cyclic AMP (cAMP)

Introduction

cAMP is a second messenger used for intracellular signal transduction. A second messenger system is a method of cellular signaling where the signaling molecule does not enter the cell, rather utilizes a cascade of events that transduces the signal into a cellular change. The main purpose of cAMP is the activation of protein kinases.

History
Sutherland and his associates discovered cAMP in 1957 as an intermediate in hyperglycemic effect of epinephrine & glucagon. Since it has been shown that cAMP mediates the effects of hormones and other biologically active agents. It is present in all animal cells. However its occurance in higher plants has been the subject of debate. Recent evidences indicates that cAMP is formed in plants and it has been suggested that in plants too, cAMP mediates the effects of hormones. cAMP have been found in green alga Chlamydomonas reinhardii and prokaryotic blue-green alga Anacystis nedulans.

Synthesis and Decompositon
cAMP is synthesised from ATP by adenylyl cyclase which is located at the cell membranes. Adenylate cyclase is activated by the hormones glucagon and adrenaline. Liver adenylate cyclase responds more strongly to glucagon, and muscle adenylate cyclase responds more strongly to adrenaline.cAMP decomposition into AMP is catalyzed by the enzyme phosphodiesterases.

Role of cAMP in Bacteria
In Bacteria, the level of cAMP varies depending on the medium used for growth. cAMP is low when glucose is the carbon source.This occurs through inhibition of the enzyme, adenylate cyclase.
An example of cAMP's function is the positive regulation of the lac operon.

• In low glucose concentration, cAMP accumulates and binds to the allosteric site on CRP and stimulate RNA transcription by 50X.
  


• In high glucose concentration, cAMP concentration decreases, and the CRP disengage from the lac operon and the transcription halts.