The Production of Ammonia by Multiheme Cytochromes c - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 217

taken up for assimilatory purposes, the cytochrome c NrfA enzyme is regarded to
contribute to nitrogen retention in the habitat.

4.1 Cytochrome c Nitrite Reductase-Containing
Organisms and Their Physiology

The NrfA enzyme is encoded on various bacterial genomes (see [ 44 ] for a com-
prehensive list of bacteria coding for a NrfA homolog). On the other hand, nrfA
genes have not been reported from archaeal or eukaryotic species. Organisms that
use cytochrome c nitrite reductase as a nitrite-ammonifying enzyme are abundantly
present in soil, water, and host-associated habitats such as the intestine or the rumen
of animals. NrfA-containing microbes are assigned to a huge variety of bacterial
phyla (including both Gram-negative and Gram-positive organisms) and vary
tremendously in their physiological lifestyles. Prominent examples are (i) growth
by anaerobic respiration using nitrate, nitrite and/or nitrous oxide as terminal
electron acceptor, (ii) growth by anaerobic respiration using non-nitrogen com-
pounds as electron acceptor such as sulfate, sulfite or fumarate, (iii) growth by
(micro)aerobic respiration, and (iv) growth by fermentation of carbohydrates.
The best characterized organisms with respect to structure and function of NrfA
are host-associated bacteria such as Escherichia coli and Wolinella succinogenes as
well as free-living species such as Shewanella oneidensis , Sulfurospirillum
deleyianum , Desulfovibrio desulfuricans , and Desulfovibrio vulgaris [ 24 ]. More
recently, nrfA genes were also reported to be present in the genomes of several
Gram-positive bacteria (both Firmicutes and Actinobacteria), for example in Bacil-
lus selenatireducens , Bacillus azotoformans , Bacillus bataviensis , Bacillus vireti ,
and Arcanobacterium haemolyticum [ 44 - 46 ].

4.2 Biochemistry and Structure of Cytochrome c
Nitrite Reductase

mol NO 2
min 1 mg 1 at optimum pH 7.0, which led to the development of an electrochem-
ical nitrite sensor. Nitric oxide (NO) and hydroxylamine (NH 2 OH) as well as its
O-methyl derivative are also transformed to ammonia, however at reduced activi-
ties. Similar findings were reported for the assimilatory sulfite reductase from
D. vulgaris , which catalyzes another important multi-electron, multi-proton trans-
fer reaction in biology, the six-electron reduction of sulfite (SO 2
3

Purified NrfA enzymes exhibit high specific activities of up to > 1,000 ʼ

) to hydrogen

sulfide (H 2 S) (equation 18 )[ 47 - 49 ].