Nitrophorins are 20 kDa ferric heme proteins from the salivary glands of the blood-sucking insect Rhodnius prolixus. In their native state, in the acidic environment of the insect saliva, nitrophorins have nitric oxide (NO) bound to heme Fe (III). The insect uses nitrophorins to deliver NO to host tissues during feeding where NO, released at neutral pH, induces vasodilation and inhibition of platelet aggregation. Additionally, nitrophorins act as antihistamine through strong binding of histamine released by the host at the site of injury.
NP1, the most abundant among four nitrophorins in Rhodnius prolixus saliva, has been expressed in E.coli, renatured and crystallized (Biochemistry 1997, 36, 4423-4428). The structure of the protein was solved by MIR methods and refined to 2.0 Å resolution.
The crystal structure of NP1 reveals heme to be sandwiched between strands of a lipocalin-like beta-barrel, and in an arrangement unlike any other gas-transport protein discovered to date. Structures of NP1 bound to NO (2.3 Å), cyanide (2.3 Å) and histamine (2.0 Å) reveal that the ligands compete for the same binding pocket, and are buried on binding.
Look here for molecular surface of the NP1-CN with the ligand shown in green. The surface