text: Adenylyl cyclase is regulated by G proteins, which can be found in the monomeric form or the heterotrimeric form, consisting of three subunits. Adenylyl cyclase activity is controlled by heterotrimeric G proteins. The inactive or inhibitory form exists when the complex consists of alpha, beta, and gamma subunits, with GDP bound to the alpha subunit. In order to become active, a ligand must bind to the receptor and cause a conformational change. This conformational change causes the alpha subunit to dissociate from the complex and become bound to GTP. This G-alpha-GTP complex then binds to adenylyl cyclase and causes activation and the release of cAMP. Since a good signal requires the help of enzymes, which turn on and off signals quickly, there must also be a mechanism in which adenylyl cyclase deactivates and inhibits cAMP. The deactivation of the active G-alpha-GTP complex is accomplished rapidly by GTP hydrolysis due to the reaction being catalyzed by the intrinsic enzymatic activity of GTPase located in the alpha subunit. It is also regulated by forskolin, as well as other isoform-specific effectors:
  * Isoforms I, III, and VIII are also stimulated by Ca/calmodulin.
  * Isoforms V and VI are inhibited by Ca in a calmodulin-independent manner.
  * Isoforms II, IV and IX are stimulated by alpha subunit of the G protein.
  * Isoforms I, V and VI are most clearly inhibited by Gi, while other isoforms show less dual regulation by the inhibitory G protein.
  * Soluble AC (sAC) is not a transmembrane form and is not regulated by G proteins or forskolin, instead acts as a bicarbonate/pH sensor. It is anchored at various locations within the cell and, with phosphodiesterases, forms local cAMP signalling domains.
  In neurons, calcium-sensitive adenylyl cyclases are located next to calcium ion channels for faster reaction to Ca influx; they are suspected of playing an important role in learning processes. This is supported by the fact that adenylyl cyclases are coincidence detectors, meaning that they are activated only by several different signals occurring together.  In peripheral cells and tissues adenylyl cyclases appear to form molecular complexes with specific receptors and other signaling proteins in an isoform-specific manner.