Unprecedented proximal binding of nitric oxide to heme: implications for guanylate cyclase

Microbial cytochromes c' contain a 5-coordinate His-ligated heme that forms stable adducts with nitric oxide (NO) and carbon monoxide (CO), but not wi th dioxygen, We report the 1.95 and 1.35 Angstrom resolution crystal struct ures of the CO- and NO-bound forms of the reduced protein from Alcaligenes xylosoxidans, NO disrupts the His-Fe bond and binds in a novel mode to the proximal face of the heme, giving a 5-coordinate species. In contrast, CO b inds 6-coordinate on the distal side. A second CO molecule, not bound to th e heme, is located in the proximal pocket. Since the unusual spectroscopic properties of cytochromes c' are shared by soluble guanylate cyclase (sGC), our findings have potential implications for the activation of sGC induced by the binding of NO or CO to the heme domain.