Has guanylyl cyclase on binding to the beta-1 subunit. Isoform 2 acts as a negative regulator of guanylyl cyclase activity as it forms non-functional heterodimers with the beta subunits. GTP = 3',5'-cyclic GMP + diphosphate Activated by nitric oxide in the presence of magnesium or manganese ions. Heterodimer of an alpha and a beta chain. Isoform 1 is expressed in fetal brain, liver, colon, endothelium and testis. Isoform 2 is expressed only in liver, colon and endothelium. There are two types of guanylate cyclases: soluble forms and membrane-associated receptor forms. Belongs to the adenylyl cyclase class-4/guanylyl cyclase family.