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

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Abstract
Microbial cytochromes c ′ contain a 5‐coordinate His‐ligated heme that forms stable adducts with nitric oxide (NO) and carbon monoxide (CO), but not with dioxygen. We report the 1.95 and 1.35 Å resolution crystal structures 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 binds 6‐coordinate on the distal side. A second CO molecule, not bound to the 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.