AMP A Second Messenger

The action of epinephrine illustrates the principles by which cyclic AMP mediates hormone action. Epinephrine is the flight or fight hormone that the adrenal glands release in response to stress. The hormone causes an increase in blood pressure and the breakdown of 

When epinephrine binds to cells, it stays outside on the membrane-bound receptor. The second messenger, cyclic AMP, is made by the enzyme adenylate cyclase. 

Adenylate cyclase is a two-component enzyme system. It ultimately catalyzes the cyclase reaction, but only when it is associated with the hormone-bound receptor and a regulatory protein called a stimulatory G-protein (guanylate nucleotide binding protein), which activates adenylate cyclase. The G-protein is the intermediate between the receptor and the synthesis of cyclic AMP. 

G-proteins exist either in an active or an inactive state, depending on the guanylate nucleotide that is bound. In the inactive state, G-protein binds to GDP. In the active state, GTP is bound to the G-pro-tein. G-proteins have an intrinsic GTPase activity, which converts bound GTP to GDP. Hydrolysis of GTP by the G-protein converts the G-protein back to an inactive state. Thus the cycle of the G-protein is as follows  

The hormone-bound receptor binds to the G-protein and causes GDP to be replaced by GTP. 

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Adenylate cyclase regulates synthesis of cyclic AMP, a second messenger that carries the signal through various cellular pathways to end points of response within the cell (Figure 14—2). 

G-proteins are so called because they bind a guanosine nucleotide, either GTP or GDP. Their transduction mechanism involves the production of a second messenger such as 3 5 cAMP, 3 5 cyclic GMP (cGMP) or IP3 and diacylglycerol (DAG), derived from AMP, GMP and phosphatidyl inositol-3,5bisphosphate respectively (Figure 4.15). It is the second messenger that initiates the downstream amplification process phase of transduction. 

Figure 12-5. Transcriptional control by CREB. Cyclic AMP is a second messenger that mediates signaling from cell-surface receptors to elicit a response from the cell, in this case, a change in expression of genes that have a cyclic AMP response element (CRE) by binding of activated CREB.

Cyclic AMP, the intracellular second messenger in this system, is short-lived. It is quickly degraded by cyclic nucleotide phosphodiesterase to 5 -AMP (Fig. 12-12, step (7)), which is not active as a second messenger ... 

Cyclic AMP Acts as a Second Messenger for a Number of Regulatory Molecules... 

Most membrane receptors generate a diffusible intracellular signal called a second messenger. Five intracellular messengers are currently known Cyclic AMP, cyclic GMP, inositol triphosphate, diacylglyc-erol, and calcium. Second messengers usually activate or inhibit the action of one or more enzymes. 

Second messenger The term applied to compounds formed within the cell, such as cyclic AMP. The second messenger initiates a series of biochemical changes within the cell following stimulation of a receptor on the cell s outer surface by drugs, hormones, and so on. 

Adenylyl cyclase is the effector enzyme that synthesizes cyclic adenosine monophosphate (cAMP) from ATP. Cyclic AMP functions as a second messenger to relay extracellular signals to intracellular enzymes such as protein kinase A. Adenylyl cyclases are integral membrane proteins that... 

Figure 5.3 shows how the second messenger, cAMP, formed fi om ATP by adenylyl cyclase, transmits the signal by activating a second-messenger-dependent serine/ threonine protein kinase, the cyclic AMP-dependent protein kinase A (PKA). 

Insulin, like epin hrine and glucagon, appears to act on a receptor on the cell membrane, but does not appear to use cyclic AMP as a second messenger. 

Hormones that use 3, 5 -cyclic AMP (cAMP) as a second messenger stimulate a mechanism, resulting in the phosphorylation of enzymes. 

Considerable work has been done on cyclic AMP as a second messenger in the metabolic control of semen spermatozoa,122 123 as nucleotide concentration is closely correlated with motility of spermatozoa from boar, bull, cock, ram, and stallion. 

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