The HNOB (Haem NO Binding) domain, is a predominantly alpha-helical domain and binds heme via a covalent linkage to histidine. It is a haem protein sensor (SONO) that displays femtomolar affinity for nitrous oxide, NO [3]. It is predicted to function as a haem-dependent sensor for gaseous ligands and to transduce diverse downstream signals in both bacteria and animals.

1: Ancient conserved domains shared by animal soluble guanylyl cyclases and bacterial signaling proteins. Iyer LM, Anantharaman V, Aravind L; BMC Genomics 2003;4:5. PMID:12590654

2: Crystal structure of an oxygen-binding heme domain related to soluble guanylate cyclases. Pellicena P, Karow DS, Boon EM, Marletta MA, Kuriyan J; Proc Natl Acad Sci U S A 2004;101:12854-12859. PMID:15326296

3: Femtomolar sensitivity of a NO sensor from Clostridium botulinum. Nioche P, Berka V, Vipond J, Minton N, Tsai AL, Raman CS; Science. 2004;306:1550-1553. PMID:15472039