. "Soluble guanylate cyclases are nitric oxide-responsive signaling proteins in which the nitric oxide sensor is a heme-binding domain of unknown structure that we have termed the heme-NO and oxygen binding (H-NOX) domain. H-NOX domains are also found in bacteria, either as isolated domains, or are fused through a membrane-spanning region to methyl-accepting chemotaxis proteins. We have determined the crystal structure of an oxygen-binding H-NOX domain of one such signaling protein from the obligate anaerobe Thermoanaerobacter tengcongensis at 1.77-angstroms resolution, revealing a protein fold unrelated to known structures. Particularly striking is the structure of the protoporphyrin IX group, which is distorted from planarity to an extent not seen before in protein-bound heme groups. Comparison of the structure of the H-NOX domain in two different crystal forms suggests a mechanism whereby alteration in the degree of distortion of the heme group is coupled to changes on the molecular surface of the H-NOX domain and potentially to changes in intermolecular interactions." . . "Proc. Natl. Acad. Sci. U.S.A." . "Kuriyan J." . "Marletta M.A." . "Pellicena P." . "Karow D.S." . "Boon E.M." . "2004"^^ . "12854-12859" . "Crystal structure of an oxygen-binding heme domain related to soluble guanylate cyclases." . "101" . "doi:10.1073/pnas.0405188101" .