Ligand Binding and Signal Transduction
Upon binding of LH to the external part of the membrane spanning receptor, a transduction of the signal takes place that activates the G protein that is bound to the receptor internally. With LH attached, the receptor shifts conformation and thus mechanically activates the G protein, which detaches from the receptor and activates the cAMP system.
It is believed that a receptor molecule exists in a conformational equilibrium between active and inactive states. The binding of LH (or CG) to the receptor shifts the equilibrium between active and inactive receptors. LH and LH-agonists shift the equilibrium in favor of active states; LH antagonists shift the equilibrium in favor of inactive states. For a cell to respond to LH only a small percentage (~1%) of receptor sites need to be activated.
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By night thy silent signal lamps to swing.”
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