Activation of soluble guanylyl cyclase (sGC) by NO correlates with scission of the proximal iron-histidine bond, as demonstrated by the application of electronic absorption and resonance Raman spectroscopy to the study of metalloporphyrin-substituted enzymes. The non-native metalloporphyrins, Mn(II)PPIX and Co(II)PPIX, can be introduced into heme-deficient sGC forming five-coordinate complexes. The similarity among Mn(II)sGC, Co(II)sGC, and the corresponding metalloporphyrin-substituted derivatives of Mb and Hb provides confirming evidence for the presence of an axial histidine ligand in sGC. Upon addition of NO, Mn(II)sGC forms a six-coordinate species with the histidine ligand still bound to the Mn, and the enzyme is not activated. In contrast, the Co(II)sGC(NO) adduct is five-coordinate and the enzyme is activated. These data imply that the activated state of sGC is attained when the proximal histidine-metal bond is broken.