In biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay a signal within a cell. 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 membrane-associated proteins. There are two basic types of second messenger molecules:
- Hydrophobic molecules like diacylglycerol 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.
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.