So three different possibilities can happen when we
consider cell surface receptors. Recall there are
two classes that we will talk about. Cell surface
receptors as well as intracellular receptors.
Cell surface receptors are required when we have a
hydrophilic molecule. And the intracellular receptors
can work when we have like a steroid hormone or
something that can pass across that hydrophobic region
of the membrane. So in the cell surface receptor
classes we have chemically gated channels. We have
enzymatic receptors. And then we'll have G-protein
coupled receptors. We'll take a brief introduction
to the G-protein coupled receptors in this lecture,
and move much more into depth in the second lecture
in this sequence. Looking at these gated channels,
we'll see them a lot in biological processes.
We have a ligand that binds to the receptor, the
receptor is closed. And when the ligand binds
it could be something like sodium or it could be
potassium or chloride or maybe even acetylcholine.
That ligand binds to the receptor, causes the receptor
to open and we have a pathway through that's favourable
to hydrophobic or hydrophilic molecules that might
want to pass into the cell. Often these are
multi-pass transmembrane proteins. So we have that
alpha helical model that this one in particularly we've got
shown an alpha helix or helix turn helix protein
binding motive. The next type of receptor we'll look at
is the enzymatic receptor. Again a cell surface
receptor. And this enzymatic receptor will actually
have an effect on the inside of the cell. So rather
than being a channel that lets things through
we have a ligand that binds to the exterior of the
protein and then have an effect on the inside of the cell.
Generally those are single pass proteins. So the
ligand binds and activates the protein.
Again we'll look at this in much more detail when we
look at receptor protein kinases in the following lecture.
But the enzymatic action is then happening inside the
cell. So this is an indirect effect with enzymatic receptors
Each of these receptor types will have some sort of
cellular response. The details of those cellular responses
we'll certainly get into in much more detail later on.
So before we move on I'm going to give you a quick
introduction to G-protein coupled receptors.
What on earth is a G-protein coupled receptor?
Well this is the first instance we'll see of second
messenger systems. Again, a cell surface receptor
that's going to have an effect on the inside of the
cell. There is a couple of different mechanisms
that can happen but they're both indirect. We have
indirect action where we have the receptor on the
outside of the cell illiciting an effect on the inside
of the cell. So G-protein coupled receptors are
a second messenger system. Again, we'll be looking
at several of these in a future lecture.
They're activated by G-proteins which are a small
class or fairly large class of proteins that are
activated by GTP rather than ATP. Again, GTP is
phosphorylating and it's phosphorylating perhaps
even a different channel. In this case we have the
receptor in one place and then the signal is moved
inside the cell. The G-protein carries that signal
to another transmembrane protein which opens up
as a channel. More details to come on that shortly
but these G-protein coupled pathways are the pathways
that many of our therapeutic drugs are targeted at.
There are so many different places in the cascade
of events leading to a cellular response that there
are lots of opportunities for us to target drugs at them.
And so that's why it's important for us to
have a good understanding of some of the major
G-protein coupled pathways.