This picture depicts a hormone receptor in
orange and a hormone in yellow that binds to it.
Now as I said earlier, most hormone receptors
are membrane bound, embedded in the membrane.
And most steroid hormone receptors are
found in the cytoplasm or nucleus.
It’s the interaction of the hormone
with the receptor that causes the signal
which the hormone is communicating ultimately
to be exerted inside of the cell.
Now this simple figure schematically
shows what’s happening.
We can see on the left that the hormone
is getting ready to bind to the receptor.
And on the right we can see the hormone
after it is bound to the receptor.
On the outside part of the cell, which
is the top layer here, that’s where
the hormone is found, we see that there's
a binding site for that hormone.
And on the bottom side of the receptor
which is the inside of the cell, we see
that after the hormone binds, that part of
the protein changes shape very slightly.
Now very slight change in shape of
that protein changes the interaction
of that protein with molecules that
are on the inside of the cell.
So what we’re seeing here is the first
step of communication, and we’re
seeing how that communication actually
ultimately will exert its effects.
So this picture depicts two different ways in
which signals can be communicated inside the cell.
On the left we see steroid hormones,
and as I said, steroid hormones are
unusual in being able to navigate
that lipid bilayer all by themselves.
They get inside of the cytoplasm which
is that middle layer that’s there.
And you see that they bind to
that purple colored molecule.
That purple colored molecule is a steroid
receptor floating around in the cytoplasm.
When that steroid receptor binds to the
hormone, you can see that it navigates
downwards, and downwards it’s moving towards
the nucleus, because the steroid hormone
receptors are going to go and directly
bind to DNA and stimulate the expression
of specific genes that cause that hormone’s
actions to ultimately be realized.
The other figure that’s shown on the screen is showing what’s
happening with a receptor that is
not a steroid hormone receptor.
Here we see that this is a
And to orient you, the outside part of the cell is upwards, and
like before, moving downwards, we’re
moving closer to the nucleus.
You can see the hormone that is floating in solution and
you can see one hormone that has bound to its receptor.
We can also see below that, a series
of different boxes and circles, and
colors and so forth; each one of those
molecules being an individual protein.
Now the binding of the membrane to the
receptor caused a complex to form.
And that complex is involved in
communicating a signal downwards.
So when we look at this membrane-bound
receptor, we see that the
complexity of the signal moving through
the cell can be quite enormous.
If we think about for example, how our computers connect
to the internet, we think it has to go through for
example, WiFi, and that WiFi has to be connected to
another system, and that to another system, etcetera.
Each of those being a node in that overall connection
of your computer to a bigger computer elsewhere.
Well that’s very much like what’s
happening here, but surprisingly there’s
quite a few proteins involved in the
process, and we’ll see how that occurs.
So after that initial receptor has bound to the
hormone, that signal has to be communicated inwards.
And as you saw in the example I
showed for one protein, that was a membrane
receptor, there were other proteins along
the way that communicated signals.
I also want to remind you that
not all of the molecules involved
in communicating messages are
proteins on the inside of the cell.
Some of the communicators as it
were, are called second messengers.
And what these messengers are, are small molecules
that do the work of communicating that information.
We can see some of them here.
First we see one called IP3.
It’s relatively small.
When I say small, I’m meaning
smaller than a protein.
Another very common one found
inside of cells is cyclic AMP.
It’s related to the molecule AMP and the molecule ATP, but has
a cyclic group as you can see on the left side of the molecule.
Calcium as I’ve talked about in other
lectures, performs functions of second
messengers as well and happens as a result
of release into the cytoplasm of the cell.
Another cyclic nucleotide that’s involved in communicating
information as a second messenger is cyclic GMP.
And we’ve talked in another lecture about how cyclic GMP
helps communicate information in the process of vision.
Now, the covalent modification occurs
during the communication of the message.
So I showed in that previous slide, a
variety of proteins, that one going to
another, to another, to another, to
another, down ultimately to the nucleus.
What’s happening there?
Well what’s happening is, each of those proteins is being
modified chemically as that information is being communicated.
That chemical modification is usually a phosphate
that’s either put onto or taken off a protein.
And that chemical modification
is essentially the signal.
So the signal you’re probably
beginning to realize is complex.
It can be coming in the form of second
messengers, the small molecules you see here.
It can be coming in the form of chemical
modifications that are happening to proteins as well.
Now, one of the things that happens
ultimately in the communication of many
messages, is alteration of the pattern
of gene expression for a given cell.
Remember that a cell in a multicellular organism has
different genes that it expresses at different times.
A bone cell for example, having different
genes expressed than a muscle cell.
So being able to control which genes are
being expressed, allows that cell to respond
appropriately to the needs of the body
and the individual needs of that cell.
Another way in which hormones can
communicate and change the path of
the cell is by changing the activity
of enzymes within it, and
we’ll see an example in just a
little bit about how the process of
glycogen metabolism can be drastically
changed by action of a hormone.
So this figure depicts a simple scheme of second
messengers involved with a protein called phospholipase C.
So phospholipase C is an enzyme that’s found in the
membrane of cells and it gets activated by hormone action.
Now this is a very fast acting process
because what’s happening in this
process is we’re seeing an enzyme
being activated, phospholipase C.
When it’s activated, phospholipase C catalyses a reaction
on a molecule called PIP2 in the cell’s lipid bilayer.
That reaction catalyzed by phospholipase
C splits PIP2 into two molecules.
One of the molecules is known as IP3, and you can
see it traversing the cytoplasm of the cell, moving
downwards in the direction of the endoplasmic reticulum,
an organelle that holds calcium for example.
When IP3 gets to a receptor on the
surface of the endoplasmic reticulum, it
binds to it and causes calcium to be
released into the cytoplasm of the cell.
Calcium is also a second messenger.
So we’ve seen, now it’s two second
messengers, one being IP3 and a
second one being the calcium that’s released
from the endoplasmic reticulum.
The calcium goes and binds to a
protein known as protein kinase C.
Now protein kinase C is a protein that
can physically alter other proteins.
Remember I said we can
covalently modify other proteins.
So that’s what is happening as
calcium binds to protein kinase C.
Well turns out for protein kinase
C to be fully active, it needs
an additional second messenger that is a third second messenger.
And the third second messenger that it uses is embedded
in the lipid bilayer of the cell and is known as DAG.
Now not coincidentally, DAG was the product
of the catalysis of phospholipase C on PIP2.
So when phospholipase C cleaves
PIP2, it broke it into IP3 and DAG.
Both of those went to different
places to exert their effects.
But the coordinated effort was to activate
the protein known as protein kinase C.
Protein kinase C can go phosphorylate other proteins
and cause the cell to change its gene expression.
Now, this pattern of modifying
proteins can be very complex.
It’s amazing the network of individual
processes that are occurring inside of cells.
So don’t worry, I’m not going
to take you through all of this.
Suffice it to say there’s an entire course we could
probably teach on what’s shown on this one slide here.
But the message I want you to take away is that in
this process that you’re seeing, there are multiple
components that are all interacting and coordinating an
effort that is focused in this case, in the nucleus.
And the nucleus of course is where the DNA is held,
and it’s there where the gene expression will be
controlled as a result of the processes that are
happening through these hormone signaling processes.
So, cellular signaling
is very complex.
And the responses in every case
though are aimed at benefiting the
organism, just like that military
where I have an individual soldier.
The soldier’s effort is to
help the unit as a whole.
So too are the actions of cells directed
to helping the organism as a whole.