• Prosthetic group features
• Nitrophorins in SWISS­PROT/TREMBL
• Nitrophorins in 3­D databases
• References

Haem type	Haem iron coordination	Axial iron ligands	Formal iron oxidation/spin states
Haem b	Pentacoordinate	NHis	FeII (S=2); FeIII (S=5/2)
	Hexacoordinate	NHis; ·NO, CN¯, NH3 or other ligand	FeII (S=0); FeIII (S=1/2)
	Hexacoordinate	NHis; Nhistamine	FeII (S=0); FeIII (S=1/2)

Nitrophorins are haemoproteins found in saliva of blood­feeding insects. Saliva of the blood­sucking bug Rhodnius prolixus contains four homologous nitrophorins, designated NP1 to NP4 in order of their relative abundance in the glands [1]. As isolated, nitrophorins contain nitric oxide (·NO) ligated to the ferric (Fe^III) haem iron. Histamine, which is released by the host in response to tissue damage, is another nitrophorin ligand. Nitrophorins transport ·NO to the feeding site. Dilution, binding of histamine and increase in pH (from pH ~5 in salivary gland to pH ~7.4 in the host tissue) facilitate the release of ·NO into the tissue where it induces vasodilatation.

Interestingly, the salivary nitrophorin from the hemipteran Cimex lectularius has no sequence similarity to Rhodnius prolixus nitrophorins. It is suggested that the two classes of insect nitrophorins have arisen as a product of the convergent evolution [2].

3­D structures of Rhodnius prolixus NP1 and NP4 complexes are known [3, 4]. The nitrophorin structures reveal lipocalin­like eight­stranded ß­barrel, three ­helices and two disulphide bonds, with haem inserted into one end of the barrel. Members of the lipocalin family are known to bind a variety of small hydrophobic ligands, including biliverdin, in a similar fashion (see [5] for review). The haem iron is ligated to His­59. The position of His­59 is restrained through water­mediated hydrogen bond to the carboxylate of Asp­70. The His­59-Fe bond is bent ~15° out of the imidazole plane. Asp­70 forms an unusual hydrogen bond with one of the haem propionates, suggesting the residue has an altered pK_a. In NP1-histamine structure (PDB 1NP1), the planes of His­59 and histamine imidazole rings lie in an arrangement almost identical to that found in oxidised cytochrome b₅ [3].

Nitrophorins in SWISS­PROT/TREMBL

Q26239	NP1_RHOPR	Nitrophorin 1 precursor (NP1); Rhodnius prolixus
Q26241	NP2_RHOPR	Nitrophorin 2 precursor (NP2) (prolixin-S); Rhodnius prolixus
Q94733	NP3_RHOPR	Nitrophorin 3 precursor (NP3); Rhodnius prolixus
O77000	O77000	Nitrophorin 3; Rhodnius prolixus
Q94734	NP4_RHOPR	Nitrophorin 4 precursor (NP4); Rhodnius prolixus

PDB	scop	BSM	RELI Base	Header	¹
1np1	1np1	1np1	1np1	Nitrophorin 1 (complex with histamine and PO43-); Rhodnius prolixus (recombinant form expressed in Escherichia coli)	-
1np4	-	1np4	1np4	Nitrophorin 4 (complex with NH3); Rhodnius prolixus (recombinant form expressed in Escherichia coli)	-
2np1	2np1	2np1	2np1	Nitrophorin 1 (complex with NH3 and H2PO4¯); Rhodnius prolixus (recombinant form expressed in Escherichia coli)	-
3np1	3np1	3np1	3np1	Nitrophorin 1 (complex with CN¯ and PO43-); Rhodnius prolixus (recombinant form expressed in Escherichia coli)	-
4np1	-	4np1	4np1	Nitrophorin 1 (complex with ·NO, PO43- and H2PO3); Rhodnius prolixus (recombinant form expressed in Escherichia coli)	-

¹ Macromolecular Structures abstract. Full text is available to BioMedNet Members

References

1. Champagne, D.E., Nussenzveig, R.H. and Ribeiro, J.M.C. (1995) Purification, partial characterization, and cloning of nitric oxide­carrying heme proteins (nitrophorins) from salivary glands of the blood­sucking insect Rhodnius prolixus. J. Biol. Chem. 270, 8691-8695.
2. Valenzuela, J.G. and Ribeiro, J.M.C. (1998) Purification and cloning of the salivary nitrophorin from the hemipteran Cimex lectularius. J. Exp. Med. 201, 2659-2664.
3. Weichsel, A., Andersen, J.F., Champagne, D.E., Walker, F.A. and Montfort, W.R. (1998) Crystal structures of a nitric oxide transport protein from a blood­sucking insect. Nature Struct. Biol. 5, 304-309.
4. Andersen, J.F., Weichsel, A., Balfour, C.A., Champagne, D.E. and Montfort, W.R. (1998) The crystal structure of nitrophorin 4 at 1.5 Å resolution: Transport of nitric oxide by a lipocalin­based heme protein. Structure 6, 1315-1327.
5. Flower, D.R. (1996) The lipocalin protein family: structure and function. Biochem. J. 318, 1-14.

Bibliography on structural studies of nitrophorins

Nitrophorin research @ University of Arizona, Tucson, USA