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 th e 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 metho ds, all of which display activation by NO. In our preparation, and in the e xpressed protein sGC(1), the resting Fe(II) state is mainly 6-coordinate an d low-spin, and the CO adduct has vibrational frequencies characteristic of a histidine-heme-CO complex in a hydrophobic environment. In contrast, the protein sGC(2) is 5-coordinate, high-spin in the resting state, and the CO adduct has perturbed vibrational frequencies indicative of a negatively po larizing residue in the binding pocket. The differences may result from the need to reconstitute sGC(1) or different isolation procedures for sGC(1) v ersus sGC(1). However, both sGC(1) and sGC(2) are activated by the same mec hanism, namely displacement of the proximal histidine ligand upon NO bindin g, and neither one is activated by CO. If CO is an activator in vivo, some additional molecular component is required.