G protein


G proteins, also known as guanine nucleotide-binding proteins.


G proteins belong to the larger group of enzymes called GTPases.


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 protein activity is regulated by factors that control their ability to bind to and hydrolyze guanosine triphosphate (GTP) to guanosine diphosphate (GDP). 


G proteins bound to GTP, become activated.


When G proteins are bound to GDP, they are inactive.


G proteins located within the cell are activated by G protein-coupled outside receptors (GPCRs) that are bound to signaling molecules that span the cell membrane.


An intracellular GPCR domain then in turn activates a particular G protein. 


G protein-coupled receptor and G proteins working together transmit signals from many hormones, neurotransmitters, and other signaling factors.


G proteins regulate metabolic enzymes, ion channels, transporter proteins, and control cell  transcription, motility, contractility, and secretion, which in turn regulate diverse systemic functions such as embryonic development, learning and memory, and homeostasis.


G proteins are important signal transducing molecules in cells. 


Malfunction of GPCR [G Protein-Coupled Receptor] signaling pathways are involved in many diseases, such as diabetes, blindness, allergies, depression, cardiovascular defects, and certain forms of cancer. 


About 30% of the modern drugs’ cellular targets are GPCRs.


The human genome encodes about 800 G protein-coupled receptors.


G protein-coupled receptors. detect photons of light, hormones, growth factors, drugs, and other endogenous ligands. 


Whereas G proteins are activated by G protein-coupled receptors, they are inactivated by  regulator of G protein signalling.


With ligand activation the G protein-coupled receptor induces a conformational change in the receptor that allows the receptor to function as a guanine nucleotide exchange factor that exchanges GDP for GTP, thus turning the GPCR  on.


The cAMP-dependent pathway is used as a signal transduction pathway for many hormones including:








CRH – Stimulates the synthesis and release of ACTH from the anterior pituitary.


ACTH – Stimulates the synthesis and release of cortisol in the zona fasciculata of the adrenal cortex in the adrenal glands.


TSH – Stimulates the synthesis and release of a majority of T4 in the thyroid gland.


LH – Stimulates follicular maturation and ovulation in women; or testosterone production and spermatogenesis in men.


FSH – Stimulates follicular development in women; or spermatogenesis in men


PTH – Increases blood calcium levels. 


Calcitonin – Decreases blood calcium levels (via the calcitonin receptor in the intestines, bones, kidneys, and brain)


Glucagon – Stimulates glycogen breakdown in the liver


hCG – Promotes cellular differentiation, and is potentially involved in apoptosis.


Epinephrine – released by the adrenal medulla  and stimulates glycogenolysis, in addition to the actions of glucagon.


ADH- Induces the synthesis and release of glucocorticoids from the Zona fasciculata of adrenal cortex, and Induces vasoconstriction.


TRH – Induces the synthesis and release of TSH from the anterior pituitary


TSH Induces the synthesis and release of a small amount of T4 (Thyroid Gland).


Angiotensin II – Induces Aldosterone synthesis and release at the zona glomerulosa of adrenal cortex


GnRH – Induces the synthesis and release of FSH and LH 








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