Metabotropic Receptors

Structure and Activation Cycle

The protein chain of metabotropic receptors traverses the cell membrane seven times. On the intracellular side, the receptor contacts a G-protein—a complex consisting of three subunits ($\alpha$, $\beta$, and $\gamma$). These proteins derive their name from their ability to bind guanine nucleotides.

Signal transduction follows a multi-step cycle:

1. Resting State: The receptor is unoccupied and no ligand is bound. The key marker of the inactive state is that the $\alpha$-subunit of the G-protein is non-covalently bound to a molecule of GDP (guanosine diphosphate).
2. Activation: An agonist binds to the receptor, inducing a conformational change. On the $\alpha$-subunit, GDP is exchanged for GTP (guanosine triphosphate).
3. Signal Transduction: The active complex consisting of the "$\alpha$-subunit + GTP" dissociates from the receptor and from the $\beta\gamma$-subunits, after which it interacts with effector proteins (enzymes or ion channels).
4. Termination: Due to its intrinsic GTPase activity, the $\alpha$-subunit hydrolyzes GTP back to GDP. The system returns to its initial resting state.

Adenylyl Cyclase Pathway (Gs and Gi Proteins)

The most thoroughly studied signal transduction mechanism involves the membrane-bound enzyme adenylyl cyclase. The active G-protein causes this enzyme to hydrolyze ATP, producing the secondary messenger cAMP (cyclic adenosine monophosphate). This messenger activates specific cAMP-dependent protein kinases, which phosphorylate cellular proteins and alter their functions.

The ultimate physiological effect depends on the type of G-protein:

• Gs-proteins stimulate adenylyl cyclase activity. An example is the sympathetic activation of myocardial $\beta_1$-adrenergic receptors. cAMP levels rise, protein kinases phosphorylate calcium channels, and calcium influx into the cell increases. Result: enhanced sinus node automaticity and positive chronotropy (increased heart rate).
• Gi-proteins inhibit adenylyl cyclase. Parasympathetic effects on the heart are mediated via $\text{M}_2$ muscarinic receptors acting through this pathway. cAMP levels drop, leading to a decreased heart rate (negative chronotropy).

Inositol Trisphosphate Pathway (Gq Proteins)

Another major signaling scenario operates via Gq-proteins, which activate the enzyme phospholipase C. This pathway is utilized, for instance, during the stimulation of $\alpha_1$-adrenergic receptors in vascular smooth muscle.

Biochemical Cascade:

1. Phospholipase C hydrolyzes membrane-bound phosphatidylinositol 4,5-bisphosphate ($\text{PIP}_2$).
2. The hydrophilic secondary messenger inositol 1,4,5-trisphosphate ($\text{IP}_3$) is synthesized.
3. $\text{IP}_3$ opens calcium channels on the sarcoplasmic reticulum.
4. $\text{Ca}^{2+}$ ions flood into the cytoplasm and form a complex with the protein calmodulin.
5. The "$\text{Ca}^{2+}$-calmodulin" complex activates myosin light-chain kinase.
6. Myosin is phosphorylated, facilitating its interaction with actin and resulting in smooth muscle contraction (vasoconstriction).

The Phenomenon of Amplification

A hallmark property of G-protein-coupled receptors is the massive amplification of the primary signal. A single activated receptor can engage multiple G-proteins, each of which drives enzymes to synthesize hundreds of secondary messenger molecules.

GPCRs represent the largest class of membrane receptors in the human body. This family includes dopamine, histamine, opioid, and certain serotonin receptors, as well as receptors for the majority of peptide hormones.