Lecture 3-13: G protein-coupled receptors

G Protein-Coupled Receptors (GPCRs)

GPCRs are seven membrane-spanning alpha helices.
They are synthesized as 7-pass integral membrane proteins.
Commonly referred to as "serpentine receptors" due to their winding structure.
Crystal structure example shows acetylcholine receptor with bound ligand (orange).

Ligand binding to GPCRs activates the associated G protein.
Function of GPCRs depends on association with trimeric G proteins, lipid-linked to the plasma membrane.
In absence of ligands, receptors have low affinity for G protein trimer (comprising alpha (α), beta (β), and gamma (γ) subunits).
Ligand binding leads to a conformational change in GPCR, allowing interaction with the α subunit of the G protein.
GPCR acts as a Guanine Exchange Factor (GEF), facilitating GDP release and GTP binding.
Activated α subunit separates from βγ complex, transmitting signals downstream.

GTP hydrolysis inactivates the α subunit, reassembling the G protein complex.
The intrinsic GTPase activity of the α subunit hydrolyzes GTP to GDP.
Regulator of G-Protein Signaling (RGS) proteins function as GTPase-Activating Proteins (GAPs).
Bound GTP serves as an activating signal, not powering G protein function.
In the GDP-bound state, the α subunit returns to the βγ complex, deactivating the G protein.

α subunits and βγ complexes have distinct functions.
When together, they mutually inhibit interactions with target proteins; upon separation, they can signal independently.
For example, the βγ complex can activate potassium channels, altering membrane potential in neuronal signaling (metabotropic signal).
The α subunit may inhibit adenylyl cyclase in specific pathways.

α subunits signal through enzymes, producing second messengers that amplify signals.
Common second messengers include cyclic AMP (cAMP), inositol triphosphate (IP3), diacylglycerol (DAG), and Ca++.
Enzyme activation can lead to rapid production of many second messenger molecules.

cAMP is derived from ATP, catalyzed by adenylyl cyclase.
Different α subunits (G_as activate; G_ai inhibit) regulate cAMP levels.
cAMP hydrolysis into AMP is catalyzed by phosphodiesterase, regulating signal duration.

cAMP activates PKA, which can phosphorylate various targets, leading to diverse cellular responses.
PKA phosphorylation can initiate cascades and alter cellular processes, such as glycogen breakdown.
PKA also phosphorylates nuclear targets affecting gene expression.

Different GPCRs modulate heart rate via adrenergic and muscarinic receptors affecting cAMP levels.
Vagal (parasympathetic) control releases acetylcholine while cardiac accelerator (sympathetic) releases norepinephrine.
cAMP influences calcium influx, enhancing cardiac contraction.

Cholera and pertussis toxins affect cAMP pathways by modifying α subunits, enhancing signaling and influencing cellular responses.
Cholera toxin locks G_as in an active state in intestinal cells, leading to water secretion and diarrhea.
Pertussis toxin prevents G_ai activation in immune cells, hypothesizing modulation of immune responses.

G_q activates PLC, which cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into IP3 and DAG.
IP3 stimulates calcium release from the endoplasmic reticulum while DAG activates PKC for further signaling.

Calcium binds to proteins such as calmodulin, altering structures and functions of target proteins.
Calmodulin-dependent protein kinase (CaMK) and transcription factors like NFAT are regulated by calcium.

G_q signaling in endothelial cells leads to nitric oxide synthesis, initiating smooth muscle relaxation.
cGMP generated from NO signaling hyperpolarizes cells by closing calcium channels.
Drugs like sildenafil prolong vasorelaxation by inhibiting cGMP breakdown.

Rhodopsin is a light-activated GPCR that depolarizes cells in darkness.
Activated by transducin, it influences cGMP levels and alters membrane potential during phototransduction.

GPCRs are fundamental for signaling through associated G proteins, with ligand binding triggering intracellular cascades.
α and βγ subunits provide versatility in signal transduction pathways affecting a wide range of physiological responses.
Second messengers such as cAMP and calcium are central to amplifying and propagating signals.
The varied functional impact of GPCR pathways reflects their significance in diverse biological processes.