher concentration.
Low on one side, higher on the other.
And I'm moving it in the direction
of the higher concentration.
That's not the way diffusion normally goes.
And so the reason energy is required for these
is because you're fighting the phenomenon of diffusion,
which normally wants to go the other way,
from high concentration to low.
Why does the cell bother with pumping,
in this case what it's doing is it's using the energy of ATP,
that's shown here, to transport out of the cell
three sodiums and to transport into the cell two potassiums.
Why does the cell bother to do this?
Well, I want to tell you the reason the cell does that.
When I talked about the lipid bilayer first,
the thing I told you about it
was the fact that surprisingly water could move
across the membrane fairly easily.
Most substances couldn't do that.
Ions can't do that.
Glucose can't do that, at least not very readily.
But water can.
So water has the ability to go across that membrane.
The membrane is not too much of a barrier for water.
Well, if you perform the experiments
that you probably did in your high school biology class
or college biology class where you learned
something about osmosis; you take a membrane bag, okay,
and you fill it full of let's say a protein,
and you close it off, and you dump it into a solution.
What happens to that membrane
if water can move across the membrane?
It starts to swell because water is diffusing
in to dilute out the protein, but the protein
can't make it out; and so the thing gets bigger,
and bigger, and bigger; it will eventually burst.
Well, we've got the same situation right here.
Water can move across this membrane,
and we've got proteins and things
on the inside that can't get out.
Uh oh, if we're not careful, we are going to burst.
Well, cells have to continually fight that battle.
And they fight that battle by adjusting ionic concentrations.
You see a prime example of it right here.
If cells stop this pumping phenomenon
that you see on the screen, the osmotic balance,
which otherwise is balanced, will get tipped
in the wrong direction, and the cell will start to swell,
and swell, and eventually explode, and die.
So one of the prices that cells pay for living,
the cost of living as it were,
is maintaining an osmotic balance
that keeps it from exploding.
Now it's a complicated thing.
I'm simplifying it here for you.
But suffice it to say that by pumping sodiums out
and potassiums in, making artificial concentration gradients
of those two ions, the cell is creating an osmotic equilibrium
as it were that keeps it from bursting.
If it stops doing this, it's in trouble.
So your cells are continually doing
exactly what you see on the screen
to keep themselves from bursting.
This is active transport because again
we're moving things against a concentration gradient.
In this case, the sodium is in lower concentration
on the inside than it is on the outside.
We're moving it against that gradient.
And potassium is higher on the inside
than it is on the outside.
We're moving it against that concentration gradient.
So in this case, we're moving both things
against the concentration gradient.
I'll stop there, take questions.
Yeah?
Male student: [Inaudible]
Professor Kevin Ahern: We are disrupting the osmotic balance,
but we're disrupting it in a way
that water is as it were confused.
So water is happy with the imbalance that we create
because we have to think about there's many,
many things that could be balanced here,
and by balancing it in this way,
water is much less likely to come in.
If we don't create this imbalance, water will come in.
And as I said, it's a complicated story
that I'm simplifying for you here.
But suffice it to say, this imbalance must be created
and maintained in order for a cell to keep from exploding.
Yes, sir.
Male student: Not to take it in a morbid direction,
but for a living, higher animal, when they pass away,
is that part of the reason that their body swells?
Professor Kevin Ahern: So for a living organism when it dies,
is this one of the reasons that a body swells?
You know, I honestly don't know the answer to that question.
But there's a lot of things that's happening in death
that I would suspect would, in fact,
probably have to do with some damage to cells, yes.
But I don't know that.
Yes, sir.
Male student: [Inaudible]
Professor Kevin Ahern: You're talking about
this transport protein?
Male student: Yeah.
Porfessor Kevin Ahern: So his question was does
this transport protein exist on a wide variety of cells,
and the answer is virtually every cell.
Male student: [Inaudible]
exist on the organelle of a membrane?
Professor Kevin Ahern: Does it exist
on the organelle membrane?
There are other considerations
for organelle membranes, no.
