AP BIO HELP!!!!! CELL SIGNALINg?

Question

ok i need help on these following question.

Q. How do the following mechanisms or molecules maintain a cell's ability to respond to fresh signals?

a. reversible binding of signal molecules

b. GTPase activity of G protein

Answer 1

Signal molecules are often called ligands, and they bind to receptors on the outside of cells. The proteins (or glycoproteins) that we call receptors are membrane bound proteins (like a G-protein coupled receptor. So, ligands (like a hormone) bind reversibly to these receptors, and begin the G-Protein signaling cascade. After stimulating the receptor, they are simply knocked off or fall off.

When a ligand stimulates a G-Protein coupled receptor, the receptor changes conformation, allowing it to associate with a 3-unit (trimeric) G-Protein. The alpha unit of the G-protein interacts with the receptor, and changes, where a bound GDP is replaced by a GTP molecule. The G-protein/GTP complex then activates a protein called adenylyl cyclase, which converts free cytoplasmic ATP into cAMP (cyclic AMP). The cAMP then activates PROTEIN KINASES, which in turn activate transcription factors, initiating transcription of some final gene product.

Let me give you an example. Fight or flight response, the body needs extra glucose to give to muscle cells, the heart, the brain, etc . . . the sympathetic system is activated, and the adrenal gland releases epinephrine. Epi then circulates where in liver cells, it binds to a G-Protein coupled receptor called a Beta-adrenergic receptor (adrenergic always refers to a receptor that epinephrine binds to). The receptor interacts with a G-Protein, the G-protein complexes with GTP, the GTP/G-protein complex activate adenylyl cyclase, ATP is converted to cAMP, transcription factors are activated, proteins that lead to glucose production are transcribed and translated. As a result, more glucose is released into the bloodstream for skeletal muscle to use as energy.

This is just one example of how a G-Protein works, there are others, but this gives you a basic idea of cell signaling. Cell signaling is EVERYWHERE, it is very important to understand it.