Proposed guanylate cyclase activation mechanism. Comparison of helix α1 in the guanylate cyclase structure with helix α1 of the active mammalian adenylate cyclase structure indicates that the guanylate cyclase structure is in an inactive state. Monomer B of the guanylate cyclase structure was superimposed onto the C2 domain of the active adenylate cyclase structure (PDB ID: 1CJU)  and monomer A was superimposed onto the C1 domain of the same adenylate cyclase structure. The C2 domain is omitted for clarity. The C1 domain of the active adenylate cyclase structure is colored blue, and the guanylate cyclase structure is colored green. The nucleotide 2',3'-dideoxyadenosine triphosphate (ddATP) and Mg2+ ions from 1CJU are shown as a stick figure and spheres: phosphorus, orange; oxygen, red; nitrogen, blue; magnesium, white.