This is on the plasma membrane.
Yeah?
Female student: [Inaudible]
Professor: Well, okay.
So her question is, is there any way for these guys
to make their ways back?
Otherwise, we just basically deprive the cell
completely of sodium and we're full of potassium.
Well, as we will see when I start talking
on the transport system, there are limits to this.
The more of a concentration gradient we make,
the harder it is to put more out or move more in.
Just like if we were trying to pump water up
to the upper layer up here and there's a big balloon.
The more water there is up there,
the harder it is to pump additional water up.
So when we fight that concentration gradient,
after awhile, the concentration gradient's gonna say,
"You're not going any further."
So you can't completely deprive a system
of ions in this way.
Okay, good.
So that's a very cool thing.
You'll see this pump come up in a thing in just a second.
Bacterial membranes are a lot more
complicated than the membranes of our cells.
Bacterial membranes, if you think about it,
bacteria have quite a wide variety of environments
in which they can find themselves.
They actually have to have a cell wall to protect them.
They don't have the rest of the body
as it were protecting them.
And that cell wall provides some strength.
It also provides them with sort of a barrier,
an even better barrier with the outside environment.
So what you see is a sort of schematic of a,
with the equivalent of a plasma membrane down here,
a lipoprotein membrane out here,
a lipopolysaccharide membrane on the outside.
And the construction of this lipopolysaccharide cell wall
as it were that bacteria have is one of
the strategies for the use of antibiotics.
Penicillin, for example, interferes with the construction
of this outer membrane of bacteria, the outer cell wall.
And by interfering with the ability of the bacterium
to make that cell wall, the bacteria actually die.
And so that's a very important consideration.
There's another depiction of it.
And internal membranes.
We look inside of organelles of cells,
related to your question over here,
the internal organelles of cells are themselves
also bounded by membranes.
And they have, in many cases, characteristics
that are different from those of the plasma membrane.
But suffice it to say, the same general idea
is involved in making those membranes.
We have a lipid bilayer, and the composition of that
will vary a bit from one membrane to the next.
The last thing I want to point out to you,
and this is something I'll come back
and talk about later in the term,
is the phenomenon that you see on the screen.
And it's a process called receptor-mediated endocytosis.
Receptor-mediated endocytosis.
The mouthful name I just gave it is shown right here.
Receptor-mediated endocytosis.
And this phenomenon turns out to be very,
very important for cells to get cholesterol in them.
We'll talk about cholesterol later in the term.
And I'll tell you at that time that there's three things
we have to consider in the cholesterol levels in an organism.
One of them is due to what you see on the screen right here.
Cholesterol can be moved in the body.
And it's moved in the body through complexes we call LDLs.
Low Density Lipoproteins.
We'll say more about those later as well.
The low density lipoproteins contain
in them a core that contains cholesterol.
And cells that are needing cholesterol
will make a receptor that will bind to LDL.
So they have a structure on their surface where a protein
on this receptor region binds to a protein on the LDL.
When these two guys attach to each other,
the LDL is internalized.
And you see this internalization happening here.
It's pulling in LDLs.
You see this thing sort of bud off, and now,
on the inside of the cell, we have these LDLs
that have been brought in that have cholesterol in them.
So this process that you see on the screen, as I said,
is called receptor-mediated endocytosis.
And it's how cells get cholesterol from the outside in.
So it's yet another way of crossing a membrane.
Another way of crossing a membrane.
So those are the general things I want
to say about membranes.
I'd like to turn our attention now to talk
about membrane transport.
And I've already introduced the topic...
Yes, sir, Lawrence.
Lawrence: What does LDL stand for?
Professor Kevin Ahern: What does LDL stand for?
LDL stands for Low Density Lipoprotein.
And I'll talk about that as I said later in the term.
LDLs your doctor will tell you is bad cholesterol,
because it's the LDLs that contain cholesterol.
Well, membrane transport.
You've already seen the introduction
now to membrane transport.
