Now let's move on and look at intracellular receptors.
With intracellular receptors, recall that the ligand
or the signal molecule must be able to pass through
the membrane. It has to pass through that hydrophobic
region. And so things like steroid hormones are
perfect examples of intracellular receptors.
When we look at how these work we will bind directly
to a receptor inside the cell and then that receptor
directly binds to DNA in order to have DNA transcribed
and translated into whatever cellular effect
we're looking for. So we'll make proteins that themselves
illicit the cellular response. Some of the ligands
will bind to intracellular receptors that are in the
cytoplasm while others will actually bind to
the receptors inside the nucleus. So, much closer to
the DNA. And the effects that they have are fairly broad.
We can see that each one though in order to bind to
DNA must have three functional domains.
The first of which is a domain that the ligand binds
to. The next is a domain that binds to the DNA.
So those are two key domains. But in order for
transcription or translation of this proteins
to actually happen, we also have to have a place for
the coactivators that start the process of
transcription and translation to bind. We'll explore
what those coactivators are. Those factors that help
the receptor actually begin its cellular effects. We are
going to be exploring those in our module on genetics.
Finally, some of our intracellular receptors can
actually act as enzymes. The nitric oxide system
is a great example of this. Nitric oxide can be
released by a neighbouring cell and then
have an effect on its neighbour. What happens here
is nitric oxide will come through and activate the
enzyme, diffuse into the neighbouring cell, activating
the enzyme guanylyl cyclase. Guanylyl cyclase
will take GTP and make it into cyclic GMP. Cyclic GMP
we'll see often in our following lecture. And it acts to
activate other proteins. So it's going to activate
protein kinase G in this case. Protein kinase G
then has its effect on blood vessels by causing them
to dilate. Now, there is another drug that's particularly
interesting. When we think about viagra, viagra uses
this nitric oxide system on a version of
cyclic GMP phosphodiasterase, no, protein kinase G
phosphodiasterase. And that phosphodiasterase
actually shuts down the pathway for blood vessel
dilation. Well the drug viagra acts on that
phosphodiasterase enzyme to shut it down so that levels
of cyclic GMP remain high and thus activate the
protein kinase that keeps blood vessels dilated.
And that phosphodiasterase is specifically
found in cells of the penis and so it works really
well only in that location. So, pretty neat trick
with nitric oxide and those pathways. So in this
lecture, you have learned about different modes of
cell signalling. By now, you should be able to describe
some of the mechanisms that cells use to signal
each other. Think about the example of being in a room
of crowded people. Do we need talk very locally or
do we need to have an endocrine long distance
signalling mechanism. And you also should be familiar
with the idea of phosphorylation. That phosphorylation
could activate or deactivate a protein kinase pathway
or a cellular response. And then finally you should be
able to differentiate between intracellular receptors
and cell surface receptors. We looked at several
different cell surface receptors and then we looked at a
couple of different forms of intracellular receptors.
In the next lecture, we'll be examining with more specifics
these second messenger systems. Thank you so much
for listening and I look forward to seing you there.