G Protein-Coupled Receptors (GPCRs) - In-Depth Notes

Intended Learning Outcomes

• Examine G-Protein Coupled Receptor (GPCR) structure and correlate this to receptor function.
• Describe how GPCRs activate cellular signaling pathways.
• Identify different G-protein subtypes and relate these subtypes to cellular signaling.

Introduction to GPCRs

• GPCRs are the largest family of related proteins, with over 900 GPCR genes predicted in humans ("GPCR-ome").
• These receptors have conserved structures across eukaryotes (from yeast to humans) and can bind a variety of stimuli.
  • Examples of Stimuli: Hormones, Ions, Light, Odorants, Proteases.
  • Functions: Neurotransmission, Cell Growth, Vision, Olfaction.
• More than 30% of prescribed drugs target GPCRs, highlighting their importance in pharmacology.
  • Notable Drugs: Salbutamol (asthma), Morphine (analgesic), Losartan (hypertension).

GPCR Structure

• The common core domain includes seven membrane-spanning $ ext{α}$-helices with an extracellular N-terminus and intracellular C-terminus.
• The helices are connected by 3 intracellular and 3 extracellular loops.

Classification of GPCRs

• GPCRs can be subdivided into six families:
  • Class A: Rhodopsin-like.
  • Class B: Secretin-like.
  • Class C: Metabotropic glutamate.
  • Class D: Pheromone receptors.
  • Class E: cAMP GPCR.
  • Class F: Frizzled GPCR.
• Each family can be distinguished by the mechanism of receptor activation.

Class A GPCRs

• Involved in phototransduction (e.g., rhodopsin).
• Bind small molecules like Epinephrine, Acetylcholine, and dopamine.
• Structural Features:
  • Extracellular N-Terminal with ligand binding to transmembrane helices form a non-polar cavity for specific ligand interactions.
  • Critical disulfide linkages stabilize the structure, and certain residues (Asp in TM2) are key for G-protein activation.

Class B and Class C GPCRs

• Class B: Activated by short peptide agonists (e.g., secretin, glucagon).
• Class C: Contains a large extracellular domain, often forms dimers, and is sensitive to glycoprotein hormones.

GPCR Signaling Mechanism

1. G-Protein Activation: GPCRs activate heterotrimeric G-proteins (composed of $ ext{α}$, $ ext{β}$, and $ ext{γ}$ subunits) by exchanging GDP for GTP.
2. Outputs of Activation: G-proteins can activate various effectors, leading to diverse cellular responses such as the generation of second messengers (cAMP, IP3, DAG).
   • Example: Thrombin activating various G-Protein pathways.

Key G-Protein Subtypes

• Over 20 G-protein subtypes exist.
• Important families include Gs, Gi, Gq, Go, G12/13.
• Each subtype selectively couples with specific receptors and effectors.

Signaling Pathways Associated with GPCRs

1. cAMP Pathway:
   • Adenylyl cyclase converts ATP to cAMP, which then activates Protein Kinase A (PKA).
   • Key regulatory molecules: phosphodiesterases (PDEs) that convert cAMP back to AMP.
2. Inositol Phosphate/DAG Pathway:
   • GPCR activation leads to the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) by phospholipase C, generating IP3 and DAG.
   • IP3 promotes calcium release from the endoplasmic reticulum (ER) while DAG activates Protein Kinase C (PKC).
3. Calcium Signaling:
   • Calcium serves as a crucial intracellular messenger that can activate various cellular pathways including contraction and enzyme activity.

Key Features of Calcium Signaling

• Calcium is rapidly mobilized, maintaining a steep gradient across the plasma membrane to allow for quick increases in intracellular $ ext{[Ca}^{2+}]$ concentrations.
• Calcium binds to proteins (e.g., calmodulin) facilitating significant conformational changes that regulate target proteins involved in various cellular responses.

Summary of GPCR Function

• GPCRs function by transducing extracellular signals into cellular responses through G-protein coupling and second messenger systems.
• The signaling pathways involve multiple molecules leading to amplification of the cellular effect, making GPCRs critical in pharmacological targets for many drugs.