Discussion
Which one of the following is NOT a second messenger in mammalian cells?
Answer: D
The one that is not a second messenger is the potassium ion.
A second messenger is a small intracellular signal whose concentration changes sharply when a hormone or neurotransmitter binds a cell-surface receptor, and which then relays the signal inside the cell. Potassium is the main intracellular cation and sets the resting membrane potential, but its level is not used to carry receptor signals in this way.
The other three are classic second messengers. Cyclic AMP is made by adenylyl cyclase and activates protein kinase A. Diacylglycerol is released from PIP2 by phospholipase C and activates protein kinase C. Calcium ions, released from the endoplasmic reticulum by IP3, act through calmodulin and many other proteins.
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