GTP = 3',5'-cyclic GMP + diphosphate Binds 1 or 2 heme groups per heterodimer. Activated by nitric oxide in the presence of magnesium or manganese ions. Heterodimer of an alpha and a beta chain. Expressed in gastric signet ring cell carcinoma, but not in the normal stomach. There are two types of guanylate cyclases: soluble forms and membrane-associated receptor forms. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family. Could be the product of a pseudogene.