Binding of extracellular ligands and other molecules to specific cell proteins that then relay information of the occurrence of binding to other targets found within the same cell.
This signal reception usually occurs at the cell surface though in the case of steroid hormones that reception occurs instead within the interior of cells. In either case, it is binding of the chemical signal to specific receptor bindings that initiates specific signal transduction pathways.
Signal binding to receptors tends to be reversible and occurs with specific affinities. As a consequence there exists a dynamic equilibrium between between signal-molecule binding and signal-molecule release. The greater the signal concentration with a cell's local environment along with the greater the affinity of receptor proteins for the signal molecule, then the more receptors that will be bound to signal molecule at any given time.
The result of this dynamic is that subsequent signal transduction ramps up with increasing signal density, is ongoing so long as signal density remains at reasonably high levels, and then declines when signal densities decline. In general, that is, cells automatically return to a non-stimulated state in the absence of signal in their environment, but remain stimulated – and therefore transducing signal – so long as signal persists at adequate levels.
This effect of signal transduction is quite similar to that of allosteric regulation of protein function with the signalling molecule in effect acting as an activator of signal transduction. Indeed, to the extent that that receptor protein is also an enzyme that is active only upon signal reception, then signal reception is truly the equivalent of allosteric activation.
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