And I hope you are starting to get an idea about
the importance of membrane transport.
One is you need membrane transport for sodium
and potassium ions to keep the cell from just blowing up.
That's pretty important.
Cells have to get nutrients.
Cells have to get glucose.
They have to get food.
They have to get amino acids.
They have to get a lot of things inside
of them so they can stay alive.
And those things don't cross membranes easily by themselves.
That means that cells have to have proteins
that help to bring them in.
And in many cases, those molecules that are being brought in,
are being brought in against a concentration gradient,
meaning that active transport is involved.
Well, there are two types of transport
that we want to consider in this class.
What's called active transport and passive transport.
Passive transport does not involve any molecule
being moved against the concentration gradient.
It simply is the process of going from
a high concentration to low concentration.
What's an example?
Well, an example would be the GLUT transporters
that we talked about before that were the ones
that allowed glucose to move across a membrane.
Our blood cells are full of GLUTs.
These are the proteins in the membrane of the blood cell
that allow glucose to move through them.
It turns out that in our bloodstream,
the concentration of glucose is higher outside
of cells than it is inside of cells.
So all that the blood cell needs is something
that only allows glucose through.
And glucose will simply diffuse through it.
Going from high on the outside to low on the inside.
Nothing is being moved against the concentration gradient.
That means, therefore, we have what we refer
to as passive transport.
Some people call it simple diffusion,
but I think that's a confusing thing because
it actually requires a transport protein.
But there's no molecule being moved
against the concentration gradient.
The only energy as such is the energy of diffusion,
high concentration to low.
Yes, sir.
Male student: When you say concentration gradient,
I've always heard it described as an electrochemical gradient.
Is that taking into consideration the electrons going as well?
Professor Kevin Ahern: So he is asking does concentration
gradient equate with the term electrochemical gradient.
In this case, in the case of glucose,
it equates with the term chemical gradient
because glucose is not charged.
But it could be, if it were an ion,
then it could be electro as well.
So it depends on the nature of the molecule.
So when I'm talking about a concentration gradient,
I'm simply talking about concentration is all.
That concentration could be charged.
That concentration could be a specific molecule.
It could be both.
Now here is a figure that I don't want
you to waste too much time on.
Basically it is telling us what?
It tells us that concentration is associated with energy.
If you think about it, that makes sense.
That's why things tend to want to move
from a high concentration to a lower concentration.
It is energetically favorable for them
to move from a high to a low.
So if we want to take something from a low to a high,
we're going to have to put energy in.
So if we have at least one thing going against
a concentration gradient, we're going to have to have
an external source of energy to make sure
that we have enough energy to fight that gradient.
If we have a situation, I talked about with the GLUTs
in the red blood cell, there's nothing to fight.
The higher concentration glucose out.
Lower concentration in.
It automatically moves in.
No additional energy is required.
and this is to bring up what Jodi just said
in the front, that what I just described to you
is a simple concentration.
This shows that the same phenomenon holds
with respect to charge.
So just as a chemical concentration
of say glucose on one side versus glucose
on the other side will have energy associated
with it, so, too, will a charge difference.
If we have more positive charge on the outside
than we have on the inside, that means the positive
is gonna wanna move in and negative is gonna wanna move out.
So chemical gradients, electrical gradients,
both of these are critical energy considerations
for moving things across membranes.
Well, I start this topic with something that your book
starts with, which is, and we don't even talk about
it until later in the book, but I'll talk about it one
of the first things, is this molecule
that you see on the screen called phosphorylaspartate.
Now this molecule is relevant because
it is a transient intermediate in the action
of certain proteins that use ATP in active transport.
This molecule is formed transiently
by some ATP-using transport proteins.
They are ATP-using, and it turns out that in the ATP use,
one of the first things that happens in the action
of these proteins is a phosphate from ATP becomes
transiently attached to the sidechain
of an aspartic acid residue.
That makes the phosphorylaspartate.
The sodium potassium ATPase
that I just showed you is one such example.
