Denitrification is a distinct means of energy conservation, making use of N oxides as terminal electron acceptors for cellular bioenergetics under anaerobic, microaerophilic, and occasionally aerobic conditions. The process is an essential branch of the global N cycle, reversing dinitrogen fixation, and is associated with chemolithotrophic, phototrophic, diazotrophic, or organotrophic metabolism but generally not with obligately anaerobic life. Discovered more than a century ago and believed to be exclusively a bacterial trait, denitrification has now been found in halophilic and hyperthermophilic archaea and in the mitochondria of fungi, raising evolutionarily intriguing vistas. Important advances in the biochemical characterization of denitrification and the underlying genetics have been achieved with Pseudomonas stutzeri, Pseudomonas aeruginosa, Paracoccus denitrificans, Ralstonia eutropha, and Rhodobacter sphaeroides. Pseudomonads represent one of the largest assemblies of the denitrifying bacteria within a single genus, favoring their use as model organisms. Around 50 genes are required within a single bacterium to encode the core structures of the denitrification apparatus. Much of the denitrification process of gram-negative bacteria has been found confined to the periplasm, whereas the topology and enzymology of the gram-positive bacteria are less well established. The activation and enzymatic transformation of N oxides is based on the redox chemistry of Fe, Cu, and Mo. Biochemical breakthroughs have included the X-ray structures of the two types of respiratory nitrite reductases and the isolation of the novel enzymes nitric oxide reductase and nitrous oxide reductase, as well as their structural characterization by indirect spectroscopic means. This revealed unexpected relationships among denitrification enzymes and respiratory oxygen reductases. Denitrification is intimately related to fundamental cellular processes that include primary and secondary transport, protein translocation, cytochrome c biogenesis, anaerobic gene regulation, metalloprotein assembly, and the biosynthesis of the cofactors molybdopterin and heme D1. An important class of regulators for the anaerobic expression of the denitrification apparatus are transcription factors of the greater FNR family. Nitrate and nitric oxide, in addition to being respiratory substrates, have been identified as signaling molecules for the induction of distinct N oxide-metabolizing enzymes.

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Cell biology and molecular basis of denitrification.

Nitric oxide (NO.), a potentially toxic molecule, has been implicated in a wide range of biological functions. Details of its biochemistry, however, remain poorly understood. The broader chemistry of nitrogen monoxide (NO) involves a redox array of species with distinctive properties and reactivities: NO+ (nitrosonium), NO., and NO- (nitroxyl anion). The integration of this chemistry with current perspectives of NO biology illuminates many aspects of NO biochemistry, including the enzymatic mechanism of synthesis, the mode of transport and targeting in biological systems, the means by which its toxicity is mitigated, and the function-regulating interaction with target proteins.

https://science.sciencemag.org/content/sci/258/5090/1898.full.pdf

Biochemistry of nitric oxide and its redox-activated forms

Functionalization of polyoxometalates: towards advanced applications in catalysis and materials science

Main-Group Element, Organic, and Organometallic Derivatives of Polyoxometalates.

The structural chemistry of metal thiolate complexes

Publisher Summary This chapter discusses the organometallic compounds containing oxygen atoms. At present, the range of organic ligands found in organometallic oxo compounds is quite restricted, the majority being either η-C 5 R 5 or alkyls or aryls having no β-hydrogen atom. Two basic routes to organometallic oxo compounds may be envisaged addition of an organic ligand to an inorganic oxo complex or addition of oxygen to an organometallic compound. Exhaustive decarbonylation with concomitant oxidation of a cyclopentadienyl metal carbonyl has proved to be a useful preparative route to cyclopentadienyl metal oxo compounds having no other ligands. In the ethylene complex, the oxo and ethylene ligands are cis to one another and the C–C axis of the ethylene is perpendicular to the M–O bond, a configuration that maximizes π bonding between the W(IV) (d 2 ) and the ethylene. Complexes containing cyclopentadienyl and oxygen as coligands are of two basic types: those containing a terminal double bond between a metal and oxygen, [M=O], and those containing one or more doubly bridging oxygen atoms, [M(μ 2 -O) n M]. The clusters are held together by M–O bonds and the M–( η 5 -C 5 H 5 ) bonding is of the usual type. Parallel to the development of organometallic clusters containing oxygen atoms has been the preparation of organometallic polyoxometallates. Although the organometallic groups are on the surface of the polyoxometallate, they are strongly and covalently bonded to the peripheral oxygen atoms.

Organometallic Compounds Containing Oxygen Atoms

Electrophilic behavior of coordinated nitric oxide

Nitrosyl complexes of ruthenium

Preparation, properties and structures of the clusters hexakis(.mu.3-oxo)pentakis(.eta.5-cyclopentadienylvanadium), [(.eta.5-C5H5)5V5O6], and tetrakis(.mu.3-oxo)tetrakis(.eta.5-cyclopentadienylchromium), [(.eta.5-C5H5)4Cr4O4]

Reactions of bis(cyclopentadienyl)- and bis(pentamethylcyclopentadienyl)titanium and bis(cyclopentadienyl)- and bis(pentamethylcuclopentadienyl)zirconium dicarbonyls with water and dihydrogen sulfide: evolution of dihydrogen and formation of oxo or sulfido monomers, dimers, or clusters

Frank Bottomley

Papers

Organometallic Compounds Containing Oxygen Atoms

Electrophilic behavior of coordinated nitric oxide

Nitrosyl complexes of ruthenium

Preparation, properties and structures of the clusters hexakis(.mu.3-oxo)pentakis(.eta.5-cyclopentadienylvanadium), [(.eta.5-C5H5)5V5O6], and tetrakis(.mu.3-oxo)tetrakis(.eta.5-cyclopentadienylchromium), [(.eta.5-C5H5)4Cr4O4]

Reactions of bis(cyclopentadienyl)- and bis(pentamethylcyclopentadienyl)titanium and bis(cyclopentadienyl)- and bis(pentamethylcuclopentadienyl)zirconium dicarbonyls with water and dihydrogen sulfide: evolution of dihydrogen and formation of oxo or sulfido monomers, dimers, or clusters