Question

How do membrane receptors work?

Answer 1

Specialized protein molecules bound to or incorporated into the cell membrane are membrane receptors. The receptors promote contact between the cell and the extracellular environment through association with particular ligands (e.g., hormones and neurotransmitters).

Complete answer:
Membrane receptors are found in plasma cell membranes (and a few intracellular membranes also). These receptors mediate signal transduction for cellular responses to extracellular stimuli. Membrane receptors are usually transmembrane proteins. Both sides of the membrane are structurally ready to transfer information from one part of the membrane to the other by means of transmembrane proteins with part of its mass. The receptor domain exposed to the external medium often has a ligand binding site.
Step 1: Initial conformation of the $G$ proteins and membrane receptors ($Gs$ and Gi). The $G$ protein complexes, with $GDP$ bound to the alpha subunit, remain in their heterotrimeric form (association of alpha, beta, and γ subunits).
Step 2: Hormone-binding changes the conformation of receptors, revealing the site of G protein binding. The hormone receptor complex is connected to the $G$ protein complex, thus activating it for $GTP$ to $GDP$ displacement.
Step 3: The exchange of $GTP$-$GDP$ helps the alpha subunit to dissociate from the $G$ protein complex, revealing a binding site for adenylate cyclase on the alpha subunit. In order to convert $ATP$ to $cAMP$, binding of the $Gs$ alpha subunit activates adenylate cyclase and Gi alpha binding inhibits adenylate cyclase.
Step 4: The alpha subunit's hydrolysis of $GTP$ allows alpha to dissociate from adenylate cyclase and to reassociate with the $\beta \gamma$ complex, leaving inactive adenylate cyclase. The receptor will continue to activate $G$ proteins as long as the hormone remains bound to the receptor.

Note:
Most of the signalling in multicellular species includes cell-surface receptors. Three general types of cell-surface receptors exist: receptors linked to the ion channel, receptors linked to the G-protein and receptors linked to the enzyme.