⚡ Core Concept: The GPCR Superfamily & Second Messenger Amplification
G Protein-Coupled Receptors (GPCRs / 7-Transmembrane Heptahelical Receptors) represent the largest receptor family in the human genome (>800 genes). Ligand binding promotes guanine nucleotide exchange on heterotrimeric G-protein α subunits ($Glphaetagamma$), producing large-scale amplification via diffusible second messengers (cAMP, $IP_3$, DAG, $Ca^{2+}$).
1. The Two Canonical Heterotrimeric G-Protein Cascades
G-protein α-subunits diverge into functional classes that stimulate or inhibit distinct enzymatic effectors:
Figure 3: Dual Second Messenger Branches: The Adenylyl Cyclase vs. Phospholipase C Pathways
GPCR Biophysics
Figure 3: Left: Gαs/Gαi stimulation/inhibition of adenylyl cyclase, cAMP synthesis, and PKA activation. Right: Gαq activation of PLC-β, cleavage of PIP2 into IP3 (releasing ER Ca2+) and DAG (activating PKC).
2. Heterotrimeric G-Protein Subunit Classification
| Subunit Class | Primary Effector Enzyme | Second Messenger Generated | Downstream Protein Kinase / Target |
|---|---|---|---|
| $Glpha_s$ | Adenylyl Cyclase (Stimulates) | Elevates $ ext{cAMP}$ | Protein Kinase A (PKA); EPAC; cyclic nucleotide-gated channels. |
| $Glpha_i / Glpha_o$ | Adenylyl Cyclase (Inhibits); $K^+$ channels | Decreases $ ext{cAMP}$; opens $GIRK$ | Lowers PKA activity; hyperpolarizes cardiac pacemaker cells ($M_2$ AChR). |
| $Glpha_q / Glpha_{11}$ | Phospholipase C-$eta$ (PLC-$eta$) | Cleaves $ ext{PIP}_2 ightarrow ext{IP}_3 + ext{DAG}$ | $IP_3$ releases $Ca^{2+}$ from ER; $Ca^{2+} + ext{DAG}$ activate Protein Kinase C (PKC). |
| $Glpha_{12} / Glpha_{13}$ | RhoGEF | Activates small GTPase RhoA | Rho-associated kinase (ROCK); induces actin stress fiber contraction. |
3. Bacterial Toxins Targeting Heterotrimeric G-Proteins
- Cholera Toxin (Vibrio cholerae): Catalyzes irreversible ADP-ribosylation of an arginine residue on $Glpha_s$, inhibiting its intrinsic GTPase activity. $Glpha_s$ remains permanently locked in the GTP-bound active state → unregulated adenylyl cyclase activity → massive cAMP spike in intestinal crypt enterocytes → continuous opening of CFTR chloride channels → catastrophic secretory diarrhea.
- Pertussis Toxin (Bordetella pertussis): Catalyzes ADP-ribosylation of a cysteine residue on $Glpha_i$, preventing its interaction with activated GPCRs. $Glpha_i$ remains trapped in the inactive GDP-bound state, unable to inhibit adenylyl cyclase → cellular cAMP accumulation.
GPCR Desensitization via GRKs & β-Arrestin
Persistent agonist exposure triggers G Protein-Coupled Receptor Kinases (GRK2/3) to phosphorylate serine/threonine residues in the GPCR C-tail. Phosphorylated receptors recruit β-arrestin, which sterically blocks further G-protein coupling (homologous desensitization) and engages AP-2/clathrin for receptor internalization.
