A principle of signal transduction is the signal amplification. The binding
of one or a few neurotransmitter molecules can enable the entry of millions
of ions. The binding of one or just a few hormone molecules can induce an enzymatic
reaction that affect many substrates. The amplification can occur at several
points of the signal pathway.
A receptor that has been activated by a hormone can activate many downstream
effector proteins. For example, a rhodopsin molecule in the plasma membrane
of a retina cell in the eye that was activated by a photon can activate up to
2000 effector molecules (in this case, transducin) per second. The total strength
of signal amplification by a receptor is determined by:
- The lifetime of the hormone-receptor-complex. The more stable the hormone-receptor-complex
is, the less likely the hormone dissociates from the receptor, the longer the
receptor will remain active, thus activate more effector proteins.
- The amount and lifetime of the receptor-effector protein-complex. The more
effector protein is available to be activated by the receptor, and the faster
the activated effector protein can dissociate from the receptor, the more effector
protein will be activates in the same amount of time.
- Deactivation of the activated receptor. A receptor that is engaged in a hormone-receptor-complex
can be deactivated, either by covalent modification (for example, phosphorylation),
or by internalization.
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