G Protein-Coupled Receptors (GPCRs)

Concept and Structure

G Protein-Coupled Receptors (GPCRs) are a large family of membrane proteins that play a crucial role in cellular signal transduction. They respond to a variety of external signals and activate intracellular pathways via G proteins.

• Definition: GPCRs are integral membrane proteins that associate with G proteins to transmit signals from extracellular ligands to intracellular effectors.

• Structure: GPCRs consist of 7 transmembrane α-helices spanning the cell membrane.

• Key Features:

  • GPCRs have an extracellular N-terminus and an intracellular C-terminus.

  • They interact with heterotrimeric G proteins composed of α, β, and γ subunits.

• Guanosine Tri-Phosphate (GTP): GTP binding and hydrolysis are central to G protein activation and function.

Example: The β-adrenergic receptor is a classic GPCR that responds to adrenaline and initiates a signaling cascade in target cells.

Essential Components of GPCR Signal Transduction

GPCR signal transduction involves several key steps and molecular components that ensure the transmission of signals from the cell surface to the interior.

• Ligand Binding: The receptor undergoes a conformational change upon ligand binding, which activates the associated G protein.

• G Protein Activation: The activated GPCR acts as a guanine nucleotide exchange factor (GEF), promoting the exchange of GDP for GTP on the Gα subunit.

• Effector Interaction: The GTP-bound Gα subunit dissociates from the βγ subunits and interacts with downstream effectors, such as enzymes or ion channels.

• Signal Termination: GTP hydrolysis by the Gα subunit returns the G protein to its inactive state.

Example: Activation of adenylyl cyclase by Gαs leads to increased production of cyclic AMP (cAMP), a secondary messenger.

Types of G Proteins and Their Functions

Stimulatory vs. Inhibitory G Proteins (Gs vs. Gi)

G proteins are classified based on their effect on effector enzymes and secondary messenger production.

• Stimulatory G Proteins (Gs): Activate effector enzymes, increasing the levels of secondary messengers (e.g., cAMP).

• Inhibitory G Proteins (Gi): Inhibit effector enzymes, decreasing the levels of secondary messengers.

Example: Gs stimulates adenylyl cyclase to produce cAMP, while Gi inhibits adenylyl cyclase, reducing cAMP levels.

Practice and Application

• GPCRs are composed of 7 transmembrane α-helices.

• G proteins are classified by their effect on secondary messenger production and effector enzyme activity.

• G proteins are involved in various cellular processes, including signal amplification, transport, and regulation of metabolic pathways.

Example: In bacterial infection, the release of toxins can affect G protein signaling, leading to altered cellular responses.

Summary Table: Types and Functions of G Proteins

Key Equations

• Activation of G protein by GPCR:

• cAMP production:

Additional info: GPCRs are involved in numerous physiological processes, including vision, taste, smell, and neurotransmission. Their dysfunction is linked to various diseases, making them important drug targets.