Structural features of the guanylate cyclase domain. A) Structural representation of a guanylate cyclase domain monomer. Elements of secondary structure are labeled according to the nomenclature depicted in Figure 1. B) The guanylate cyclase catalytic domain. Monomer A is colored green, and monomer B is multi-colored, ranging from blue at the N-terminus to red at the C-terminus. Two of eight phosphate ions are shown and are depicted as stick figures: phosphorus, orange; oxygen, red.