Nitrophorin

The crystal structures of several nitrophorin complexes are known. The Rhodnius prolixus nitrophorin structures reveal lipocalin-like eight-stranded β-barrel, three α-helices and two disulfide bonds, with heme 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. The heme iron is ligated to histidine residue (His-59). The position of His-59 is restrained through water-mediated hydrogen bond to the carboxylate of aspartic acid residue (Asp-70). The His-59–Fe bond is bent ~15° out of the imidazole plane. Asp-70 forms an unusual hydrogen bond with one of the heme propionates, suggesting the residue has an altered pK_a. In NP1-histamine structure, the planes of His-59 and histamine imidazole rings lie in an arrangement almost identical to that found in oxidized cytochrome b₅.

The fold of nitrophorin from Cimex lectularius consists of central 11-stranded β-sandwich and seven peripheral α-helices. The heme is positioned between β-sheet and an α-helix, with heme iron ligated to cysteinate residue. NO can bind both to heme Fe³⁺ and to proximal Cys-60 ligand causing reversible S-nitrosylation.

• Montfort, W.R., Weichsel, A. and Andersen, J.F. (2000). "Nitrophorins and related antihemostatic lipocalins from Rhodnius prolixus and other blood-sucking arthropods". Biochimica et Biophysica Acta (BBA) - Protein Structure and Molecular Enzymology. 1482 (1–2): 110–118. doi:10.1016/S0167-4838(00)00165-5. PMID 11058753.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Valenzuela, J.G. & Ribeiro, J.M.C. (1998). "Purification and cloning of the salivary nitrophorin from the hemipteran Cimex lectularius". Journal of Experimental Biology. 201 (Pt 18): 2659–2664. doi:10.1242/jeb.201.18.2659. PMID 9716517.
• Weichsel, A., Maes, E.M., Andersen, J.F., Valenzuela, J.G., Shokhireva, T.Kh., Walker, F.A. and Montfort, W.R. (2005). "Heme-assisted S-nitrosation of a proximal thiolate in a nitric oxide transport protein". Proceedings of the National Academy of Sciences. 102 (3): 594–599. Bibcode:2005PNAS..102..594W. doi:10.1073/pnas.0406549102. PMC 545542. PMID 15637157.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• PDB: 1NP1​ — X-ray structure of Rhodnius prolixus nitrophorin 1 complexed with histamine and PO3−4
• PDB: 1Y21​ — X-ray structure of Cimex lectularius nitrophorin in complex with NO
• Montfort Lab - Nitrophorin research @ University of Arizona, Tucson, USA