G Protein Coupled Receptors

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G Protein Coupled Receptors

G Proteins-Coupled Receptors

The largest family of cell surface receptors.
Have 7 transmembrane segments ⟶ called 7-pass receptors.
Zigzag arrangement in like a snake ⟶ called serpentine receptors.
On extracellular side: Bind with a ligand.
On intracellular side: Associate with G protein.

G Proteins

Bind with GTP/GDP ⟶ Called GTP binding proteins or G protein.
Have GTPase activity.
Switch between two states:
GDP bound state: Inactive
GTP bound state: Active
Broad types:
Heterotrimeric G proteins.
Monomeric G proteins.

Heterotrimeric G Proteins

Composed of 3 subunits
α
β
γ
α subunit has GTPase activity.
Entire complex is anchored to the membrane with α and γ subunits.
Types:
Gs
Gi
Gq
Go
Different types have different sets of α, β, and γ subunits.
Mediates response to a wide range of signaling molecules including hormones, neurotransmitters, odorants, tastants etc.

Monomeric or Small G Proteins

Composed of a single subunit.
The subunit is similar to α subunit that is seen in heterotrimeric G proteins ⟶ has GTPase activity.
Families:
Ras
Rho
Rab
Arf
Ran
Regulate gene expression, dell proliferation, differentiation and survival

Cyclic Journey of Heterotrimeric G Protein

During signaling, subunits of G protein go through a cyclic path.
As they go through the cycle, they affect different downstream second messengers.

Resting Condition

Trimer of α, β and γ subunit is associated with the receptor.
α subunit is bound to GDP

Activation and Signaling

Ligand binds with the receptor

↓

Conformational change

↓

Release of GDP and binding of GTP to α subunit

↓

Dissociation of the complex from receptor & separation of subunits

↓

Formation of free α subunit and βγ complex

↓

Interact with different downstream molecules e.g. α subunit interacts with adenylyl cyclase and βγ complex interacts with ion channels

↓

The signal is carried further downstream

↓

Response

Inactivation

GTPase activity of α subunit

↓

Hydrolyses GTP into GDP and phosphate

↓

Inactive α subunit

↓

Dissociates from the downstream messenger

↓

Re-associates with βγ complex

↓

Termination of the signal