These are proteins, and they have a name that are called
P- type transporters P, the letter P, -type transporters.
P - type transporters will have this phosphorylaspartate
as an intermediate in their action.
So phosphorylaspartate appears as an intermediate
in the action of P-type ATP transport proteins.
Yes, sir.
Male student: Is that aspartate [inaudible]
by itself or is it...
Professor Kevin Ahern: It's part of the protein.
So the aspartate is a sidechain of the protein.
And so like we saw in the serine proteases,
where we had the alkoxide ion that played that role,
it was a sidechain of the serine, this is a sidechain
of an aspartic acid, and it plays a role in this process.
We're not gonna go through the mechanism of that.
But suffice it to say that this is an
intermediate in that process.
Let's look at a simple pump.
This is a very simple pump.
It depicts a protein in a membrane.
It is specific for a specific molecule.
And yes, we generally see that proteins that
do transport are very specific for very specific molecules.
It will only allow certain molecules to pass through them.
Now, as we look at this,
we see here's the molecule being transported.
It comes in.
It fits nicely into here.
And then, once it gets in there, the protein sort
of changes its shape so that the open region,
instead of pointing outwards,
points inwards; and the molecule comes in.
This is a very simple depiction of transport.
This could be either active or passive transport.
If this guy is being moved against
a concentration gradient, it is an active transport.
If it is moving simply with a concentration gradient,
it is a passive transport.
The only energy being input, if it's a passive,
is just the energy of diffusion.
There's no other energy that's needed in passive transport.
If it's active transport, then there are a,
and you might want to underline this,
if it's active transport,
there are a variety of possible energy sources.
A variety of them.
We're gonna talk about a couple of them.
ATP is a common one, but ATP is not the only one.
Let's take a look at a P-type ATP-using transport protein.
This is a protein that is involved in transporting calcium.
And it's moving calcium against a concentration gradient.
Moving calcium against a concentration
gradient across a membrane.
This could be outside the cell, inside the cell,
or even within an organelle within the cell.
For our purposes, it doesn't matter at this point.
Here's the membrane, whether it's the organelle
membrane or the cell membrane,
and what this is trying to do is move calcium
from the lower place down here to outside the membrane here.
So what's happening?
Well, the first thing we see in that process
is in this case two calcium ions bind
to specific locations inside of a transport protein.
ATP comes in.
And right there is our little sidechain of aspartic acid.
The phosphate gets transferred to the aspartic acid,
and the transfer of that phosphate
to this aspartic acid causes, and here we go again,
a slight shape change in the protein.
That slight shape change in the protein
is what is responsible for now opening
this thing towards the outside instead of the inside.
So here's the phosphate on there.
Shape change.
And now we've got the opening pointing outside.
And calcium goes out.
Calcium may get kicked out in that process,
which is what it's gonna have to physically do
if it is working against the concentration gradient.
Everything else then, is back to resolving
the initial condition.
Water comes in, releases the phosphate,
the release of the phosphate causes the shape change,
back to the original state, and we're back where we were.
Now I'm not so worried about you memorizing
the steps in this process.
It doesn't really matter to me if you know that
calcium comes in first, and then ATP, etc., etc.
But I do want you to understand the role of ATP here.
ATP is an energy source that's putting a phosphate on here,
and that's what's driving the whole process.
The addition of a phosphate to that aspartic acid
residue is causing a shape change that results
in the expulsion in this case of calcium.
Yes, sir.
Male student: If the concentration of the ATP transporter
gets high enough, are these theoretically reversible?
Could you generate ATP, or is there a one way stop sign?
Professor Kevin Ahern: Very good question.
So he just asked if the concentration of this got so high,
could this reverse and actually make ATP.
Some pumps can do that.
Some pumps can do that.
Male student: [Inaudible]
Professor Kevin Ahern: What's that?
Male student: Can the calcium come back
from outside of the...?
Professor: It is possible if we had something else
that was making the outside calcium
concentration be very high.
It won't go for very long, but it's possible
that you can do that.
