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Publication Abstract from PubMed

The interaction of ferriheme proteins with nitrite has recently gained interest as a source for NO or other nitrogen oxides in mammalian physiology. However, met-hemoglobin (metHb), which was suggested as a key player in this process, does not convert nitrite unless small amounts of NO are added in parallel. We have recently reported that, in contrast, nitrophorins (NPs) convert nitrite as the sole substrate to form NO even at pH 7.5, which is an unprecedented case among ferrihemes [He, C., Knipp, M. (2009) <i>J. Am. Chem. Soc. 131</i>, 12042-12043]. NPs, which comprise a class of unique heme <i>b</i> proteins from the saliva of the blood feeding insect <i>Rhodnius prolixus</i>, appear in a number of concomitant isoproteins. Herein, the first spectroscopic characterization of the initial complexes of the two isoproteins NP4 and NP7 with nitrite is presented and compared to the data reported for metHb and met-myoglobin (metMb). Because upon nitrite binding, NPs, in contrast to metHb and metMb, continue to react with nitrite, resonance Raman spectroscopy and cw-EPR spectroscopy were applied to frozen samples. As a result, the existence of two 6-coordinate ferriheme low-spin complexes was established. Furthermore, X-ray crystallography of NP4 crystals soaked with nitrite revealed the formation of a <i>eta</i>¹-N nitro complex, which is in contrast to the <i>eta</i>¹-O bound nitrite in metMb and metHb. Stopped-flow kinetic experiments show that although the ligand dissociation constants of NP4 and NP7 (15 to 190 M^{horizontal line 1}) are comparable to those of the metHb and metMb, the rates of ligand binding and release are significantly slower. Moreover, the reaction kinetics, but also EPR spectroscopy exhibits notable differences between the two isoproteins.

Complex Formation of Nitrite with the Ferriheme <i>b</i> beta-Barrel Proteins Nitrophorin 4 and Nitrophorin 7.,He C, Ogata H, Knipp M Biochemistry. 2010 Jun 4. PMID:20524697^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.