title: Vibrational analysis of mononitrosyl complexes in hemerythrin and flavodiiron proteins: relevance to detoxifying NO reductase.
abstract: Flavodiiron proteins (FDPs) play important roles in the microbial nitrosative stress response in low-oxygen environments by reductively scavenging nitric oxide (NO). Recently, we showed that FMN-free diferrous FDP from Thermotoga maritima exposed to 1 equiv NO forms a stable diiron-mononitrosyl complex (deflavo-FDP(NO)) that can react further with NO to form N(2)O [Hayashi, T.; Caranto, J. D.; Wampler, D. A; Kurtz, D. M., Jr.; Moënne-Loccoz, P. Biochemistry 2010, 49, 7040-7049]. Here we report resonance Raman and low-temperature photolysis FTIR data that better define the structure of this diiron-mononitrosyl complex. We first validate this approach using the stable diiron-mononitrosyl complex of hemerythrin, Hr(NO), for which we observe a ν(NO) at 1658 cm(-1), the lowest ν(NO) ever reported for a nonheme {FeNO}(7) species. Both deflavo-FDP(NO) and the mononitrosyl adduct of the flavinated FPD (FDP(NO)) show ν(NO) at 1681 cm(-1), which is also unusually low. These results indicate that, in Hr(NO) and FDP(NO), the coordinated NO is exceptionally electron rich, more closely approaching the Fe(III)(NO(-)) resonance structure. In the case of Hr(NO), this polarization may be promoted by steric enforcement of an unusually small FeNO angle, while in FDP(NO), the Fe(III)(NO(-)) structure may be due to a semibridging electrostatic interaction with the second Fe(II) ion. In Hr(NO), accessibility and steric constraints prevent further reaction of the diiron-mononitrosyl complex with NO, whereas in FDP(NO) the increased nucleophilicity of the nitrosyl group may promote attack by a second NO to produce N(2)O. This latter scenario is supported by theoretical modeling [Blomberg, L. M.; Blomberg, M. R.; Siegbahn, P. E. J. Biol. Inorg. Chem. 2007, 12, 79-89]. Published vibrational data on bioengineered models of denitrifying heme-nonheme NO reductases [Hayashi, T.; Miner, K. D.; Yeung, N.; Lin, Y.-W.; Lu, Y.; Moënne-Loccoz, P. Biochemistry 2011, 50, 5939-5947 ] support a similar mode of activation of a heme {FeNO}(7) species by the nearby nonheme Fe(II).
authors.affiliation: Division of Environmental and Biomolecular Systems, Institute of Environmental Health, Oregon Health and Science University, 20000 NW Walker Road, Beaverton, Oregon 97006-8921, USA.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Hayashi
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Caranto
authors.forename: Hirotoshi
authors.initials: H
authors.lastname: Matsumura
authors.forename: Donald M
authors.initials: DM
authors.lastname: Kurtz
authors.forename: Pierre
authors.initials: P
authors.lastname: Moënne-Loccoz
mesh_terms: D006422:Hemerythrin / Q000737:chemistry*; D033862:Iron-Binding Proteins / Q000737:chemistry*; D009569:Nitric Oxide; D010088:Oxidoreductases; D017550:Spectroscopy, Fourier Transform Infrared; D013059:Spectrum Analysis, Raman
chemical_list: D006422:Hemerythrin; D033862:Iron-Binding Proteins; D009569:Nitric Oxide; D010088:Oxidoreductases; C034747:nitric-oxide reductase