Resonance Raman spectra have been recorded for the ferrous heme of soluble guanylate cyclase (sGC), the only receptor known thus far for .NO. On the basis of the frequencies of porphyrin core sensitive vibrations in the high frequency region of the Raman spectrum, we conclude that the ferrous heme is five-coordinate, high spin, when no exogenous ligands are present. We assign a prominent vibration that occurs at 204 cm-1 in the reduced enzyme to the heme Fe(2+)-proximal histidine stretching vibration. In the .NO bound form of the enzyme, the heme Fe2+ retains a five-coordinate geometry. Assuming that .NO binds to the distal side of the heme, this observation indicates that the weak Fe-His bond breaks when .NO binds. Two isotope-sensitive vibrations are observed in the .NO bound enzyme, one at 1677 cm-1, attributed to the N-O stretching vibration, and one at 525 cm-1, attributed to the Fe-NO stretching vibration. When CO is bound to the ferrous heme, the heme ligation is six-coordinate. From this, we conclude that the Fe-His bond is still intact and that, if cleavage of the Fe-proximal ligand bond is necessary for complete activation of sGC, then CO should only weakly activate the enzyme, which has been shown to be the case. In the carbonmonoxy enzyme, the Fe-CO stretching vibration is observed at 472 cm-1 and the Fe-C-O bending vibration is detected at 562 cm-1. These frequencies are the lowest yet observed for the Fe-CO stretching and Fe-C-O bending modes in heme proteins or model systems with imidazole as the proximal ligand and suggest that there is significant negative polarity in the distal pocket. The negative polarity and the low frequency of the Fe-His stretching vibration may account for the very low O2 affinity of sGC.