Small, non-protein molecules that are important propagators and amplifiers of signals during signal transduction.
Because second messengers are small, soluble (that is, hydrophilic), and relatively inexpensive to produce, they can be enzymatically created in large numbers and then diffuse rapidly to their targets. In this manner they act similarly to the flooding of cytoplasms with ions, such as one sees with ion channels as part of signal transduction pathways (including as ion channel receptors). Indeed, ions such as Ca++ can be included among second messengers.
Alternatively, hydrophobic second messengers exist that are able to diffuse within lipid bilayers. Lastly, bridging the two types, are gaseous second messengers which are able to diffuse rapidly within both cytosol and membranes.
Second messengers include cyclic AMP, which is hydrophilic second messenger, and nitric oxide, which is a gas.
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