Structural Insight into H-NOX Gas Sensing and Cognate Signaling Protein Regulation

Heme-nitric oxide/oxygen binding (H-NOX) proteins are a family of gas-binding hemoproteins that bind diatomic gas ligands such as nitric oxide (NO) and oxygen (O-2). In bacteria, H-NOXs are often associated with signaling partners, including histidine kinases (HKs), diguanylate cyclases (DGCs) or methyl-accepting chemotaxis proteins (MCPs), either as a stand-alone protein or as a domain of a larger polypeptide. H-NOXs regulate the activity of cognate signaling proteins through ligand-induced conformational changes in the H-NOX domain and protein/protein interactions between the H-NOX and the cognate signaling partner. This review summarizes recent progress toward deciphering the molecular mechanism of bacterial H-NOX activation and the subsequent regulation of H-NOX-associated cognate sensor proteins from a structural and biochemical point of view.