The Production of Ammonia by Multiheme Cytochromes c - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 228

the active site have been overlooked by these methods until recently [ 44 ] since
previous primer pairs were designed to target a nucleotide stretch encoding the
NrfA Cys-X-X-Cys-Lys signature [ 105 ].
Likewise, metagenomic data have not been rigorously investigated with respect
to the presence of ammonia-producing multiheme cytochromes c . Given that these
enzymes are generally present at the outside of the cytoplasmic membrane, for
example, in the periplasm of Gram-negative bacteria, any produced ammonia
would not be readily available to assimilatory cytoplasmic enzymes and had to be
taken up by suitable ammonium transporters. Consequently, multiheme cyto-
chromes c might contribute to the release of ammonia into the atmosphere,
depending on environmental parameters such as dioxygen availability, temperature,
and pH. To our knowledge, this is an as yet largely unaccounted feature in global-
scale models of ammonia production in habitats such as anaerobic soil or marine
oxygen minimum zones [ 29 , 30 , 106 - 108 ]. The situation is reminiscent to the
discovery that bacterial ammonifiers of nitrate and nitrite, i.e., NrfA-containing
microorganisms, might significantly contribute to atmospheric N 2 O production, in
addition to the denitrifying microbial community [ 109 - 112 ]. In this case, any NO
formed either enzymatically or abiotically from nitrite is apparently detoxified
under anaerobic conditions to N 2 O by a variety of NO-reactive enzymes. Taken
together, it seems that the biogeochemical nitrogen cycle still offers a lot of
surprising and fascinating discoveries, possibly with many more to come.

Abbreviations

anammox an aerobic amm onium ox idation ( ¼ comproportionation of nitrite and

ammonium to form dinitrogen)

DFT density functional theory

DNA deoxyribonucleic acid

EPR electron paramagnetic resonance

Fe/S iron-sulfur center

Hao hydroxylamine oxidoreductase

HOMO highest occupied molecular orbital

LUMO lowest unoccupied molecular orbital

MCC multiheme cytochrome c family

MK menaquinone

MKH 2 menaquinol/menahydroquinone

NapA periplasmic nitrate reductase

Nrf nitrite reduction by formate

NrfA pentaheme cytochrome c nitrite reductase

Onr octaheme cytochrome c nitrite reductase

Otr octaheme cytochrome c tetrathionate reductase

PCR polymerase chain reaction