Second messenger

Intracellular signal transduction is largely carried out by second messenger molecules, usually low-weight and diffusible. They are synthesized or released by specific enzymatic reactions, usually as a result of an external signal that was received by a transmembrane receptor and pre-processed by other membrane-associated proteins. There are three basic types of second messenger molecules:
- Hydrophobic molecules like diacylglycerol, InsP3 and phosphatidylinositols are membrane-associated and diffuse from the plasma membrane into the juxtamembrane space where they can reach and regulate membrane-associated effector proteins.
- Hydrophilic molecules are water-soluble molecules, like cAMP, cGMP, and Ca2+, that are located within the cytosol.
- gases, nitric oxide (NO) and carbon monoxide (CO), that can diffuse both through cytosol and across cellular membranes.

These intracellular messengers have some properties in common:
- They can be synthesized/released and broken down again in specific reactions by enzymes.
- Some (like Ca2+) can be stored in special organelles and quickly released when needed.
- Their production/release and destruction can be localized, enabling the cell to limit space and time of signal activity.


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