Cells have to transmit and receive different signals, which
can in turn coordinate cellular activities and physiological processes
occurring inside the body.
In fact, there are 4 ways of which activation of receptors
can control the activity of target cells. The first way is by the direct
control of ion channels. Ion channels may be opened and signals can be
transmitted by initiating a change in membrane potential or by the influx of
calcium ions. Nicotinic Ach receptors is an example. The second way of control
is by sending signals onto intracellular receptors which can moderate the
transcription of genes in our body. The oestrogen receptor uses this method. As
for the third method, the effector enzyme can be directly controlled upon its
activation. An example of this would be the Insulin receptor. The fourth way of
control is the activation of G-protein coupled receptors that alters the
intracellular concentration of second messenger, or regulates ion channels.
Muscurinic Ach receptors is an example.
In this essay we shall delve into the discussion of a large class
of receptor proteins inside mammalian bodies that use the fourth way of control
of target cells, which are the G-protein coupled receptors. The activation of
G-protein coupled receptors is directly associated with the control of second
messengers. As the G-protein coupled receptors are activated, signals will be
sent from a plasma membrane to a specific enzyme or intracellular receptors,
initiating a flow of signaling cascades vital to maintain cellular processes
taking place inside the body.
The G-protein coupled receptors are called heterotrimeric G
proteins because they possess 3 subunits, namely (?, ?, and ?). The 3 subunits
are sticking together as the ? subunit of the protein binds to GDP. As the G-
protein coupled receptor is being activated, GDP is being exchanged for GTP,
which causes the ? subunit of the G-protein to separate from the other ??
subunit complex. These allows the activity of enzymes and ion channels to be
are various of G-protein ?
subunits and they are designed to interact with different effectors. Below I
shall discuss the Gs, Gi and Gq ? subunits, as well as their signaling
The Gs Receptor
The binding of Gs receptors activates adenylyl cyclase,
hence generating cyclic AMP. The responses initiated by cyclic AMP are
different in different cells, largely dependent on the type of enzymes the cell
possesses. The signal produced can be amplified accordingly. An enzyme, called
phosphodiesterases can be used to stop the change of cyclic AMP into AMP. Hence
the concentration of cyclic AMP in cells is controlled by the balance between
phosphodiesterases and adenylyl cyclase. In fact, the use of Gs receptors are
be illustrated in action of adrenaline on our skeletal muscles. The chemical
signal adrenaline activates the receptor, ?-adrenoceptor. The G-protein
subsequently dissociates and activates the adenylyl cyclase. Then, as the
message is being generated, intracellular cyclic AMP increases in
concentration. An enzyme, protein kinase A, is being activated, leading to the
phosphorylation of glycogen phosphorylase. Glycogen stored is being broken down
into glucose. Glucose is being mobilized quickly and immediately upon the
amplification of the original signals.