Variable forms of soluble guanylyl cyclase: Protein-ligand interactions and the issue of activation by carbon monoxide

Soluble guanylyl cyclase (sGC) is known to be activated by NO binding to the heme moiety; previous studies have shown that CO does not activate sGC to the same extent as NO. Resonance Raman spectroscopy reveals different heme pocket structures for soluble guanylyl cyclase prepared by alternate methods, all of which display activation by NO. In our preparation, and in the expressed protein sGC₁, the resting Fe(II) state is mainly 6-coordinate and low-spin, and the CO adduct has vibrational frequencies characteristic of a histidine-heme-CO complex in a hydrophobic environment. In contrast, the protein sGC₂ is 5-coordinate, high-spin in the resting state, and the CO adduct has perturbed vibrational frequencies indicative of a negatively polarizing residue in the binding pocket. The differences may result from the need to reconstitute sGC₁ or different isolation procedures for sGC₁ versus sGC₂. However, both sGC₁ and sGC₂ are activated by the same mechanism, namely displacement of the proximal histidine ligand upon NO binding, and neither one is activated by CO. If CO is an activator in vivo, some additional molecular component is required.