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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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