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nitrite reduction by menaquinol unlikely [
24
]. Electron transport between NrfCD
and NrfA in
E. coli
is thought to be mediated by the pentaheme cytochrome
c
NrfB,
which is distantly related to NrfH [
24
,
82
]. Taken together, it seems that different
electron transfer routes have been established during evolution in order to transport
electrons from the reduced quinone pool to NrfA, albeit without any influence on
the overall bioenergetics of the process.
These findings are a prominent example of the emerging picture of electron
transport module families that are involved in the build-up of functionally diverse
prokaryotic respiratory chains [
24
,
83
]. Furthermore, differences in the described
electron transport chains are also reflected in the fact that
E. coli
and
W. succinogenes
use different so-called cytochrome
c
biogenesis systems for the
maturation of NrfA and other cytochromes
c
in the periplasmic space [
84
]. In both
organisms, the unconventional Cys-X-X-Cys-Lys heme
c
attachment motif is
processed by a dedicated cytochrome
c
synthase isoenzyme that recognizes
its cognate NrfA by an unknown mechanism [
65
,
85
-
87
]. The corresponding
cytochrome
c
synthases of
E. coli
and
W. succinogenes
are largely unrelated in
their primary structures and it seems that they have been derived from the respec-
tive general cytochrome
c
biogenesis system (named System I or Ccm system in
E. coli
and System II or Ccs system in
W. succinogenes
) in order to enable NrfA
maturation [
84
,
88
,
89
]. Cys-X-X-Cys-Lys-specific cytochrome
c
synthases are
apparently missing in organisms that contain NrfA proteins carrying five conven-
tional Cys-X-X-Cys-His motifs [
65
].
4.4 Role of Cytochrome c Nitrite Reductase in Stress Defense
Although originally described as a key enzyme of anaerobic nitrite respiration, a
prominent function in nitrosative and peroxidative stress defense has been eluci-
dated for NrfA enzymes more recently due to the fact that NrfA reduces NO,
hydroxylamine, and hydrogen peroxide efficiently to ammonia and water, respec-
tively, in addition to nitrite, which is also a cytotoxic substance [
90
-
92
]. This
widens the physiological function of NrfA considerably as it appears that the
enzyme represents a unique periplasmic means to combat nitrosative stress, thus
supporting the many known cytoplasmic NO-reactive proteins present in aerobic as
well as anaerobic microorganisms such as (flavo)hemoglobins, flavorubredoxins,
and other flavodiiron proteins [
93
]. Furthermore, NrfA might functionally replace
catalase in some microaerobic bacteria [
92
,
94
].
Taken together, NrfA appears to carry out a versatile role in handling the above
mentioned stressors as well as in nitrite tolerance, which has been shown either by
physiological experiments using intact cells or with purified enzymes [
92
]. It might
also explain why NrfA is constitutively present in many bacteria irrespective of the
presence of nitrite.
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