Second Messengers

Second messengers are of such importance that they require a closer look. You will find this information essential for your later understanding of hormone and neuro-transmitter action. Let's consider how the hormone epinephrine stimulates a cell. Epinephrine, the "first messenger," cannot pass through plasma membranes, so it binds to a surface receptor. The receptor is linked on the intracellular side to a peripheral protein called a G protein (fig. 3.9). G proteins are named for the ATP-like chemical,

First messenger

First messenger

Second Messenger System

ATP cAMP (second messenger) {

ATP cAMP (second messenger) {

Inactive Activated kinase kinase *

Inactive Activated enzymes enzymes j

Various metabolic effects

Figure 3.9 A Second-Messenger System. (1) A messenger such as epinephrine (red triangle) binds to a receptor in the plasma membrane. (2) The receptor releases a G protein, which then travels freely in the cytoplasm and can have various effects in the cell. (3) The G protein binds to an enzyme, adenylate cyclase, in the plasma membrane. Adenylate cyclase converts ATP to cyclic AMP (cAMP), the second messenger. (4) cAMP activates a cytoplasmic enzyme called a kinase. (5) Kinases add phosphate groups (P|) to other cytoplasmic enzymes. This activates some enzymes and deactivates others, leading to varied metabolic effects within the cell.

Is adenylate cyclase an integral protein or a peripheral protein? What about the G protein?

guanosine triphosphate (GTP), from which they get their energy. When activated by the receptor, a G protein relays the signal to another membrane protein, adenylate cyclase (ah-DEN-ih-late SY-clase). Adenylate cyclase removes two phosphate groups from ATP and converts it to cyclic AMP (cAMP), the second messenger. Cyclic AMP then activates enzymes called kinases (KY-nace-es) in the cytosol. Kinases add phosphate groups to other cellular enzymes. This activates some enzymes and deactivates others, but either way, it triggers a great variety of physiological changes within the cell.

G proteins play such an enormous range of roles in physiology and disease that Martin Rodbell and Alfred Gilman received a 1994 Nobel Prize for discovering them. Up to 60% of currently used drugs work by altering the activity of G proteins.

Was this article helpful?

0 0
How To Boost Your Metabolism

How To Boost Your Metabolism

In The Next 45 Minutes You're Going To Discover How To Boost Your Metabolism And Lose Weight. Who Else Wants To Boost Their Metabolism And Shed Pounds Fast?

Get My Free Ebook


  • simone
    What are second messengers in physiology?
    3 years ago
    What is the second messenger effect in physiology?
    1 year ago
  • Adriano Dellucci
    What is a secondhand messenger in physiology?
    4 months ago

Post a comment