Figure 2.

Schematic diagram of adenylyl and guanylyl cyclases. Mammalian adenylyl cyclases (left-hand side) are typically transmembrane proteins activated by heterotrimeric G proteins. Mammalian guanylyl cyclases (right-hand side) are receptor enzymes directly activated by hormones, such as nitric oxide (NO), or other factors that bind to its regulatory domains (for simplicity, the regulatory domains are not shown).