G proteins act as molecular switches inside cells that transmit signals from stimuli outside the cell to its interior. They were discovered when researchers found that adrenaline receptors stimulate G proteins, which then stimulate enzymes inside the cell rather than the receptors stimulating enzymes directly. There are two classes of G proteins: monomeric small GTPases and heteromeric G protein complexes composed of α, β, and γ subunits. The G protein subclasses Gαs, Gαq, Gαi, and Gαt each activate or inhibit different intracellular signaling pathways.

1. G Proteins
Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET
Assistant Professor
Department of Microbiology
Sacred Heart College (Autonomous)
Tirupattur, Vellore District.
Mobile: 9994146964
E.mail: microsaranraj@gmail.com

2. G Proteins
G proteins, also known as Guanine nucleotide-binding
proteins.
They are a family of proteins that act as molecular
switches inside cells, and are involved in transmitting signals
from a variety of stimuli outside a cell to its interior.
G proteins were discovered when Alfred G. Gilman and Martin
Rodbell investigated stimulation of cells by Adrenaline. They
found that when adrenaline binds to a receptor, the receptor does
not stimulate enzymes (inside the cell) directly. Instead, the
receptor stimulates a G protein, which then stimulates an enzyme.
Their activity is regulated by factors that control their ability to bind
to and hydrolyze GTP to GDP.
G proteins belong to the larger group of enzymes called GTPase.

3. There are two classes of G proteins. The first function as
Mononetic small GTPase, while the second form and
function as Heteromeric G Protein complexes (made up of
α, β and γ sub-units).
Gαs
 Stimulates Adenyl cyclase.
 It is associated with the receptors of many hormones (i)
Adrenaline, (ii) Glucagon, (iii) Lutenizing hormone (LH) and
(iv) Parathyroid hormone.
Gαq
 Activates Phospholipase C during IP3 and DAG Pathway.
 It is found in G Protein coupled receptor for (i) Vasopressin,
(ii) Thyroid stimulating hormone and (iii) Angiotensin.

4. Gαi
Inhibits the Adenyl cyclase and lowers the level of
cAMP in cell.
It is activated by the receptor for Somatostatin.
Gαt
The "t" is for transducin, the molecule responsible for
generating a signal.
Gαt triggers the breakdown of cyclic GMP (cGMP).
Gβγ
This complexes sometimes also have active functions.
Examples include coupling to and activating G protein-coupled
and rectify the problems in potassium channels.