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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

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▶️ Adenylyl Cyclase: cAMP Pathway