Transmembrane receptors are proteins that span the thickness of the plasma membrane of the cell, with one end of the receptor outside (extracellular domain) and one inside (intracellular domain) the cell. When the extracellular domain recognizes the hormone, the whole receptor undergoes a structural shift that affects the intracellular domain, leading to further action. In this case the hormone itself does not pass through the plasma membrane into the cell.
The binding of the hormone by the hormone receptor uses the same non-covalent mechanisms, such as hydrogen bonds, electrostatic forces, hydrophobic and Van der Waals forces. The binding affinity of a hormone to its cognate hormone receptor is expressed as a Kd:
[R] is concentration of receptor, [H] is concentration of free hormone, [HR] is concentration of receptor-bound hormone.
The important value for the strength of the signal relayed by the receptor is the concentration of the hormone-receptor complex, which is defined by the affinity of the hormone for the receptor, the concentration of the hormone and, of course, the concentration of the receptor. The concentration of the circulating hormone is the key value for the strength of the signal, since the other two values are constant. For fast reaction, the hormone-producing cells can store prehormones, and quickly modify and release them if necessary. Also, the recipient cell can modify the sensitivity of the receptor, for example by phosphorylation; also, the variation of the number of receptors can vary the total signal strength in the recipient cell.
Signal transduction across the plasma membrane is possible only by many components working together. First, the receptor has to recognize the hormone with the extracellular domain, then activate other proteins within the cytosol with its cytoplasmic domain, which the protein does through a shift in conformation. The activated effector proteins usually stay close to the membrane, or are anchored within the membrane by lipid anchors, a posttranslational modification. Many membrane-associated proteins can be activated in turn, or come together to form a multi-protein complex that finally sends a signal via a soluble molecule into the cell.
A ligand-activated ion channel will recognize its ligand, and then undergo a structural change that opens a gap (channel) in the plasma membrane through which ions can pass. These ions will then relay the signal. An example for this mechanism is found in the receiving cell of a synapse.
An ion channel can also open when the receptor is activated by a change in cell potential, that is, the difference of the electrical charge on both sides of the membrane. If such a change occurs, the ion channel of the receptor will open and let ions pass through. In neurons, this mechanism underlies the action potential impulses that travel along nerves.
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