Figure 2.

Alternative hypothesis of the allosteric effect of additional NO. In the presence of NO excess, the five-coordinate NO-heme adduct (state A) may bind a second NO to form a transient ternary complex (state B), which rapidly loses the distal NO and converts into a five-coordinate NO-heme adduct with NO bound on the proximal side of heme (state C). Alternatively, the allosteric effect of additional NO is explained by S-nitrosylation of an unspecified cysteine residue (state D), resulting in a full stimulation cGMP-forming activity.