Alpha amidating enzyme
Receptor structure is varied: some receptors consist of a single polypeptide chain with a domain on either side of the membrane, connected by a membrane-spanning domain.
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.
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.
Activated PLCβ hydrolyzes membrane phospholipids (as described below) resulting in increased levels of IP and DAG.
Downstream signaling proteins are phosphorylated on serine and threonine by PKA and DAG-activated protein kinase C (PKC) leading to alterations in their activities.
Plasma carrier proteins exist for all classes of endocrine hormones.
Carrier proteins for peptide hormones prevent hormone destruction by plasma proteases.
GPCRs also couple to G-protein activation of phospholipase C-β (PLCβ).
In a separate but related system, exocrine tissues secrete their products into ducts and then to the outside of the body or to the intestinal tract.
Classically, endocrine hormones are considered to be derived from amino acids, peptides, or sterols and to act at sites distant from their tissue of origin.
Representative pathway for the activation of c AMP-dependent protein kinase, PKA.
In this example glucagon binds to its' cell-surface receptor, thereby activating the receptor.
The resultant complex of steroid and receptor bind to response elements of nuclear DNA, regulating the production of m RNA for specific proteins.