FoundationalConceptsNeuroscience

with a molecule of GTP (guanosine triphosphate, a chemical relative
of ATP) taken from the intracellular medium, and the G-protein
breaks apart into two subunit proteins. These pieces of G-protein
then move around on the inner surface of the cell membrane and
bind to other things, resulting in a variety of possible effects. One
type of protein to which an activated G-protein (more accurately, a
piece of the G-protein, the alpha subunit) might attach is an enzyme
called adenylate cyclase. The binding of the G-protein alpha subunit
to adenylate cyclase changes the activity of the enzyme, either up or
down. This particular enzyme catalyzes the formation of molecules
of cyclic-adenosine monophosphate (cAMP, another chemical relative
of ATP). cAMP molecules move around inside the cell and interact
with various proteins, altering their enzymatic activities. Among the
proteins that are activated by the binding of cAMP are members of a
class of enzymes called protein kinases. Protein kinases catalyze the
attachment of phosphate groups (a process called phosphorylation)
to still other (substrate) proteins, changing the activity of these latter
proteins as a result. Among the proteins that can be phosphorylated
by cAMP-activated protein kinases are various ion-channel proteins.
Phosphorylated ion channels may open or close with different
kinetics than their nonphosphorylated forms, thus bringing about
changes in the membrane potential and thereby moving the cell to-
ward greater or lesser excitability. Also among the proteins that can
be phosphorylated by cAMP-activated protein kinases are various
transcription factors. These proteins interact with DNA in the cell nu-
cleus and influence the transcription of genes. Transcription factors
will promote or inhibit the transcription of specific genes, and their
ability to do this will be influenced by whether or not they have been
phosphorylated by activated protein kinases. Okay, enough for the
moment.