In vitro, CyaB catalyzes the biosynthesis of cyclic AMP (cAMP) from ATP. It seems that under the physiological conditions CyaB has no function in cAMP processes. In vitro, it is also able to hydrolyze substrates such as thiamine triphosphate (ThTP) and inorganic triphosphate (PPPi) at a low rate. It has a slight preference for ThTP over ATP and PPPi in the presence of manganese ions. This PPPase activity is probably not of physiological importance. ATP = 3',5'-cyclic AMP + diphosphate Inhibited by GTP. Optimum pH is 9.5. Optimum temperature is 65 degrees Celsius. Cells lacking this gene show no impair of cAMP biosynthesis. Belongs to the adenylyl cyclase CyaB family.