The Interaction between the Ferriheme Protein Nitrophorin and Nitrite

摘要

The ferriheme proteins nitrophorin 4 and 7 convert nitrite into nitric oxide at neutral pH, which is an unprecedented case among the proteins. The initial complex formed is a 6-coordinate low-spin FeIII--N nitro complex (S = 1/2). Determination of the reaction kinetics of the initial nitrite binding step allows the estimation of the nitrite affinity being comparable to that of met-hemoglobin and met-myoglobin, which indicates that nitrite may be a substrate of nitrophorins in vivo.Nitrophorins (NPs) comprise a unique class of ferriheme b proteins that are found in the saliva of the blood-sucking insect Rhodnius prolixus [1]. Four NPs, NP1-4, were isolated from the saliva of these insects [2]. A fifths isoprotein, NP7, was recently found in a cDNA library [3] characterized [4-7]. The major biological function of these proteins is the delivery of NO to a host species throughout a blood meal. Recently, the interaction of ferriheme proteins with NO2~ gained much interest because in the presence of small amounts of NO, a nitrite anhydrase/reductase reaction appears with met-hemoglobin (metHb), which may be important for the production of N2O3 from blood NO2~ [8, 9]. However, in the absence of NO, both metHb and met-myoglobin (metMb) do not exhibit a reaction with NO2~ other than NO2~ binding [10]. In case of NP4 and NP7, we now have conducted a detailed study of the interaction with nitrite.