Second Messengers in Cell Signaling

     Second messengers (SMs) are intracellular substances that mediate cell activity by relaying signals from an extracellular molecule bound to a cell's surface.  A second messenger can be a neurotransmitter or a hormone.  The second messenger travels from receptor to effector.  (National Center for Biotechnology Information)

     SMs are abundant. SMs are present in low concentrations in a cell at rest.  Once the resting cell is stimulated, SMs are rapidly produced.  SMs are released as fast as they're made. Microseconds.  SMs specifically bind to target proteins.  Cellular signaling spreads downstream.

     SMs four major classes: 

1. Cyclic nucleotides that signal within the cytosol.  cAMP/PKA. cGMP/PKG.  PDE regulated.
2. Lipid messengers that signal within cell membranes.  IP3, and DAG.  PIP3, and AKt signaling.  Sphingolipid controlled apoptosis. Lysolipids, prostaglandins, and other eicosanoids.
3. Ions that signal within and between cells.   Intracellular calcium levels are controlled by an assortment of channels, pumps, transporters, buffers, and effector moieties.  Magnesium.
4. Gasesous free radicals that signal throughout the cell and neighboring cells.

     SMs bind to specific proteins, alter the proteins,  and relay signals.  These are homeostatically controlled mechanisms. (Brittanica)

     Enzymes catalytic activity is modified by the direct binding of the SMs. Amplification takes place simultaneously with the activation of multiple target enzymes by a single second messenger molecule.

     Let's glance at caffeine. (IUPAC)

     Caffeine enhances the action of cAMP by inhibiting the enzyme phosphodiesterase.  Phosphodiesterase functions to degrade cAMP.  The enhancement of cAMP activity contributes to the action of caffeine.  Prolonged excitement.

     Second messengers (SMs) are centralized in the ability to respond rapidly to information.