Nitrophorins: Nitrite disproportionation reaction and other novel functionalities of insect heme‐based nitric oxide transport proteins

Abstract

SummaryNitrophorins (NPs) comprise a unique class of heme proteins used by the blood-sucking insect Rhodnius prolixus to deliver the signaling gas molecule NO into the blood vessel of a host during feeding. Upon NO release, histamine can be scavenged by coordination to the heme iron. Although the protein is of similar size as the mammalian globin monomers and shares the same cofactor and proximal histidine coordination, nitrophorin structure, in contrast, is almost entirely composed of a b-barrel. Comparison of the NO and histamine association constants with the concentrations of both compounds in vivo raises concerns about the very simple ligand release model in case of at least some of the NPs. Therefore, novel functionalities of the NPs were sought. As a result, catalysis of the nitrite disproportionation reaction was found, which leads to the formation of NO with nitrite as the sole substrate. This is the first example of a ferriheme protein that can perform this reaction. Furthermore, although NPs stabilize the ferriheme state, a peroxidase reactivity of the cofactor involving the higher oxidation state iron (Compound I/II) was studied with the potential to catalyze the oxidation of histamine and norepinephrine. In contrast to many other heme proteins including the globins, the ferroheme state was found to be extremely sensitive to O 2 , which is a consequence of the much lower reduction potential of the NPs, so that the 1-electron reduction of O 2 to O À 2 becomes a thermodynamically favored process. Altogether, the detailed study of the NPs gives insight into the structure-function relationships required for the targeted delivery of diatomic gas molecules in biology. Moreover, the comparison of the structure-function relationships of the NPs (NO transporters) with those of the globins (O 2 transporters) will help to elucidate the architectural requirement for the respective tasks.

Designed multi-stranded heme binding β-sheet peptides in membrane

“…However, the design and characterization of atomic-resolution structures of β-sheet proteins that would bind to heme are yet to be reported. Beta barrel water soluble and membrane proteins are known to confer heme coordination 36,37. A heme-iron transporter beta barrel membrane protein, Has R, with 22 antiparallel beta-strands, binds heme at the surface of the barrel through bis-histidine coordination (Fig.…”

Mechanisms of nitrite bioactivation

“…The mechanisms shown here are based on those that are heme-based, molybdopterin-based, and those that function as nitrite anhydrases including nitrophorin [131, 132]. …”

Insertion of Heme b into the Structure of the Cys34‐Carbamidomethylated Human Lipocalin α1‐Microglobulin: Formation of a [(Heme)2(α1‐Microglobulin)]3 Complex

“…During a blood meal, the insects pump their saliva into the tissue of a host, where NO is released and serves as a vasodilator and blood‐coagulation inhibitor. In this respect, although expressed in insect tissue, the function of NPs is deployed in vertebrate blood 18c…”