Carriers for steroid and thyroid hormones allow these very hydrophobic substances to be present in the plasma at concentrations several hundred-fold greater than their solubility in water would permit.
Carriers for small, hydrophilic amino acid-derived hormones prevent their filtration through the renal glomerulus, greatly prolonging their circulating half-life.
The classic interactions between receptors, G-protein transducer, and membrane-localized adenylate cyclase are illustrated below using the pancreatic hormone glucagon as an example.
When G-proteins bind to receptors, GTP exchanges with GDP bound to the α subunit of the G-protein.
Some receptors are comprised of a single polypeptide chain that is passed back and forth in serpentine fashion across the membrane, giving multiple intracellular, transmembrane, and extracellular domains.The generation of c AMP occurs via activation of G-protein coupled receptors (GPCRs) whose associated G-proteins activated adenylate cyclase.For more information on GPCRs and G-proteins visit the Signal Transduction page.Activation of these receptors by hormones (the first messenger) leads to the intracellular production of a second messenger, such as c AMP, which is responsible for initiating the intracellular biological response.Steroid and thyroid hormones are hydrophobic and diffuse from their binding proteins in the plasma, across the plasma membrane to intracellularly localized receptors.