to translate information contained in hormonal messages, when these are
sensed by a receptor, into a language that can be interpreted and acted
upon by target cells.
proteins, peptides, and hormones with a significant ionic charge at neutral
pH, receptors are usually integral membrane proteins located at the cell
surface. When hormones bind to the receptors, the receptors interact
with membrane-bound or intracellular transducer proteins to begin the cascade
of events leading to cellular response.
membrane receptors, e.g., the acetyl-choline receptor, act as ion
channels that open or close in response to hormone binding and induce changes
via changes of the intracellular ion/charge balance.
many lipophilic hormones, e.g., steroids or thyronines, receptors
are intracellular, usually intranuclear, proteins. When their specific
ligands bind, the hormone-receptor complexes undergo conformational
changes that allow them to interact with specific hormone recognition sites
(HREs) in the DNA of the regulatory regions of certain genes.
transduction processes involve allosteric changes in receptor and/or
transducer protein shape. The signaling cascades of membrane-bound
receptors almost universally involve protein phosphorylation by
kinases. These kinases may be activated initially via generation
of secondary messengers produced by allosteric activation of enzymes like
adenylyl or guanylyl cyclase or via unmasking of kinase activities that
are part of the cytoplasmic portions of the receptor proteins themselves.
allosteric changes and/or phosphorylation, which often changes protein
charge, alter protein shape and/or intercellular location and protein function.
These are exactly the kind of changes that would be needed to trigger a
biochemical and cellular response. This may occur without the intervention
of protein synthesis and therefore may be very rapid, milliseconds to minutes.
or closing of ion channels will also precipitate rapid responses either
directly or via the intervention of phosphorylation cascades with their
associated changes in protein functions.
receptor transduction also involves allosteric and phosphorylation changes.
It normally triggers changes in gene transcription and subsequent protein
production and frequently modulates changes during a longer time course
of minutes to days.