G-Protein Coupled Receptors

G-PROTEIN COUPLED RECEPTORS

INTRODUCTION:

• Large family of cell membrane receptors linked to effector.
• To enzyme/channel/carrier protein).
• Large heterotrimeric GTP-binding proteins (G proteins).
• All have seven transmembrane segments.
• Loops in & out of cell membrane.
• (I.e.7 α-helical) membrane spanning.
• Hormone binding site – Extracellular domain.
• G-protein binding site – Intracellular domain.

• G-protein has three sub-units α, β & γ.
• In inactive state, subunits form complex binding GDP on α-subunit.

On extracellular ligand (hormone) binding → Conformational change occur in receptor → G-Protein activated.

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GDP exchanged by GTP on α-subunit.

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GDP displacement dissociates α-subunit from other two subunits.

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Activated α-subunit carrying GTP induces intracellular signals.

Signal termination:

• On hormone removal → Signaling event terminated.
• α-subunit inactivates itself by converting bound GTP to GDP.
• Since α-subunit has GTPase activity.
• α-subunit once again combines with β & γ subunits.
• Forms inactive membrane-bound trimeric G protein.

TYPES OF Gα – SUBUNITS:

• Action of active G protein – Either “Effector activation/inactivation”.
• Because of different α-subunits (Gα subunits),
• Gαs (Gs alpha) → Stimulates adenylyl cyclase & ↑es cAMP.
• Gαi (Gi alpha) → Inhibits adenylyl ryclase & ↓es cAMP.
• Gαq(Gq alpha) → Activates phospholipase C (PLC) generating second messangers IP3 & DAG.
• Gαt (Gt alpha) → “t” – “Transducin” – Responsible for signal generation in retinal rods.

MECHANISM OF ACTION OF G-PROTEIN:

G protein activation acts in 2 ways – 

• Stimulatory G protein – Stimulates; 
• Inhibitory G protein – Inhibits.

1. Adenylyl cyclase (AC):

AC Activation –

• Increases synthesis & intracellular accumulation of cAMP.
• Due to stimulatory G protein action.
• cAMP acts through cAMP-dependent Protein Kinase ‘A’.
• In turn, phosphorylate & alters functions of many enzymes, ion channels & structural proteins.
• Note: Inhibitory G protein inhibits AC & has opposite effects. 

Examples:

• Corticotropin-releasing hormone (CRH), FSH, LH, TSH.
• ACTH (corticotropin).
• ADH (V2 receptors).
• Parathormone.
• Catecholamines (α2, β2) – Adrenaline (Most actions).
• Glucagon.
• hCG.
• Calcitonin.
• Somatostatin.
• Acetylcholine.
• Dopamine (D1 & D2).
• Angiotensin II (epithelial cells).
• GABAB.
• Histamine (H2).

2. PHOSPHOLIPASE IP3 – DAG SYSTEM:

• Phospholipase activation – By stimulatory G protein.
• Hydrolyzes membrane phospholipid phosphatidylinositol 4, 5 bisphosphates (PIP2).
• Generating second messenger “Inositol I, 4, 5 triphosphate” (IP3) & “Diacylglycerol” (DAG).
• IP3 mobilizes Ca2+ from intracellular organelles → Increased cytosolic Ca2+.
• DAG enhances Protein Kinase ‘C’ activation by Ca2+.
• Ca2+ acts as “Third messenger” for this transduction mechanism.
• Also mediates physiological effects of drugs.
• Protein kinase-C phosphorylates various intracellular proteins.
• Eg: Threonine, serine or tyrosine residue).
• Causing their activation/inactivation.

Example:

• Growth Hormone-Releasing Hormone (GHRH).
• Thyrotropin-Releasing Hormone (TRH).
• GnRH.
• ADH/Vasopressin (V1 receptor – vasopressor action).
• Oxytocin.
• Cholecystokinin.
• PDGF.
• Gastrin.
• Catecholamines (some actions via α1 receptors).
• Angiotensin II (vascular smooth muscle).
• Substance P.
• Histamine – H1.
• Muscarinic (M1 & M3).

3. CHANNEL REGULATION:

• Activated G-proteins can open/close ion channels – Ca2+, K+ or Na2+.

Examples:

• Increased Ca2+-β1 adrenergic.
• Decreased Ca2+-Dopamine D2. 
• Increased K+-Adrenergic – α2.
• Muscarinic M2.
• Dopamine D2.
• GABAB.

• G-protein has seven transmembrane segments.
• (I.e.7 α-helical) membrane spanning.
• Hormone binding site – Extracellular domain.
• G protein activation results in exchange of GDP by GTP on α-subunit.
• α-subunit dissociates from other two subunits.
• α-subunit inactivates itself by converting its bound GTP to GDP, with help of GTPase activity.
• Active G protein may either be effector activator/inactivator, because of different α-subunits (Gα – subunits).
• Gαs (Gs alpha) → Stimulates adenylyl cyclase & ↑es cAMP.
• Activation of AC results in increased synthesis & intracellular accumulation of cAMP.
• cAMP acts through “cAMP-dependent Protein Kinase’A’ phosphorylates.

Examples: 

• Corticotropin-releasing hormone (CRH).
• FSH.
• LH.
• ACTH (corticotropin).
• Catecholamines (α2, β2).
• Glucagon.
• Dopamine (D1 & D2).
• Histamine (H2).
• Phospholipase activation by stimulatory G protein hydrolyzes “Membrane Phospholipid Phosphatidylinositol 4, 5-Bisphosphates (PIP2), in turn, generating second messenger “Inositol I, 4, 5 triphosphate” (IP3) & “Diacylglycerol” (DAG).
• Ca2+ acts as “Third messenger”.

Protein kinase-C phosphorylates various intracellular proteins.

• Examples of IP-DAG system:
• Cholecystokinin.
• Catecholamines (some actions via α1 receptors).
• Histamine – H1.