Reaction of Carbon Monoxide with the Reduced Active Site of Bacterial Nitric Oxide Reductase†

Nitrogen is an essential element for life, with the availability of fixed nitrogen limiting productivity in many ecosystems. The return of oxidized nitrogen species to the atmospheric N2 pool is predominately catalyzed by microbial denitrification (NO3- → NO2- → NO → N2O → N2). Incomplete denitrification can produce N2O as a terminal product, leading to an increase in atmospheric N2O, a potent greenhouse and ozone depleting gas2. The production of N2O is catalyzed by nitric oxide reductase (NOR) members of the heme-copper oxidoreductase (HCO) superfamily3. Here we propose that a number of uncharacterized HCO families perform nitric oxide reduction and demonstrate that an enzyme from Rhodothermus marinus, belonging to one of these families does perform nitric oxide reduction. These families have novel active-site structures and several have conserved proton channels, suggesting that they might be able to couple nitric oxide reduction to energy conservation. They also exhibit broad phylogenetic and environmental distributions, expanding the diversity of microbes that can perform denitrification. Phylogenetic analyses of the HCO superfamily demonstrate that nitric oxide reductases evolved multiple times independently from oxygen reductases, suggesting that complete denitrification evolved after aerobic respiration.

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Unexpected weak magnetic exchange coupling between haem and non-haem iron in the catalytic site of nitric oxide reductase (NorBC) from Paracoccus denitrificans1

Biochemical Journal ◽

10.1042/bj20121406 ◽

2013 ◽

Vol 451 (3) ◽

pp. 389-394 ◽

Cited By ~ 6

Author(s):

Jessica H. Van Wonderen ◽

Vasily S. Oganesyan ◽

Nicholas J. Watmough ◽

David J. Richardson ◽

Andrew J. Thomson ◽

...

Keyword(s):

Nitric Oxide ◽

Active Site ◽

Exchange Coupling ◽

Paracoccus Denitrificans ◽

Magnetic Circular Dichroism ◽

Variable Temperature ◽

Field Variable ◽

Nitric Oxide Reductase ◽

Magnetic Exchange ◽

Haem Iron

Bacterial NOR (nitric oxide reductase) is a major source of the powerful greenhouse gas N2O. NorBC from Paracoccus denitrificans is a heterodimeric multi-haem transmembrane complex. The active site, in NorB, comprises high-spin haem b3 in close proximity with non-haem iron, FeB. In oxidized NorBC, the active site is EPR-silent owing to exchange coupling between FeIII haem b3 and FeBIII (both S=5/2). On the basis of resonance Raman studies [Moënne-Loccoz, Richter, Huang, Wasser, Ghiladi, Karlin and de Vries (2000) J. Am. Chem. Soc. 122, 9344–9345], it has been assumed that the coupling is mediated by an oxo-bridge and subsequent studies have been interpreted on the basis of this model. In the present study we report a VFVT (variable-field variable-temperature) MCD (magnetic circular dichroism) study that determines an isotropic value of J=−1.7 cm−1 for the coupling. This is two orders of magnitude smaller than that encountered for oxo-bridged diferric systems, thus ruling out this configuration. Instead, it is proposed that weak coupling is mediated by a conserved glutamate residue.

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Synthesis and Spectroscopy of μ-Oxo (O2-)-Bridged Heme/Non-heme Diiron Complexes: Models for the Active Site of Nitric Oxide Reductase

Inorganic Chemistry ◽

10.1021/ic0348143 ◽

2004 ◽

Vol 43 (2) ◽

pp. 651-662 ◽

Cited By ~ 26

Author(s):

Ian M. Wasser ◽

Constantinus F. Martens ◽

Claudio N. Verani ◽

Eva Rentschler ◽

Hong-wei Huang ◽

...

Keyword(s):

Nitric Oxide ◽

Active Site ◽

Nitric Oxide Reductase

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Kinetic and thermodynamic resolution of the interactions between sulfite and the pentahaem cytochrome NrfA from Escherichia coli

Biochemical Journal ◽

10.1042/bj20100866 ◽

2010 ◽

Vol 431 (1) ◽

pp. 73-80 ◽

Cited By ~ 28

Author(s):

Gemma L. Kemp ◽

Thomas A. Clarke ◽

Sophie J. Marritt ◽

Colin Lockwood ◽

Susannah R. Poock ◽

...

Keyword(s):

Nitric Oxide ◽

Escherichia Coli ◽

Active Site ◽

Dissociation Constants ◽

Cellular Activity ◽

Nitric Oxide Reductase ◽

Wide Range ◽

Nitric Oxide Reduction ◽

Nitric Oxide Detoxification ◽

Kinetic And Thermodynamic Studies

NrfA is a pentahaem cytochrome present in a wide-range of γ-, δ- and ε-proteobacteria. Its nitrite and nitric oxide reductase activities have been studied extensively and contribute to respiratory nitrite ammonification and nitric oxide detoxification respectively. Sulfite is a third substrate for NrfA that may be encountered in the micro-oxic environments where nrfA is expressed. Consequently, we have performed quantitative kinetic and thermodynamic studies of the interactions between sulfite and Escherichia coli NrfA to provide a biochemical framework from which to consider their possible cellular consequences. A combination of voltammetric, spectroscopic and crystallographic analyses define dissociation constants for sulfite binding to NrfA in oxidized (~54 μM), semi-reduced (~145 μM) and reduced (~180 μM) states that are comparable with each other, and the Km (~70 μM) for sulfite reduction at pH 7. Under comparable conditions Km values of ~22 and ~300 μM describe nitrite and nitric oxide reduction respectively, whereas the affinities of nitrate and thiocyanate for NrfA fall more than 50-fold on enzyme reduction. These results are discussed in terms of the nature of sulfite co-ordination within the active site of NrfA and their implications for the cellular activity of NrfA.

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