Zumft WG, Kroneck PMH
Respiratory transformation of nitrous
oxide (N2O) to dinitrogen by Bacteria and Archaea
Adv. Microb. Physiol. 52:107-227 (2006)
N2O is a
potent greenhouse gas and stratospheric reactant that has been steadily on the
rise since the beginning of industrialization. It is an obligatory inorganic metabolite
of denitrifying bacteria, and some production of N2O is found also
in nitrifying and methanotrophic bacteria. We focus
this review on the respiratory aspect of N2O transformation
catalysed by the multicopper enzyme nitrous oxide reductase (N2OR) that provides the bacterial
cell with an electron sink for anaerobic growth. Two types of Cu centers discovered in N2OR were both novel
structures among the Cu proteins: the mixed-valent dinuclear CuA species
at the electron entry site of the enzyme, and the tetranuclear
CuZ center as
the first catalytically active Cu-sulfur complex
known. Several accessory proteins function as Cu chaperone and ABC transporter
system for the biogenesis of the catalytic centre. We describe here the
paradigm of Z-type N2OR, whose characteristics have been studied in
most detail in the genera Pseudomonas and Paracoccus.
Sequenced bacterial genomes now provide an invaluable additional source of
information. New strains harbouring nos genes
and capability of N2O utilization are being uncovered. This reveals
previously unknown relationships and allows pattern recognition and
predictions. The core nos genes, nosZDFYL, share a common phylogeny. Most principal
taxonomic lineages follow the same biochemical and genetic pattern and share
the Z-type enzyme. A modified N2OR is found in Wolinella
succinogenes, and circumstantial evidence also
indicates for certain Archaea another type of
N2OR. The current picture supports the view of evolution of N2O
respiration prior to the separation of the domains Bacteria and Archaea. Lateral nos
gene transfer from an e-proteobacterium
as donor is suggested for Magnetospirillum magnetotacticum and Dechloromonas
aromatica. In a few cases nos
gene clusters are plasmid borne. Inorganic N2O metabolism is associated
with a diversity of physiological traits and biochemically challenging
metabolic modes or habitats, including halorespiration,
diazotrophy, symbiosis, pathogenicity,
psychrophily, thermophily,
extreme halophily, and the marine habitat down to the
greatest depth. Components for N2O respiration cover topologically
the periplasm and the inner and outer membranes. The
Sec and Tat translocons share the task of exporting Nos components to their functional sites. Electron donation
to N2OR follows pathways with modifications depending on the host
organism. A short
chronology of the field is also presented.