It's not a major energy source for the cell
because it's not gonna last very long.
So that is how a P-type ATPase, I call it a PTP,
a P-type ATP transport protein.
Some people also refer to it as a P-type ATPase
is what they call it.
So both of those terms are acceptable.
Here shows you that sodium potassium gradient
that I talked about before.
There it is doing its thing,
and now we see it in the context of this cell
doing something else.
And this is kind of cool.
This is kind of cool.
Notice that we're increasing the concentration
of sodium outside the cell by the action
of that pump that you've already seen.
The concentration of sodium is getting high.
What did I tell you to start this about
the concentration gradient in terms of energy?
There's potential energy there, right?
We have a high concentration of sodium outside the cell.
Does sodium want to come inside the cell?
Yes, it does.
That's an energy source.
It turns out that especially in heart cells,
not in heart cells but in some cells,
they use the sodium gradient as a way of bringing in glucose.
So cells that are in a portion of the body
that don't have a high concentration of glucose
outside may need to actively transport glucose in,
that is to move glucose against
the concentration gradient in.
Those cells can use a sodium gradient
as their energy source to bring in glucose.
So a concentration gradient can actually be
an energy source for a transporter,
for an active transporter.
That's a little confusing to students sometimes.
How can a concentration gradient
play a role in an active transport?
And that's where we come back to that definition
that I gave you.
The definition I gave you was we have at least
one molecule being moved against a concentration gradient.
In this case, glucose is being moved
against a concentration gradient.
Sodium is moving with a concentration gradient.
And so it's the movement of the sodium
that's helping to bring that glucose in.
I'm saying a lot, so let me slow down and take questions.
There's usually a lot of questions about this guy right here.
I can tell you guys have been gone on break.
Karen, is that a question?
Karen: Yeah, well, does the sodium,
is it just associated with the glucose molecule
over there or does it bind?
Professor: So the only place where the sodium
and the glucose have any interaction at all
is at the place where the protein,
where they're coming in.
Out here, no.
They have no association at all.
Yes?
Male student: [Inaudible]
Professor Kevin Ahern: That's a very good question.
Does it have two channels?
It has the ability yes, to bring both in.
And we see this commonly happen in proteins.
We saw it actually right here.
There's one channel for sodium and one channel for potassium.
And each one is specific for each one.
Yes?
Male student: Would the actual mechanism [inaudible]?
Professor: So I think what you said
that is the mechanism such that this cannot happen
unless both have loaded.
And the answer is that's correct.
So these are basically requiring both things
to bind before any one of them will come through.
Yes, Neil.
Neil: [Inaudible]
Professor: The energy ultimately is coming from ATP,
but the energy of all things are ultimately coming from ATP.
Or if we really want to go ultimate, we say to the sun.
Or we say to, you know, it's where we draw the line.
But in this particular process,
it's specifically sodium ions.
But yes, ATP was driving that.
So we have to draw a line where we're saying
that energy source is.
So that's kind of a cool process.
I want to tell you about a cool molecule, digitoxigenin.
Digitoxigenin is something that's made by foxglove.
And it is a nasty compound if it is not used properly
because what it does is it blocks the action
of the sodium potassium ATPase,
the sodium potassium transporter,
the thing that was keeping our cells happy
so that they don't blow up.
This guy's a poison.
But it actually has medicinal uses in small quantities.
And I want to tell you one of those medicinal uses
that's really interesting if you're interested
in how the heart works.
Now, the heart, as you know,
depends upon calcium like any muscle does for contraction.
It depends on calcium for contraction.
When we see the contraction of a muscle,
we see calcium stimulating the contraction,
and then we see the pumping out of calcium to reduce
that concentration so the contraction can happen again.
The heart is no different than anything else.
The heart, like other cells in the body,
have a sodium potassium ATPase.
So they have a system that's very much
like this guy right here.
In addition, they have another transporter
that's of considerable interest.
This other transporter that they have uses
a sodium gradient to pump calcium out.
