title: Is histidine dissociation a critical component of the NO/H-NOX signaling mechanism? Insights from X-ray absorption spectroscopy.
abstract: The H-NOX (Heme-Nitric oxide/OXygen binding) family of diatomic gas sensing hemoproteins has attracted great interest. Soluble guanylate cyclase (sGC), the well-characterized eukaryotic nitric oxide (NO) sensor is an H-NOX family member. When NO binds sGC at the ferrous histidine-ligated protoporphyrin-IX, the proximal histidine ligand dissociates, resulting in a 5-coordinate (5c) complex; formation of this 5c complex is viewed as necessary for activation of sGC. Characterization of other H-NOX family members has revealed that while most also bind NO in a 5c complex, some bind NO in a 6-coordinate (6c) complex or as a 5c/6c mixture. To gain insight into the heme pocket structural differences between 5c and 6c Fe(ii)-NO H-NOX complexes, we investigated the extended X-ray absorption fine structure (EXAFS) of the Fe(II)-unligated and Fe(II)-NO complexes of H-NOX domains from three species, Thermoanaerobacter tengcongensis, Shewanella woodyi, and Pseudoalteromonas atlantica. Although the Fe(II)-NO complex of TtH-NOX is formally 6c, we found the Fe-N(His) bond is substantially lengthened. Furthermore, although NO binds to SwH-NOX and PaH-NOX as a 5c complex, consistent with histidine dissociation, the EXAFS data do not exclude a very weakly associated histidine. Regardless of coordination number, upon NO-binding, the Fe-N(porphyrin) bond lengths in all three H-NOXs contract by ~0.07 Å. This study reveals that the overall heme structure of 5c and 6c Fe(II)-NO H-NOX complexes are substantially similar, suggesting that formal histidine dissociation may not be required to trigger NO/H-NOX signal transduction. The study has refined our understanding of the molecular mechanisms underlying NO/H-NOX signaling.
authors.affiliation: Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, USA.
authors.forename: Zhou
authors.initials: Z
authors.lastname: Dai
authors.forename: Erik R
authors.initials: ER
authors.lastname: Farquhar
authors.forename: Dhruv P
authors.initials: DP
authors.lastname: Arora
authors.forename: Elizabeth M
authors.initials: EM
authors.lastname: Boon
mesh_terms: D001665:Binding Sites; D005296:Ferrous Compounds / Q000737:chemistry; D006162:Guanylate Cyclase / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006639:Histidine / Q000378:metabolism*; D006860:Hydrogen Bonding; D009569:Nitric Oxide / Q000737:chemistry / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D031161:Pseudoalteromonas / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry / Q000378:metabolism*; D020592:Shewanella / Q000378:metabolism; D015398:Signal Transduction; D000071756:Soluble Guanylyl Cyclase; D046968:Thermoanaerobacter / Q000378:metabolism; D056928:X-Ray Absorption Spectroscopy
chemical_list: D005296:Ferrous Compounds; D018160:Receptors, Cytoplasmic and Nuclear; D009569:Nitric Oxide; D006418:Heme; D006639:Histidine; D006162:Guanylate Cyclase; D000071756:Soluble Guanylyl Cyclase