Final answer:
G-protein-linked receptors are involved in multiple cellular signaling pathways and each receptor has a distinct structure with seven transmembrane domains. They are activated by ligand binding, which leads to GTP binding by the G-protein and activation of downstream proteins, such as enzymes or ion channels. Disruption of G-protein signaling can lead to diseases like cholera due to bacterial toxins affecting these receptors.
Step-by-step explanation:
G-protein-linked receptors are crucial components in cell signaling pathways. They operate via a specific mechanism: when a ligand binds to the receptor, this action induces a conformational change, which then activates the associated G-protein. The G-protein, in turn, can interact with various membrane proteins such as ion channels or enzymes, initiating a cascade of cellular events.
All G-protein-linked receptors share a common structure of seven transmembrane domains, but each has a unique extracellular domain as well as a specialized G-protein-binding site. Activation of the G-protein involves the exchange of GDP for GTP on the alpha subunit, leading to the separation of the alpha subunit from the beta and gamma subunits. These subunits can then go on to activate or inhibit other cellular proteins, such as adenylate cyclase, which when activated, can lead to the production of the secondary messenger cAMP and the subsequent activation of Protein Kinase A (PKA).
In the context of health and disease, the role of G-protein-linked receptors is underscored by their involvement in conditions caused by toxins from pathogenic bacteria; these toxins can interfere with G-protein functions, causing serious health issues such as cholera, where a toxin causes continuous activation of a chloride channel leading to severe dehydration.