Heterodimers with Gyc-89Da and Gyc-89Db are activated in response to changing oxygen concentrations, alerting flies to hypoxic environments. Under normal oxygen concentrations, oxygen binds to the heme group and results in low levels of guanylyl cyclase activity. When exposed to reduced oxygen concentrations, the oxygen dissociates from the heme group resulting in activation of the enzyme. GTP = 3',5'-cyclic GMP + diphosphate Binds 1 or 2 heme groups per heterodimer. Probably not activated by nitric oxide (NO). Heterodimer also exhibits some stimulation, some compounds (SIN-1 and two of the NONOates) that were ineffective at stimulating Gyc-88E alone did stimulate the heterodimer. Heterodimer; with Gyc88E, in the presence of magnesium or manganese. There are two types of guanylate cyclases: soluble forms and membrane-associated receptor forms. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family.