So part of that flushing of the muscle cell
to get the calcium out requires sodium coming in
at the same time calcium is going out.
They go in opposite directions.
Digitoxigenin in small doses is used to stimulate
a more vigorous heartbeat.
How does it work?
Well, based on what I just told you,
I'd hope that you'd be able to figure it out,
but I'll tell you.
What's happening is we give digitoxigenin in low doses,
maybe we target the heart for this.
What are we gonna do to the sodium potassium ATPase?
It's going to slow down.
Right?
If it slows down, what's gonna happen
to the concentration of sodium outside the cell?
It's gonna fall.
If the concentration of sodium outside the cell falls,
what happens to the ability to pump calcium
out of the cell?
It will also fall.
Digitoxigenin is keeping this calcium concentration
higher in the heart cell to cause it to contract harder.
It's a way to stimulate the heart to beat harder.
And it's used sometimes for people who have congestive
heart failure where their heart is not beating hard enough.
So it's artificially keeping that calcium
concentration in the heart muscle cell high.
It can't do this forever obviously.
But over a short period of time,
it can actually stimulate the heart to beat harder
and do the pumping that's necessary to keep the person alive.
A very cool application of our knowledge
of membrane transport.
Questions about that?
Usually there are.
You're a quiet group today.
This is the quietest of I've seen the class the whole term.
Yes, sir.
Male student: Is there another [inaudible]?
Professor: Are there other names for it?
There may will be.
But the one we'll use is digitoxigenin.
Digitoxin.
You see that some.
Let's do one last thing, and then we'll call it a day.
And the last thing I want to talk about here,
actually I have two things, are another class
of proteins that use ATP to do active transport.
So I've described the P-type ATPases to you as one example.
They were the ones that used that phosphoaspartate
or phosphorylaspartate, same thing.
What you see on the screen is a schematic representation
of another class of ATP-using transporters.
They are called ABC transporters.
And ABC transporters use a different mechanism.
They do not use phosphoaspartate.
We're not going to go into their mechanism.
They have a schematic structure of something
like what you see on the screen.
And my only purpose in showing you this
is just to let you know that there are other ways
that ATP energy can be used to do pumping.
Now, there are some very interesting proteins
that are in fact ATP type, I'm sorry, ABC type transporters.
One of the most interesting ones is this guy right here,
multidrug-resistance protein, also called MDR.
There are many versions of this type of protein
that are found in cells almost across all of biology.
We have a protein like this in our body.
Some bacteria have a protein like this.
And what they do is they use the energy
of ATP to pump toxic substances outside of them.
And they have a wide variety of substances
that they will grab.
They're not very specific.
It's one of the few transporters
that's not very specific in terms of what they will grab.
Well, imagine that you are trying to kill
a bacterium with an antibiotic,
and the bacterium gets one of these transporters
and has the ability to kick the antibiotic outside
of itself where it needs to be inside to work.
What's going to happen to the effectiveness
of that antibiotic?
It's gonna go down, right?
The same thing happens in our bodies
with some types of chemotherapy.
The chemotherapeutic drugs that we give
are very good at killing cells.
But cancer cells that figure out,
"Oh, this is the thing that is killing me.
"Let's pump it all on out,"
become resistant to it as a result of that.
So multidrug-resistance protein is a very important
protein that's an ABC type transporter.
Yes, sir.
Male student: So what's the basis of its
selectivity for exporting things?
Professor Kevin Ahern: Yeah, What's the basis
of its selectivity for selecting things?
It tends to favor fairly nonpolar molecules.
It doesn't tend to favor very specific
shapes or anything like that.
But it tends to favor nonpolar molecules.
And many drugs are in that category.
So that tends to just kick them out, and they're set.
But it's not a very specific protein at all.
Male student: Do they have any tags or flags
on their own proteins so they don't just
turn themselves inside out?
Professor Kevin Ahern: I would assume they have some way
of knowing what's me and what's not me,
but I can't tell you that.
Let's call it a day there, and I'll see you guys on Friday.
[END]